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Animal Essay

500 Words Essay on Animal

Animals carry a lot of importance in our lives. They offer humans with food and many other things. For instance, we consume meat, eggs, dairy products. Further, we use animals as a pet too. They are of great help to handicaps. Thus, through the animal essay, we will take a look at these creatures and their importance.

Types of Animals

First of all, all kinds of living organisms which are eukaryotes and compose of numerous cells and can sexually reproduce are known as animals. All animals have a unique role to play in maintaining the balance of nature.

A lot of animal species exist in both, land and water. As a result, each of them has a purpose for their existence. The animals divide into specific groups in biology. Amphibians are those which can live on both, land and water.

Reptiles are cold-blooded animals which have scales on their body. Further, mammals are ones which give birth to their offspring in the womb and have mammary glands. Birds are animals whose forelimbs evolve into wings and their body is covered with feather.

They lay eggs to give birth. Fishes have fins and not limbs. They breathe through gills in water. Further, insects are mostly six-legged or more. Thus, these are the kinds of animals present on earth.

Importance of Animals

Animals play an essential role in human life and planet earth. Ever since an early time, humans have been using animals for their benefit. Earlier, they came in use for transportation purposes.

Further, they also come in use for food, hunting and protection. Humans use oxen for farming. Animals also come in use as companions to humans. For instance, dogs come in use to guide the physically challenged people as well as old people.

In research laboratories, animals come in use for drug testing. Rats and rabbits are mostly tested upon. These researches are useful in predicting any future diseases outbreaks. Thus, we can protect us from possible harm.

Astronomers also use animals to do their research. They also come in use for other purposes. Animals have use in various sports like racing, polo and more. In addition, they also have use in other fields.

They also come in use in recreational activities. For instance, there are circuses and then people also come door to door to display the tricks by animals to entertain children. Further, they also come in use for police forces like detection dogs.

Similarly, we also ride on them for a joyride. Horses, elephants, camels and more come in use for this purpose. Thus, they have a lot of importance in our lives.

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Conclusion of Animal Essay

Thus, animals play an important role on our planet earth and in human lives. Therefore, it is our duty as humans to protect animals for a better future. Otherwise, the human race will not be able to survive without the help of the other animals.

FAQ on Animal Essay

Question 1: Why are animals are important?

Answer 1: All animals play an important role in the ecosystem. Some of them help to bring out the nutrients from the cycle whereas the others help in decomposition, carbon, and nitrogen cycle. In other words, all kinds of animals, insects, and even microorganisms play a role in the ecosystem.

Question 2: How can we protect animals?

Answer 2: We can protect animals by adopting them. Further, one can also volunteer if one does not have the means to help. Moreover, donating to wildlife reserves can help. Most importantly, we must start buying responsibly to avoid companies which harm animals to make their products.

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• Biology Article

Animal Kingdom

Animals are eukaryotic, multicellular, species belonging to the Kingdom Animalia. Every animal has its own unique characteristics. They obtain their energy either by feeding on plants or on other animals. There are millions of species which have been identified, few share similar characteristics while others differ drastically.

Also Read:  Lower Invertebrates

Classification of Animal Kingdom

Animals are classified based on their characteristics. They are eminent from algae , plants, and fungus where rigid cell walls are absent. Some are also heterotrophic, in general, they digest their food within the internal chambers which again distinguish them from algae and plants. Another elite character of these species is that they are motile, except in certain life stages.

Vertebrates

• Organ Level of Organization: Animal tissues comprising of similar capacity are classified into shaped organs. Every organ is definite for particular capacity. For example Platyhelminthes.
• Tissue Level of Organization: Animal cells displaying division of exercises among themselves.Cells performing the same function cooperate to form tissues.
• Organ framework Level of Organization: The organ framework level of organization are displayed in those organisms where organs define the shape of functional frameworks and each framework is with a distinct physiological capacity.
• Cellular Level of Organization:  This organization consists of animals with cells which are formed as free cell lumps.

Recommended Video:

Organ Systems Patterns

Circulatory System: They are 2 types of the Circulatory framework – open type and closed type.

• Open Type: In this type of circulatory system the blood is pumped out of the heart. For example Mollusca and Arthropods.
• Closed Type: In this type of circulatory system the blood flows through a progression of vessels that is capillaries, arteries, and veins.

Digestive System:  There are 2 types of digestive system.  Complete and Incomplete digestive systems.

• Complete Digestive System: In this type of digestive system there are 2 openings to the outside of the body, a rear-end and a mouth. For instance: Chordates and Arthropods.
• Incomplete Digestive System:  It consists of only one open to the outside of the body a solitary opening which serves as both rear-end and mouth. For example Platyhelminthes.

Body Symmetry:  There are 3 types of symmetry. Bilateral, Radial, and Asymmetrical.

• Bilateral Symmetry: Animals, where a body can be partitioned into indistinguishable left and right parts, are known to be bilaterally symmetrical.
• Radial Symmetry: Animals tend to display spiral symmetry. For example Coelenterates, Echinoderms, and Ctenophores.
• Asymmetrical: Asymmetry is the finished nonappearance of symmetry. That is a few animals cannot be divided into two equivalent parts along with any plane going through the focal point of the organism. For example Sponges.

Also Read:  Kingdom Plantae, Animalia, Viruses

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The Biggest Myths About Motherhood in the Animal Kingdom

M y closest brush with motherhood was an intense 24 hours fostering an orphaned baby owl monkey in the Peruvian Amazon in 2009. According to Charles Darwin, my maternal drive should have transformed me into an intuitively wise and selfless nurse. But the truth was I felt quite traumatized—fretful, exhausted, and for the sake of my defiled and defecated hair alone (the baby was happiest when clinging to my head), uninclined to repeat the ordeal ever again. I was 39 at the time and wrestling with whether I should be having children myself. My night with the owl monkey reinforced my suspicion that I was not cut out for motherhood.

Females have long been equated with motherhood, as if no other role existed. But my research about motherhood in the animal kingdom taught me that maternal instinct is a long-standing myth, created by men, that reduces females to identikit automatons and belittles the complexities of motherhood.

Firstly, maternal instinct assumes caring for young is the sole responsibility of the female. In the owl monkey’s case, his mother would have suckled him every few hours. But after each feeding she would have driven him away, quite unsentimentally, by biting his tail, leaving his father to take on the heavy job of carrying him 90% of the time.

The commitment to childcare demonstrated by owl monkey fathers is admittedly not the norm amongst mammals (only one in ten species exhibit direct male care) but once females are liberated from the physiological responsibilities of pregnancy and lactation, dads become much more devoted. Amongst birds, biparental care is the overwhelming majority with 90% of avian couples sharing the load. Slide back along the evolutionary scale and paternal care becomes not only more common, but customary. Amongst fish, it’s single dads that do all the nursing in almost two thirds of species, with moms doing little more than dumping eggs and disappearing. Some, like the male seahorse even give birth.

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Read More: There Are Amazing Fathers in the Animal Kingdom

It’s a similar story with amphibians, which display a range of parental care strategies from single dads to single moms to co-parenting. Flashy little poison frogs, for example, make for surprisingly dedicated parents transporting tadpoles to a safe water source on their back like a wriggling knapsack. This marathon is mostly performed by males but can be females or even both. Lauren O’Connell, assistant professor of biology at Stanford recognized this variability offered a unique opportunity to examine the neural circuitry controlling parental care. She discovered it is identical across the sexes.

The same story is true in mammals. Catherine Dulac Higgins, professor of molecular and cellular biology at Harvard, uncovered the same switch for parenting in the brains of mice. So, it’s not that one sex is programmed to provide care, both males and females retain the neural architecture to drive this urge. Dulac has yet to discover the trigger for this parental instinct, but she assumes it will be a complex mixture of internal and external cues.

The impulse to parent may be hard-wired, but the bespoke actions it elicits go way beyond mere instinct. “We are so simplistic in the way we see things as being either male-specific or female-specific” Higgins told me. “If you look around, whether it's in humans or in any animal, not all behave the same. Not all females are equally maternal. There is enormous variability.”

The pioneering scientist Jeanne Altmann was the first to provide evidence. Her 40-year baboon study has revealed these working moms spend 70% of each day making a living by walking several kilometres a day in search of food. When a female gives birth, there is no down time to recover. Though exhausted from the effort, she must keep up with her troop, carrying her infant while walking on her remaining three limbs. If the infant is not carried in the correct position, it cannot suckle and can quickly dehydrate and die.

Mastering this technique can be especially challenging for first time moms, which are often puzzled by their infant’s distress. Altmann remembers one young mother whose struggle to nurse had fatal consequences. “Vee’s first infant, Vicky, was not able to get on the nipple during her first day of life; her mother carried her upside down, even dragging her and bumping her on the ground for much of the day.” Although Vee, like most first-timers picked up the ropes within a few days, it was too late. Vicky died within a month. Such deaths are not unusual. Amongst primates, mortality rates for first born infants are up to 60% higher than for subsequent siblings.

But not all baboon mothers are born equal. Males may duke it out for their alpha position, but females also inhabit a rigid female aristocracy, worthy of British nobility. Status is inherited and laced with privilege.

Daughters born into baboon nobility have the advantage of their mother’s social connections—a network of protective benevolence. This support system means mothers don’t have to be the be-all and end-all for their kids, which is especially helpful for first-timers surfing a brutal maternal learning curve. Altmann found that daughters surrounded by high-ranking kin give birth at an earlier age to offspring more likely to survive, giving them a lifetime reproductive advantage over mothers in the lower ranks.

This social privilege has a massive impact on a baboon’s mothering style. Noble-born mothers have what Altmann described as a “laissez-faire” approach. They let their infants roam far and wide and exhibit tough love early on when it comes to weaning. This hands-off approach makes for self-sufficient and socially integrated juveniles, which gives them a higher chance of survival as adults.

Low-ranking females are put upon by just about everybody. Without the social standing to protect them and their baby, they compensate with what Altmann describes as “restrictive” parenting, keeping their infant constantly within arm’s length. Their young develop independence more slowly and place more demand on the mother’s critical resources.

Faced with non-stop potential threats their anxiety increases. This stress, detected in hormones excreted in the mother’s faeces, lowers their immune response and makes them more vulnerable to disease. It can also manifest as depression and even infant abuse. Humans are not the only primates to suffer from post-natal depression. In olive baboons low ranking mothers rank exhibited higher levels of abusive behaviour during the postpartum period. In wild populations of macaques, 5-10% of mothers have been observed biting, throwing or crushing their infants to the ground. Some have been known to perish as a result. Those that don’t, are psychologically scarred and more likely to mistreat their own young, ensuring that this abusive behaviour ricochets down the generations.

Although it might appear that low-ranking baboons are doomed by birth. Altmann’s team discovered that if they are able to forge strategic friendships with other baboons, both male and female, they can gain much needed assistance when running the brutal Darwinian gauntlet.

“We showed that those females who have more friends do live longer and their kids survive better.” Altmann told me.

Baboon moms have another destiny-cheating tactic: They can manipulate the sex of their offspring. Altmann discovered that low-ranking females had more sons than daughters. This plays to their advantage. While daughters remain shackled to their mother’s social status, a son can rise to the top and breed with a high-ranking female. In contrast, high-bred female baboons produce more daughters.

When Altmann exposed the female baboon’s trick, many found it hard to believe such a calculating, albeit unconscious move was possible. But sex-manipulation is utilized by mothers from fig wasps to kakapos . How baboons do it isn’t clear. But in other mammals, like coypu and red deer, their method is selective abortion.

These animal moms teach us that motherhood is much more than a one-size-fits-all knee-jerk response to nurture, but a multifarious life-or-death business with a treacherous learning curve.

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• Social Sci LibreTexts - Zoology
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• Table Of Contents

zoology , branch of biology that studies the members of the animal kingdom and animal life in general. It includes both the inquiry into individual animals and their constituent parts, even to the molecular level, and the inquiry into animal populations, entire faunas, and the relationships of animals to each other, to plants, and to the nonliving environment . Though this wide range of studies results in some isolation of specialties within zoology, the conceptual integration in the contemporary study of living things that has occurred in recent years emphasizes the structural and functional unity of life rather than its diversity .

Prehistoric man’s survival as a hunter defined his relation to other animals, which were a source of food and danger. As man’s cultural heritage developed, animals were variously incorporated into man’s folklore and philosophical awareness as fellow living creatures. Domestication of animals forced man to take a systematic and measured view of animal life, especially after urbanization necessitated a constant and large supply of animal products.

Study of animal life by the ancient Greeks became more rational, if not yet scientific, in the modern sense, after the cause of disease—until then thought to be demons—was postulated by Hippocrates to result from a lack of harmonious functioning of body parts. The systematic study of animals was encouraged by Aristotle ’s extensive descriptions of living things, his work reflecting the Greek concept of order in nature and attributing to nature an idealized rigidity.

In Roman times Pliny brought together in 37 volumes a treatise , Historia naturalis , that was an encyclopaedic compilation of both myth and fact regarding celestial bodies, geography, animals and plants, metals, and stone. Volumes VII to XI concern zoology; volume VIII, which deals with the land animals, begins with the largest one, the elephant. Although Pliny’s approach was naïve, his scholarly effort had a profound and lasting influence as an authoritative work.

Zoology continued in the Aristotelian tradition for many centuries in the Mediterranean region and by the Middle Ages, in Europe, it had accumulated considerable folklore, superstition, and moral symbolisms, which were added to otherwise objective information about animals. Gradually, much of this misinformation was sifted out: naturalists became more critical as they compared directly observed animal life in Europe with that described in ancient texts. The use of the printing press in the 15th century made possible an accurate transmission of information. Moreover, mechanistic views of life processes ( i.e., that physical processes depending on cause and effect can apply to animate forms) provided a hopeful method for analyzing animal functions; for example, the mechanics of hydraulic systems were part of William Harvey ’s argument for the circulation of the blood—although Harvey remained thoroughly Aristotelian in outlook. In the 18th century, zoology passed through reforms provided by both the system of nomenclature of Carolus Linnaeus and the comprehensive works on natural history by Georges-Louis Leclerc de Buffon; to these were added the contributions to comparative anatomy by Georges Cuvier in the early 19th century.

Physiological functions, such as digestion, excretion, and respiration, were easily observed in many animals, though they were not as critically analyzed as was blood circulation.

Following the introduction of the word cell in the 17th century and microscopic observation of these structures throughout the 18th century, the cell was incisively defined as the common structural unit of living things in 1839 by two Germans: Matthias Schleiden and Theodor Schwann . In the meanwhile, as the science of chemistry developed, it was inevitably extended to an analysis of animate systems. In the middle of the 18th century the French physicist René Antoine Ferchault de Réaumer demonstrated that the fermenting action of stomach juices is a chemical process. And in the mid-19th century the French physician and physiologist Claude Bernard drew upon both the cell theory and knowledge of chemistry to develop the concept of the stability of the internal bodily environment , now called homeostasis .

The cell concept influenced many biological disciplines , including that of embryology , in which cells are important in determining the way in which a fertilized egg develops into a new organism. The unfolding of these events—called epigenesis by Harvey—was described by various workers, notably the German-trained comparative embryologist Karl von Baer , who was the first to observe a mammalian egg within an ovary. Another German-trained embryologist, Christian Heinrich Pander , introduced in 1817 the concept of germ, or primordial , tissue layers into embryology.

In the latter part of the 19th century, improved microscopy and better staining techniques using aniline dyes, such as hematoxylin, provided further impetus to the study of internal cellular structure.

By this time Darwin had made necessary a complete revision of man’s view of nature with his theory that biological changes in species occur through the process of natural selection . The theory of evolution —that organisms are continuously evolving into highly adapted forms —required the rejection of the static view that all species are especially created and upset the Linnaean concept of species types. Darwin recognized that the principles of heredity must be known to understand how evolution works; but, even though the concept of hereditary factors had by then been formulated by Mendel, Darwin never heard of his work, which was essentially lost until its rediscovery in 1900.

Genetics has developed in the 20th century and now is essential to many diverse biological disciplines. The discovery of the gene as a controlling hereditary factor for all forms of life has been a major accomplishment of modern biology. There has also emerged clearer understanding of the interaction of organisms with their environment. Such ecological studies help not only to show the interdependence of the three great groups of organisms—plants, as producers; animals, as consumers; and fungi and many bacteria, as decomposers—but they also provide information essential to man’s control of the environment and, ultimately, to his survival on Earth . Closely related to this study of ecology are inquiries into animal behaviour , or ethology . Such studies are often cross disciplinary in that ecology, physiology , genetics, development , and evolution are combined as man attempts to understand why an organism behaves as it does. This approach now receives substantial attention because it seems to provide useful insight into man’s biological heritage—that is, the historical origin of man from nonhuman forms.

The emergence of animal biology has had two particular effects on classical zoology. First, and somewhat paradoxically, there has been a reduced emphasis on zoology as a distinct subject of scientific study; for example, workers think of themselves as geneticists, ecologists, or physiologists who study animal rather than plant material. They often choose a problem congenial to their intellectual tastes, regarding the organism used as important only to the extent that it provides favourable experimental material. Current emphasis is, therefore, slanted toward the solution of general biological problems; contemporary zoology thus is to a great extent the sum total of that work done by biologists pursuing research on animal material.

Second, there is an increasing emphasis on a conceptual approach to the life sciences. This has resulted from the concepts that emerged in the late 19th and early 20th centuries: the cell theory; natural selection and evolution; the constancy of the internal environment; the basic similarity of genetic material in all living organisms; and the flow of matter and energy through ecosystems. The lives of microbes, plants, and animals now are approached using theoretical models as guides rather than by following the often restricted empiricism of earlier times. This is particularly true in molecular studies, in which the integration of biology with chemistry allows the techniques and quantitative emphases of the physical sciences to be used effectively to analyze living systems.

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The Animal Kingdom: A Very Short Introduction

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The Animal Kingdom: A Very Short Introduction presents a modern tour of the animal kingdom. Beginning with the definition of animals, this VSI goes on to show the high-level groupings of animals (phyla) and new views on their evolutionary relationships based on molecular data, together with an overview of the biology of each group of animals. This phylogenetic view is central to zoology today. The animal world is immensely diverse, and our understanding of it has been greatly enhanced by analysis of DNA and the study of evolution and development.

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• 10 facts about the animal kingdom
• Animal evolution

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Essays on Animal Kingdom

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Essay on Animals 500+ words

Animals, our fellow inhabitants of planet Earth, form a diverse and fascinating part of our natural world. In this essay, we will argue for the importance of animals, highlighting their vital role in ecosystems, their unique qualities, and the need for their protectio

Biodiversity and Ecosystem Balance

Animals are an integral part of Earth’s biodiversity. They come in countless shapes, sizes, and species, each playing a specific role in maintaining the balance of ecosystems. For example, bees pollinate flowers, aiding in plant reproduction and the production of fruits and vegetables that sustain us.

Sources of Scientific Knowledge

Animals have been subjects of study and observation for centuries, providing valuable insights into biology, behavior, and adaptation. Scientists have learned about genetics, communication, and survival strategies through the study of animals, benefiting not only our understanding of nature but also medical and technological advancements.

Companionship and Emotional Bonds

Pets, such as dogs and cats, offer companionship and emotional support to countless individuals. Studies have shown that interaction with animals can reduce stress, anxiety, and loneliness. The unconditional love and loyalty of pets enhance our overall well-being.

Economic and Agricultural Contributions

Animals are essential in agriculture, providing us with meat, milk, eggs, wool, and other products. They also play critical roles in the livelihoods of millions of people worldwide. For instance, cattle support dairy and beef industries, while chickens are primary sources of eggs and poultry.

Conservation and Wildlife Protection

Many animals are endangered or threatened due to habitat loss, poaching, and climate change. Conservation efforts, often led by organizations and experts, aim to protect and preserve these species. The work of conservationists, like those saving the giant panda or African elephant, ensures the survival of Earth’s incredible biodiversity.

The Moral Responsibility of Humans

As the dominant species on Earth, humans bear a moral responsibility to treat animals with kindness and respect. Ethical treatment includes proper care of pets, humane farming practices, and the preservation of wildlife habitats. Ensuring animals’ welfare reflects our own moral values.

A Source of Wonder and Awe

Animals captivate our imaginations with their extraordinary abilities and behaviors. From the intelligence of dolphins to the grace of eagles in flight, the animal kingdom never ceases to inspire awe and wonder. These creatures remind us of the beauty and complexity of the natural world.

Conclusion of Essay on Animals

In conclusion, animals are not merely passive inhabitants of our planet; they are active participants in the intricate web of life. From their crucial roles in ecosystems to the bonds they form with humans, animals enrich our world in countless ways. It is our duty to protect and preserve the magnificent diversity of life on Earth.

As we navigate the challenges of the modern world, let us remember the importance of animals in our lives and the broader ecosystem. Let us support conservation efforts, advocate for ethical treatment, and nurture our curiosity and empathy toward all living beings. In doing so, we honor the invaluable contributions of animals to our planet and ensure a harmonious coexistence for generations to come. Animals, in all their majesty, remind us of our responsibility to protect and cherish the wonders of our shared home, Earth.

Also Check: The Essay on Essay: All you need to know

27.4 The Evolutionary History of the Animal Kingdom

Learning objectives.

• Describe the features that characterized the earliest animals and when they appeared on earth
• Explain the significance of the Cambrian period for animal evolution and the changes in animal diversity that took place during that time
• Describe some of the unresolved questions surrounding the Cambrian explosion
• Discuss the implications of mass animal extinctions that have occurred in evolutionary history

Many questions regarding the origins and evolutionary history of the animal kingdom continue to be researched and debated, as new fossil and molecular evidence change prevailing theories. Some of these questions include the following: How long have animals existed on Earth? What were the earliest members of the animal kingdom, and what organism was their common ancestor? While animal diversity increased during the Cambrian period of the Paleozoic era, 530 million years ago, modern fossil evidence suggests that primitive animal species existed much earlier.

Pre-Cambrian Animal Life

The time before the Cambrian period is known as the Ediacaran period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era ( Figure 27.14 ). It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time. Some protist species called choanoflagellates closely resemble the choanocyte cells in the simplest animals, sponges. In addition to their morphological similarity, molecular analyses have revealed similar sequence homologies in their DNA.

The earliest life comprising Ediacaran biota was long believed to include only tiny, sessile, soft-bodied sea creatures. However, recently there has been increasing scientific evidence suggesting that more varied and complex animal species lived during this time, and possibly even before the Ediacaran period.

Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. These sponge-like fossils, named Coronacollina acula , date back as far as 560 million years, and are believed to show the existence of hard body parts and spicules that extended 20–40 cm from the main body (estimated about 5 cm long). Other fossils from the Ediacaran period are shown in Figure 27.15 ab .

Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. Until this discovery, most scientists believed that there was no animal life prior to the Ediacaran period. Many scientists now believe that animals may in fact have evolved during the Cryogenian period.

The Cambrian Explosion of Animal Life

The Cambrian period, occurring between approximately 542–488 million years ago, marks the most rapid evolution of new animal phyla and animal diversity in Earth’s history. It is believed that most of the animal phyla in existence today had their origins during this time, often referred to as the Cambrian explosion ( Figure 27.16 ). Echinoderms, mollusks, worms, arthropods, and chordates arose during this period. One of the most dominant species during the Cambrian period was the trilobite, an arthropod that was among the first animals to exhibit a sense of vision ( Figure 27.17 abcd ).

The cause of the Cambrian explosion is still debated. There are many theories that attempt to answer this question. Environmental changes may have created a more suitable environment for animal life. Examples of these changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period ( Figure 27.18 ). Some scientists believe that an expansive, continental shelf with numerous shallow lagoons or pools provided the necessary living space for larger numbers of different types of animals to co-exist. There is also support for theories that argue that ecological relationships between species, such as changes in the food web, competition for food and space, and predator-prey relationships, were primed to promote a sudden massive coevolution of species. Yet other theories claim genetic and developmental reasons for the Cambrian explosion. The morphological flexibility and complexity of animal development afforded by the evolution of Hox control genes may have provided the necessary opportunities for increases in possible animal morphologies at the time of the Cambrian period. Theories that attempt to explain why the Cambrian explosion happened must be able to provide valid reasons for the massive animal diversification, as well as explain why it happened when it did. There is evidence that both supports and refutes each of the theories described above, and the answer may very well be a combination of these and other theories.

However, unresolved questions about the animal diversification that took place during the Cambrian period remain. For example, we do not understand how the evolution of so many species occurred in such a short period of time. Was there really an “explosion” of life at this particular time? Some scientists question the validity of the this idea, because there is increasing evidence to suggest that more animal life existed prior to the Cambrian period and that other similar species’ so-called explosions (or radiations) occurred later in history as well. Furthermore, the vast diversification of animal species that appears to have begun during the Cambrian period continued well into the following Ordovician period. Despite some of these arguments, most scientists agree that the Cambrian period marked a time of impressively rapid animal evolution and diversification that is unmatched elsewhere during history.

View an animation of what ocean life may have been like during the Cambrian explosion.

Post-Cambrian Evolution and Mass Extinctions

The periods that followed the Cambrian during the Paleozoic Era are marked by further animal evolution and the emergence of many new orders, families, and species. As animal phyla continued to diversify, new species adapted to new ecological niches. During the Ordovician period, which followed the Cambrian period, plant life first appeared on land. This change allowed formerly aquatic animal species to invade land, feeding directly on plants or decaying vegetation. Continual changes in temperature and moisture throughout the remainder of the Paleozoic Era due to continental plate movements encouraged the development of new adaptations to terrestrial existence in animals, such as limbed appendages in amphibians and epidermal scales in reptiles.

Changes in the environment often create new niches (living spaces) that contribute to rapid speciation and increased diversity. On the other hand, cataclysmic events, such as volcanic eruptions and meteor strikes that obliterate life, can result in devastating losses of diversity. Such periods of mass extinction ( Figure 27.19 ) have occurred repeatedly in the evolutionary record of life, erasing some genetic lines while creating room for others to evolve into the empty niches left behind. The end of the Permian period (and the Paleozoic Era) was marked by the largest mass extinction event in Earth’s history, a loss of roughly 95 percent of the extant species at that time. Some of the dominant phyla in the world’s oceans, such as the trilobites, disappeared completely. On land, the disappearance of some dominant species of Permian reptiles made it possible for a new line of reptiles to emerge, the dinosaurs. The warm and stable climatic conditions of the ensuing Mesozoic Era promoted an explosive diversification of dinosaurs into every conceivable niche in land, air, and water. Plants, too, radiated into new landscapes and empty niches, creating complex communities of producers and consumers, some of which became very large on the abundant food available.

Another mass extinction event occurred at the end of the Cretaceous period, bringing the Mesozoic Era to an end. Skies darkened and temperatures fell as a large meteor impact and tons of volcanic ash blocked incoming sunlight. Plants died, herbivores and carnivores starved, and the mostly cold-blooded dinosaurs ceded their dominance of the landscape to more warm-blooded mammals. In the following Cenozoic Era, mammals radiated into terrestrial and aquatic niches once occupied by dinosaurs, and birds, the warm-blooded offshoots of one line of the ruling reptiles, became aerial specialists. The appearance and dominance of flowering plants in the Cenozoic Era created new niches for insects, as well as for birds and mammals. Changes in animal species diversity during the late Cretaceous and early Cenozoic were also promoted by a dramatic shift in Earth’s geography, as continental plates slid over the crust into their current positions, leaving some animal groups isolated on islands and continents, or separated by mountain ranges or inland seas from other competitors. Early in the Cenozoic, new ecosystems appeared, with the evolution of grasses and coral reefs. Late in the Cenozoic, further extinctions followed by speciation occurred during ice ages that covered high latitudes with ice and then retreated, leaving new open spaces for colonization.

Link to Learning

Watch the following video to learn more about the mass extinctions.

Career Connection

Paleontologist Natural history museums contain the fossil casts of extinct animals and information about how these animals evolved, lived, and died. Paleontogists are scientists who study prehistoric life. They use fossils to observe and explain how life evolved on Earth and how species interacted with each other and with the environment. A paleontologist needs to be knowledgeable in biology, ecology, chemistry, geology, and many other scientific disciplines. A paleontologist’s work may involve field studies: searching for and studying fossils. In addition to digging for and finding fossils, paleontologists also prepare fossils for further study and analysis. Although dinosaurs are probably the first animals that come to mind when thinking about paleontology, paleontologists study everything from plant life, fungi, and fish to sea animals and birds.

An undergraduate degree in earth science or biology is a good place to start toward the career path of becoming a paleontologist. Most often, a graduate degree is necessary. Additionally, work experience in a museum or in a paleontology lab is useful.

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Essay on Animals

Essay generator.

Animals have been an integral part of our planet’s diverse ecosystem for millions of years. They come in all shapes, sizes, and species, each contributing to the balance and beauty of our natural world. The animal kingdom is a fascinating realm filled with remarkable creatures, each with its unique adaptations, behaviors, and roles in the ecosystem. In this essay, we will delve into the incredible world of animals, exploring their diversity, significance, and the crucial role they play in maintaining the delicate balance of our planet.

Diversity of the Animal Kingdom:

The animal kingdom is incredibly diverse, encompassing a vast array of species, each uniquely adapted to its environment. From the smallest microscopic organisms to the largest mammals, the variety of life in this kingdom is awe-inspiring. Animals can be broadly categorized into several groups:

• Invertebrates: These animals lack a backbone and include creatures like insects, arachnids, mollusks, and crustaceans. They constitute the majority of animal species on Earth, with insects alone accounting for over a million identified species.
• Fish: The aquatic world is teeming with fish, which come in various shapes, sizes, and colors. They play a crucial role in aquatic ecosystems and are a vital food source for many other animals, including humans.
• Amphibians: Amphibians, such as frogs, toads, and salamanders, are known for their ability to live both in water and on land. They are important bioindicators, helping scientists monitor the health of ecosystems.
• Reptiles: Reptiles, like snakes, lizards, turtles, and crocodiles, are characterized by their scaly skin and cold-blooded nature. They have been on Earth for millions of years and have adapted to various environments.
• Birds: Birds are known for their feathers, beaks, and ability to fly. They come in diverse shapes, sizes, and colors and have a profound impact on ecosystems through pollination, seed dispersal, and predation on insects and small animals.
• Mammals : Mammals, including humans, are characterized by features like hair or fur, live birth, and the ability to nurse their young with milk. They exhibit remarkable diversity, from tiny shrews to massive elephants.

The Significance of Animals:

Animals hold immense significance in our lives, the environment, and the world at large. Here are some key reasons why animals matter:

Biodiversity: Animals contribute to the rich tapestry of life on Earth. Their diversity is essential for maintaining healthy ecosystems and ensuring the survival of countless other species.

Ecosystem Services: Animals provide crucial ecosystem services, such as pollination by bees, seed dispersal by birds, and nutrient cycling by decomposers like insects and microbes. These services are vital for maintaining the balance of nature.

Scientific Research: Animals have been instrumental in scientific research, helping us gain insights into genetics, behavior, and physiology. They have been used in medical studies, leading to significant advancements in human healthcare.

Cultural and Aesthetic Value: Animals have cultural and aesthetic value, inspiring art, literature, and folklore throughout human history. They are symbols of identity and heritage for many communities.

Economic Importance: Many industries rely on animals for economic purposes, such as agriculture (livestock and poultry), tourism (wildlife safaris), and the pet trade.

Education and Conservation: Studying animals enhances our understanding of the natural world, leading to better conservation efforts. Zoos, wildlife documentaries, and educational programs teach people about the importance of animal preservation.

Role of Animals in Ecosystems:

• Animals play vital roles in various ecosystems, ensuring their proper functioning. These roles are interconnected and essential for the health of the environment:
• Pollinators: Bees, butterflies, and birds are key pollinators, facilitating the reproduction of plants by transferring pollen from one flower to another. This process is critical for the production of fruits and vegetables, supporting agriculture and food security.
• Seed Dispersers: Animals like birds, bats, and rodents aid in seed dispersal by consuming fruits and then dispersing the seeds in different locations. This helps plants colonize new areas and maintain genetic diversity.
• Predators and Prey: Predators help control prey populations, preventing overgrazing and ensuring the survival of plant species. The prey, in turn, serve as a food source for predators, forming intricate food webs within ecosystems.
• Decomposers: Scavengers and decomposers, such as vultures, insects, and bacteria, break down dead organic matter, recycling nutrients back into the ecosystem. This decomposition process is crucial for soil health and nutrient cycling.
• Ecosystem Engineers: Some animals, like beavers and termites, modify their environments by building dams and nests. These modifications can create new habitats and affect the water flow and nutrient cycling of ecosystems.

Conservation and Animal Welfare:

Despite the critical roles animals play in our world, many species face threats from habitat destruction, pollution, poaching, and climate change. Conservation efforts are essential to protect endangered species and preserve biodiversity. These efforts involve creating and maintaining protected areas, implementing sustainable practices, and raising awareness about the importance of animal conservation.

Animal welfare is another crucial aspect that concerns the ethical treatment of animals in various settings, including agriculture, research, and entertainment. Ethical treatment includes providing animals with adequate living conditions, proper nutrition, and protection from harm.

In conclusion, animals are a fundamental part of our planet’s intricate web of life. Their incredible diversity, significance, and roles in ecosystems make them indispensable to the health and well-being of our world. As responsible stewards of the Earth, it is our duty to protect and conserve these amazing creatures for future generations. By understanding and appreciating the wonders of the animal kingdom, we can better appreciate our interconnectedness with all living beings and work towards a harmonious coexistence with the natural world.

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Kingdom Animalia

Kindome Animalia n., [ˈkɪŋdəm ˌæn əˈmeɪ li ə ] Definition: A taxonomic kingdom comprising all living or extinct animals

Table of Contents

Kingdom Animalia Definition

Each person can say that they know of or can name at least one animal. However, do people know that animals are not merely a group but a kingdom? What does Animalia kingdom mean? What defines kingdom Animalia? To define the kingdom Animalia, one must think of it in a biological sense. Kingdom Animalia or just Animalia is a huge kingdom consisting of eukaryotic , multicellular animals that are heterotrophic in nature.

Characteristics of Animalia Kingdom

What are the characteristics of Kingdom Animalia? Members of kingdom Animalia lack a cell wall , which is found in plant cells , despite the fact that they are unable to create their own food, which is one of the most distinguishing traits of plants. The majority of animals , with the exception of a few, are motile, which helps them to successfully respond to stimuli and obtain food, among other things. One can also go more into depth about the structure and reproduction and growth characteristics of animals.

Kingdom Animalia Classification

Let’s find out about the classification of animals. The animal kingdom classification chart helps see clearly the different categories animals are put into. The animal kingdom chart also helps with the different animal kingdom facts that the group possesses.

In general, animals are separated into two groups:

• Vertebrates (animals with a backbone)
• Invertebrates (animals without a backbone) (animals that lack a backbone)

They are, however, separated into various phyla , which will be examined in further depth below.

A. Vertebrates

Vertebrates are all creatures that belong to the Vertebrata subphylum. They are members of the Chordata phylum and have a backbone ( vertebrae ) (where the spinal cord is located). In addition, they have an internal skeletal system ( endoskeleton ) to which muscles are joined. Examples of vertebrates are seen in Figure 1 below.

1. Mammalia

Mammary glands , which produce milk for nourishing (nursing) their young in females, a neocortex (a portion of the brain), fur or hair, and three middle ear bones characterize mammals (from Latin mamma , ‘breast’). These features set them apart from reptiles (including birds), from whom they separated about 300 million years ago in the Carboniferous.

There are around 6,400 living mammalian species. Rodents, bats, and Eulipotyphla are the three biggest orders (hedgehogs, moles, shrews, and others). The Primates which consist of humans, apes, monkeys, and others, the Artiodactyla which include cetaceans and even-toed ungulates, and the Carnivora are the next three groups consisting of cats, dogs, seals, and others.

2. Reptilia

Reptiles are the creatures in the class Reptilia, a paraphyletic grouping that includes all sauropsid amniotes save Aves (birds). Turtles, crocodilians, squamates (lizards and snakes), and rhynchocephalians are examples of living reptiles (tuatara). Classified separately from other reptiles, birds are in the classical Linnaean classification system.

Crocodilians, on the other hand, are more closely related to birds than to other extant reptiles, hence recent cladistic categorization schemes include birds inside Reptilia, redefining the term as a clade. Other cladistic definitions drop the name reptile entirely in favor of the clade Sauropsida, which includes all creatures that are more closely related to current reptiles than to mammals. Herpetology is the study of classical reptile orders, traditionally mixed with contemporary amphibians.

3. Amphibia

Amphibians are ectothermic tetrapod animals of the Amphibia class. The group Lissamphibia includes all live amphibians. They live in a broad range of habitats, with the majority of species inhabiting terrestrial, fossorial, arboreal, or freshwater aquatic settings. As a result, most amphibians begin as larvae in water, although certain species have evolved behavioral adaptations to avoid this.

The larvae with gills typically metamorphose into an adult air-breathing species with lungs. Amphibians utilize their skin as a supplementary respiratory surface, and some tiny terrestrial salamanders and frogs lack lungs and rely only on their skin for respiration. They resemble lizards on the surface, but reptiles, like mammals and birds, are amniotes and do not need bodies of water to procreate.

Agnatha is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, with both living and extinct species (cyclostomes and ostracoderms). Cyclastomes are the sister group to all vertebrates with jaws, known as gnathostomes . Recent molecular evidence from rRNA and mtDNA, as well as embryological data, substantially support the concept that the cyclostomes, or live agnathans, are monophyletic.

The first fossil agnathans occurred in the Cambrian period, and two families of agnathans still exist today: lampreys and hagfish, with a total of roughly 120 species. Because hagfish lost vertebrae secondarily, they are classified as members of the subphylum Vertebrata; before molecular and developmental data concluded this happening, Linnaeus created the group Craniata (which is still sometimes used as a strictly morphological descriptor) to refer to hagfish plus vertebrates.

5. Osteichthyes

Osteichthyes is a taxonomic group of fish whose skeletons are mostly formed of bone tissue, they are sometimes called bony fish as well. They are distinguished from the Chondrichthyes by their cartilage-based skeletons. The great majority of fish are members of the Osteichthyes order, which is an exceptionally varied and plentiful organization that includes 45 orders, over 435 families, and 28,000 species.

It is the most numerous class of vertebrates on the planet today. Osteichthyes is separated into two groups: ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). The oldest known fossils of bony fish are around 425 million years old, and they are also transitional fossils, with a tooth arrangement that is intermediate between a shark and bony fish tooth rows.

6. Chondrichthyes

Chondrichthyes is a class of about 1050 extant cartilaginous fishes that includes skates, sharks, rays, and chimeras. The class consists of 12 orders separated into two monophyletic subclasses, the Elasmobranchii (sharks, rays, and skates) and the Holocephali (chimeras).

With just 40 live species of chimera in the sole order Chimaeriformes representing the Holocephali, the great majority of extant chondrichthyans are elasmobranchs grouped into 11 orders. Although previous classifications divided the elasmobranchs into two categories, sharks and batoids (rays and skates), Campagno recognized four unique groupings based on phenetics (overall similarity) and classified them as the four elasmobranch superorders.

B. Invertebrates

In contrast to cartilaginous or bony vertebrates, invertebrates are any animals that lack a vertebral column or backbone. Invertebrates account for more than 90% of all extant animal species. They are found all over the world and include species such as sea stars, sea urchins, earthworms, sponges, jellyfish, lobsters, crabs, insects, spiders, snails, clams, and squid. Invertebrates are particularly significant as agricultural pests, parasites, or agents for parasitic infection transfer to humans and other vertebrates.

Invertebrates provide food for humans, are essential components of food chains that feed birds, fish, and a variety of other vertebrate species, and play critical roles in plant pollination. Despite providing crucial environmental services, invertebrates are frequently overlooked in wildlife study and conservation, with large vertebrate studies taking precedence.

Furthermore, numerous invertebrate taxa (including many types of insects and worms) are seen purely as pests, and by the early twenty-first century, widespread pesticide usage had resulted in significant population losses among bees, wasps, and other terrestrial insects.

Levels of Organization

It is interesting to note that despite all animal kingdom species being multicellular, not all of their cellular arrangements abide by this rule. Animal levels of the organization are categorized into the following categories based on cellular organization patterns:

• The Cellular Level of Organization: Cells in animals with this kind of cell organization are grouped in loose cell aggregates. Sponge organization is a good example of this.
• Tissue Level Organization: Animal cells exhibit divisions in cell activity. Cells that complete the same job are tissues . Coelenterates is one example.
• Organ Level of Organization: Tissues of a certain animal group that perform the same function are grouped together to create an organ. Each organ has a distinct purpose. Platyhelminthes is an example. Organ System Level of Organization: Organ system level of organization has been found in animals where organs have been coupled to create functional systems, each system concerned with a certain physiological function. Chordates, Annelids, Mollusks, Echinoderms, and Arthropods are a few examples.

Certain species, most notably sponges and ameboid protozoans, lack symmetry , having either an irregular shape that varies from individual to individual or one that undergoes continual changes of form. The great majority of creatures, on the other hand, have a distinct symmetrical shape. Animals have four types of symmetry: spherical, radial, biradial, and bilateral .

Body Cavity/Coelom

The coelom (or celom) is the primary body cavity that surrounds and houses the digestive tract and other organs in most animals. It is lined by mesothelium in certain mammals. It is undifferentiated in other species, such as mollusks. Coelom features have previously been used to categorize bilaterian animal phyla into informal groupings for practical purposes.

Acoelomata is essentially a subgroup (or super-phylum) of creatures that lack a real body cavity. The body cavity, also known as a coelom, is a fluid-filled region positioned between the body wall and the digestive system.

The ventral (where the heart, lungs, and intestines, among other things, are placed) and dorsal cavities are two of the greatest examples of human bodily cavities (where the brain and spinal cord are located). The area between the body wall and the intestine in acoelomates is generally made up of mesenchyme (or some muscle fibres) rather than coelomic fluid (the fluid that separates and protects various organs).

Pseudocoelomates (animals that have a false coelom)

A pseudocoelomate is an organism having a body cavity that is not generated from the mesoderm, as is the case with a real coelomate. Since the blastocoel, or cavity within the embryo, becomes the body cavity, a pseudocoelomate is also known as a blastocoelomate. A real coelom is bordered with a peritoneum, which serves to keep fluid from entering the body cavity. The bodily fluids bathe the organs and obtain nutrients and oxygen from the fluid in the cavity in a pseudocoelomate.

Coelomate creatures, also known as Coelomata (sometimes known as eucoelomates – “genuine coelom”), have a bodily cavity called a coelom with a full lining produced from mesoderm called peritoneum (one of the three primary tissue layers). The entire mesoderm lining permits organs to be linked to one another and hung in a certain sequence while still moving freely inside the space. Coelomates make up the majority of bilateral creatures; this includes all vertebrates.

Different Phyla under the Kingdom Animalia

The kingdom organisms are divided into many phylum animals in the kingdom Animalia phylum. What is a phylum? What is it for ? A phylum ( plural: phyla, not phylums ) is a major taxonomic rank below Kingdom. The members of the animal kingdom are categorized into numerous phyla and subpyla. These divisions help to form the animal kingdom hierarchy and include the eukaryotic kingdom chart.

1. Kingdom Animalia: Phylum Chordata

A member of the Chordata phylum is called a chordate. At some time during their larval or maturity phases, all chordates have 5 synapomorphies or main traits that separate them from all other species. The five synapomorphies are a notochord, then the dorsal hollow nerve cord, endostyle – which is also known as the thyroid. There are also pharyngeal slits and a post-anal tail. The term “chordate” is derived from the first of these synapomorphies, the notochord, which is important in chordate structure and movement. Chordates are bilaterally symmetric, have a coelom, a circulatory system, and metameric segmentation.

Main characteristics of chordates

Chordates possess very distinct anatomical features.

• A notochord is a rigid cartilage rod that runs down the interior of the body. The notochord develops into the spine in the vertebrate subgroup of chordates, and in fully aquatic animals, this allows the animal to swim by bending its tail.
• A neural tube’s dorsal end. This develops into the spinal cord, the major communication trunk of the nervous system, in fish and other vertebrates.
• Slits in the pharynx. The area of the throat behind the mouth is known as the pharynx. The slits in fish are changed to create gills, but in certain other chordates, they are part of a filter-feeding system that collects food particles from the water in which the animals dwell.
• The post-anal tail. A powerful tail that extends behind the anus.
• A type of endostyle. This is a groove in the pharynx’s ventral wall. It generates mucus to capture food particles in filter-feeding animals, which aids in food delivery to the esophagus. It also stores iodine and is thought to be a forerunner of the vertebrate thyroid gland.

2. Kingdom Animalia: Phylum Porifera

Sponges, members of the phylum Porifera (meaning ‘pore bearer,’) are a basic animal group that is related to the Diploblasts. They are multicellular creatures with pores and channels that let water move through their bodies, which are made up of jelly-like mesohyl sandwiched between two thin layers of cells. Sponges have unspecialized cells that can change into other types and travel between the major cell layers and the mesohyl regularly. Sponges lack neurological, digestive, and circulatory systems. Instead, most rely on a steady flow of water through their bodies to get food, oxygen, and waste removal. Sponges were the first creatures to split out from the last common ancestor of all animals, making them the sister group of all other animals.

3. Kingdom Animalia: Phylum Platyhelminthes

Flatworms, Platyhelminthes , or platyhelminths are a phylum of very basic bilaterian, unsegmented, soft-bodied invertebrates. They are acoelomates (without a body cavity) and lack specific circulatory and respiratory organs, limiting them to flattened geometries that enable oxygen and nutrients to move through their bodies by diffusion. Because the digestive cavity only has one opening for both ingestion (nutrient intake) and egestion (removal of unprocessed wastes), food cannot be processed constantly.

4. Kingdom Animalia: Phylum Cnidaria

Cnidaria is a phylum under the kingdom Animalia that contains approximately 11,000 species of aquatic organisms found in both freshwater and marine settings, with a focus on the latter. Their distinctive characteristic is cnidocytes, which are specialized cells used mostly for prey capture. Mesoglea, a non-living jelly-like substance sandwiched between two layers of epithelium, each roughly one cell thick, makes up their bodies.

5. Kingdom Animalia: Phylum Annelida

The annelids (Annelida, from Latin anellus, “small ring”), often known as ringed worms or segmented worms, are a vast phylum that includes ragworms, earthworms, and leeches. The species live in and have evolved to a variety of ecologies, including some in marine habitats such as tidal zones and hydrothermal vents, some in freshwater, and yet others in wet terrestrial areas.

Annelids are bilaterally symmetrical, triploblastic, and coelomate invertebrates. They have parapodia for movement as well. Most textbooks continue to utilize the conventional classification of polychaetes (nearly all marine), oligochaetes (including earthworms), and leech-like animals.

Since 1997, cladistic research has drastically altered this system, with leeches now considered a subgroup of oligochaetes and oligochaetes considered a subgroup of polychaetes. Furthermore, the Pogonophora, Echiura, and Sipuncula, which were formerly considered different phyla, are now considered sub-groups of polychaetes. Annelids are part of the Lophotrochozoa, a protostome “super-phylum” that also contains molluscs, brachiopods, and nemerteans.

6. Kingdom Animalia: Phylum Mollusca

After the Arthropoda, Mollusca is the second-largest phylum of invertebrates. Mollusks (or molluscs) are the members. There are around 85,000 known species of mollusks. The number of new fossil species is believed to be between 60,000 and 100,000. The fraction of unnamed species is quite high. Many taxa are still understudied.

Mollusks are the most numerous marine phylum, accounting for around 23% of all identified marine creatures. Freshwater and terrestrial environments are where most mollusks are. They are extremely different, not only in terms of size and physical structure but also in terms of behavior and environment. The phylum is often classified into seven or eight taxonomic groups, two of which are extinct. Cephalopod mollusks like squid, cuttlefish, and octopuses are among the most neurologically sophisticated invertebrates, with the giant squid or gigantic squid being the biggest known invertebrate species. The gastropods (snails and slugs) are by far the most common mollusks, accounting for over 80% of all identified species.

7. Kingdom Animalia: Phylum Arthropoda

Arthropods are invertebrates with an exoskeleton, segmented bodies, and paired jointed limbs. Arthropods are classified as members of the phylum Arthropoda. They are recognized by their jointed limbs and chitin cuticle, which is frequently mineralized with calcium carbonate.

An arthropod’s body plan is made up of segments, each with a pair of appendages. Arthropods have a bilaterally symmetrical body and an external skeleton. To continue developing, they must go through moulting stages, in which they lose their exoskeleton to expose a new one. Some animals have wings. They are a hugely varied group, with up to ten million different species.

8. Kingdom Animalia: Phylum Hemichordata

Hemichordata is a phylum of marine deuterostome organisms that is commonly regarded as the echinoderms’ sister group. They first emerge in the Lower or Middle Cambrian and are divided into two groups: Enteropneusta (acorn worms) and Pterobranchia. Planctosphaeroidea, a third class, is known exclusively from the larva of a single species, Planctosphaera pelagica. Graptolithina, an extinct class, is connected to pterobranchs.

Acorn worms are worm-like invertebrates that live alone. They mainly live in burrows (the earliest produced tubes) and are deposit feeders, however, some species are pharyngeal filter feeders, and the Torquaratoridae family is a free-living detritivore. Many are well recognized for producing and accumulating halogenated phenols and pyrroles. Pterobranchs are filter feeders that live in a collagenous tube structure known as a coenecium.

9. Kingdom Animalia: Phylum Echinodermata

An echinoderm is any marine animal that belongs to the group Echinodermata. Adults include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as sea lilies or “stone lilies,” which have radial symmetry (usually five points). Adult echinoderms can be found from the intertidal zone to the abyssal zone at all ocean depths.

The phylum has over 7,000 extant species, making it the second-largest grouping of deuterostomes (a superphylum) after chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles). Echinoderms are the biggest phylum with no members in freshwater or on land.

10.  Kingdom Animalia: Phylum Ctenophora

Ctenophora is a phylum of marine invertebrates known as comb jellies that live in seawater all around the world. They are famous for the clusters of cilia they utilize for swimming (often referred to as “combs”), and they are the biggest creatures that swim using cilia.

Adult ctenophores range in size from a few millimetres to 1.5 m (5 ft) depending on the species. Only 100 to 150 species have been verified, and another 25 may yet be incompletely characterized and designated. Cydippids, which have egg-shaped bodies and retractable tentacles fringed with tentilla (“small tentacles”) that are coated with colloblasts, sticky cells that trap prey, are textbook examples.

11. Kingdom Animalia: Phylum Aschelminthes

Archaeocyatha is a taxon of extinct, sessile, reef-building marine sponges that lived during the Cambrian Period in warm tropical and subtropical seas. The Archaeocyatha origin is presently thought to be in East Siberia, where they have been recognized since the beginning of the Cambrian Tommotian Age, 525 million years ago (mya).

They first arose in other parts of the world much later, during the Atdabanian period, and soon diversified into over a hundred families. They were the planet’s earliest reef-building creatures and are a global index fossil for the Lower Cambrian.

Kingdom Animalia examples

Animalia is a kingdom of eukaryotic creatures. Parthenogenesis is the process through which they reproduce sexually or asexually . When you think of animals, you typically think of species from the phylum Chordata, but there are many others.

• Jellyfish – For millions of years, long before dinosaurs existed, jellyfish have been traveling about on ocean currents. The jelly-like creatures may be found in both cold and warm ocean water, as well as deep-sea and along coasts, and they pulsate with ocean currents. Jellyfish, despite their name, are invertebrates, or organisms without backbones, not fish.

• Dogs – A domesticated ancestor of the wolf, the dog ( Canis familiaris or Canis lupus familiaris ) is recognized by an upturned tail. The dog is descended from an extinct wolf, and the current wolf is the dog’s closest living relative. Before the development of agriculture, hunter-gatherers domesticated the dog for the first time some 15,000 years ago. Dogs have increased to a vast number of domestic individuals as a result of their long contact with humans, and they have developed the capacity to live on a starch-rich diet that would be insufficient for other canids. Dogs evolved to be particularly attuned to human behavior over millennia, and the human-canine link has been the subject of much research.

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Essay on Animals: Samples in 100, 200 and 300 Words

• Updated on  
• Dec 27, 2023

Animals are an important part of the natural world. Their existence in our environment is as important as ours. Some of the common animals that we see regularly are dogs, cats, cows, birds, etc. From small insects to blue whales, there are millions of species of animals in our environment, each having their habitat and way of living. Some animals live in seas, while others on land. Our natural environment is so diverse that there are more than 7 million species of animals currently living. Today, we will provide you with some essay on animals. Stay tuned!

Table of Contents

• 1 Essay on Animals in 100 Words
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• 3 Essay on Animals in 300 Words

Also Read: Essay on New Education Policy in 500 Words

Essay on Animals in 100 Words

Animals are part of our natural world. Most of the animal specials are related to humans in direct or indirect ways. In agricultural and dairy production, animals play an important role. Our food, such as eggs, milk, chicken, beef, mutton, fish, etc. all come from animals. Animals are generally of two types; domestic and wild. 

Domestic animals are those that we can keep at our homes or use their physical strength for activities like agriculture, farming, etc. Wild animals live in forests, where they have different ways of survival. There is an interdependence between humans and animals. Without animals, our existence would be impossible. Therefore, saving animals is as important as saving ourselves.

Also Read: Essay on Cow: 100 to 500 Words

Essay on Animals in 200 Words

Animals play a major role in maintaining the balance of our ecosystem. They contribute to our biodiversity by enriching the environment with their diverse species. Animals range from microscopic organisms to majestic mammals with their unique place in the intricate web of life.

Animals provide essential ecosystem services, such as pollination, seed dispersal, and pest control, which are vital for the survival of many plant species. Animals contribute to nutrient cycling and help in maintaining the health of ecosystems. Animals have an interdependency on each other which creates a delicate equilibrium. Our activities often disturb his balance, which affects the entire ecosystem.

There are a lot of animals that we can domesticate, such as dogs, cats, cows, horses, etc. These animals bring joy and companionship to our lives. We also domesticate milch animals, such as cows, goats, camels, etc. for services like milk or agricultural activities. Wild animals living in forests contribute to our cultural and aesthetic aspects, inspiring art, literature, and folklore.

In recent years, animal species have faced threats due to habitat destruction, climate change, and human activities. Conservation efforts are crucial to protecting endangered species and preserving the diversity of life on Earth.

Animals are integral to the health of our planet and contribute to the overall well-being of human societies. It is our responsibility to appreciate, respect, and conserve the rich tapestry of animal life for the benefit of present and future generations.

Also Read: How to Prepare for UPSC in 6 Months?

Essay on Animals in 300 Words

Scientific studies say there are 4 types of animals; mammals, fish, birds, reptiles, and amphibians. All these types of animals are important in maintaining the balance of our ecosystem. From the smallest insects to the largest mammals, each species has a unique role to play in the web of life.

One of the fundamental roles of animals is in ecosystem services. Bees and butterflies, for example, are crucial pollinators for many plants, including crops that humans rely on for food. Birds and mammals contribute to seed dispersal, facilitating the growth of various plant species. Predators help control the population of prey animals, preventing overgrazing and maintaining the health of ecosystems.

Beyond their ecological contributions, animals also have immense cultural significance. Throughout history, animals have been revered and represented in art, mythology, and religious beliefs. They symbolize traits such as strength, agility, wisdom, and loyalty, becoming integral to human culture. Domesticated animals, such as dogs and cats, have been companions to humans for thousands of years, providing emotional support and companionship.

However, the impact of human activities on animals is a growing concern. Habitat destruction, pollution, climate change, and poaching pose significant threats to many species. Conservation efforts are crucial to safeguarding biodiversity and ensuring the survival of endangered animals.

Moreover, the well-being of animals is closely linked to human welfare.  Livestock and poultry contribute to the global food supply, and advancements in medical research often rely on animal models. Ethical considerations surrounding animal welfare are increasingly important, leading to discussions on responsible and humane treatment.

Animals are essential components of our planet’s ecosystems and contribute significantly to human culture and well-being. Balancing our interactions with animals through conservation, ethical treatment, and sustainable practices is imperative to ensure a harmonious coexistence and preserve the diversity of life on Earth.

Tree: trimmed. ✔ Goats are skilled climbers who don't limit their search for food to the ground. #goat #greatestholidayofalltime #Morocco pic.twitter.com/eQrwHPWSPr — Animal Planet (@AnimalPlanet) December 19, 2023

Ans: Animals are an important part of our natural environment. Humans and animals depend on each other for their survival. We humans depend on animals for food, agricultural activities, etc. Domestic animals are those that we can keep at our homes or use their physical strength for activities like agriculture, farming, etc. Wild animals live in forests, where they have different ways of survival. There is an interdependence between humans and animals. Without animals, our existence would be impossible. Therefore, saving animals is as important as saving ourselves.

Ans: Some of the domesticated animals are dogs, cats, cows, goats, camels, etc.

Ans: Mammals, fish, birds, reptiles, and amphibians.

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Animal Kingdom

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With the omission of both prokaryotes and protists, Kingdom Animalia generally contains all sorts of animal species. It is estimated that anywhere from nine to ten million exist on Earth – the exact number is not precisely known. Kingdom Animalia includes the following species: mammals, insects, birds, echinoderms, and etcetera. This paper examines the following in particular: elephants, armadillos, termites, butterflies, penguins, hummingbirds, starfishes, and sand dollars.

These species not only divide upon the two the same equivalence in characteristics, setting them apart from other species, but includes the fact that all are generally eukaryotic, multicellular, heterotrophic, lack cell walls, are motile, and usually pass through a blastula stage as animals. Elephants and armadillos are just two species out of the vast majority of existing mammals. These two share the same characteristics: warm-blooded; their young are born alive; and have lungs to breathe air. On the other hand, the two have apparent differences.The body characteristics are the most evident. An elephant’s physical characteristics consist of its enormous size, have tusks, and even a trunk.

An armadillo, however, is of moderate size, has hard armor-like skin, short legs, and have claws. Also, an elephant’s diet is usually plant flood, considering that it is an herbivore, while an armadillo uses its sharp claws to dig for grubs or insects. The elephant belongs to the Proboscidea, which only contains one family of living animals, the elephants – includes three species: African Bush Elephant, African Forest Elephant, and Asian Elephant.The Proboscidea order classifies animals that feed by its trunk.

The armadillo is only family in the order Cingulata, as well as a superorder Xenartha, which also includes anteaters and sloths. The Cingulata order classifies animals with girdlelike shells. Insects include many species, in particular, the termite and butterfly. These two insects generally share these characteristics: have an exoskeleton, have three main body parts (head, thorax, and abdomen), and have antennas. On the other hand, they differ as well.A termite’s diet usually feeds upon dead plant material (wood, leaf litter, soil, animal dung, crops, etcetera), while a butterfly usually sips liquid food through a tube-like proboscis, such as from rotting fruits or some even prefer animal flesh or fluids.

Another major difference exists within the fact that termites are typically seen as pests to humans, as they cause much damage to buildings, crops, and plantation forests, while butterflies are viewed as being pleasant and attractive to the eye.A termite is classified under the order Isoptera, which includes only termites, but is related to cockroaches and mantids in the superorder Dictyoptera. The order Isoptera simply means ‘termites’. Butterflies are under the order Lepidoptera, which classifies insects as having a life cycle that includes stages of larval caterpillar, pupal, and metamorphism. Penguins and hummingbirds are both birds, and share the following characteristics: endothermic vertebrates, covered with feathers, and lay eggs.On the other hand, chief differences exist as well.

A penguin is aquatic, flightless birds that live in cold temperatures. Hummingbirds, however, hover in mid-air by rapidly flapping their wings, can fly backwards, and feed from nectar or tiny anthropods from flower blossoms. Penguins belong to the order Sphenisciformes, which includes eighteen species such as chinstrap penguins, king penguins, and emperor penguins. The order Sphenisciformes classifies birds as flightless birds within cold temperatures – thus, penguins.Hummingbirds belong to the order Apodiformes, which includes two other families – the swifts, Apodiadae and the tree swifts, Hemiprocnidae.

The order Apodiformes, of the Greek word meaning “a pous” or “without foot”, are generally with small feet and short legs and cannot walk. Echinoderms have a variety of species, but we will only focus on two: starfishes and sand dollars. They share the following characteristics: two embryonic openings, the anus and the mouth; have no cephalisation; as larva, they are free-swimming bilaterally symmetrical organisms.On the other hand, apparent contrasts exist between the two as well.

Starfish have five or more arms, which extend from an indistinct disc, have a diet that includes shelled animals such as oysters and clams, and even are able to redevelop missing arms. However, sand dollars do not have “arms” and have a round, flat shape. They have fire pores that move seawater within its vascular system, which permit the organism to move. They generally feed upon crustacean larvae, tiny copepods, detritus, algae, diatoms, and organic particles.

A starfish belongs to seven orders, which are the following: Brisingida, Forcipulatida, Paxillosida, Notomyotida, Spinulosida, Valvatida, and Velatida. Their shape, legs, and ability to regenerate classify them within these orders. Other species may include sea urchins and brittle stars. Sand dollars, on the other hand, are classified under the order Clypeasteroida, which have organisms with irregular echinoids and multiple ambulacral pores and tube feet for movement.

Without animal life, along with other living organisms of this world, life would not be the same – it’d be safe to say that it wouldn’t be the world we know now. These animals are as equally important as its neighbor, considering that we all exist together on one enormous planet. Regardless of what particular species, we all exist within a natural balance, and essentially, we are all one part or another of a food chain.

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Essay On Animals

500+ words essay on animals, the different animal species and their importance.

The planet we live on is home to both humans and animals. An animal is a living creature, which is part of a group of multicellular eukaryotic organisms. These organisms have special sense organs and nervous systems and are capable of locomotion and reproduction. All animals breathe in oxygen and breathe out carbon dioxide and with the exception of a few, most animals consume organic matter. 

Animals are very important for the environment. We need them for several things from companionship to food and even balancing the ecosystem. There are several species of animals in the world and they live on land and water. Each of these animals has a unique place in the environment and are crucial to maintain the balance of the ecosystem. And each of them has a purpose for their existence. The study of animals is called biology.

In this essay on animals, you’ll learn about the different species of animals and how they’re classified. This essay on animals also talks about the importance of animals.

Essay On Animals: The Different Species Of Animals

Animals are divided into different groups or species in Biology. It is estimated that the world has over 7 million species of animals. According to biology, animals can be classified into two groups, vertebrates and invertebrates. 

Vertebrates

All animals that have a backbone are called vertebrates. Vertebrates can be further classified into 5 groups, mammals, birds, fish, amphibians and reptiles.

• Mammals: These are warm-blooded animals that have hair or fur and vertebrates (a backbone). Most mammals give birth to their young ones and produce milk to feed and nourish their young ones. Some examples of mammals are human beings, cats, dogs, cows, lions, dolphins, whales etc.
• Birds: Birds are warm-blooded animals with feathers, wings and a light skeleton, which helps them fly. But, some birds like ostriches, penguins, emus, kiwis, cassowary etc cannot fly. Birds lay eggs and hatch them to give birth to their young ones. Some examples of birds are crows, ducks, swans, geese, chickens, pigeons, peacocks etc.
• Fish: Fish are cold blooded vertebrates that live in water. They have fins and scales that help them swim in the water. Like birds, fish also lay eggs to reproduce. Some examples of fish are sharks, clownfish, salmon, eels, seahorses etc.
• Amphibians: Amphibians are vertebrates that live on both land and water. These cold blooded animals need a moist environment to survive. They breathe through their skin by absorbing water. Like birds and fish, amphibians also reproduce by laying eggs. Some examples of amphibians are frogs, toads, salamanders, etc
• Reptiles:  Reptiles are cold blooded animals with a backbone and live on land and water. Their skin is covered with scales or bony plates. Reptiles give birth to their young ones by laying eggs. Some examples of reptiles are snakes, lizards, geckos, crocodiles, turtles etc.

Invertebrates

Invertebrates are animals, which do not have a backbone. About 95% of the animal kingdom is made up of invertebrates, which are mostly insects. The eight different types of invertebrates, which can be found today are: annelida, arthropoda, cnidaria, echinodermata, mollusca, nematoda, platyhelminthes and porifera. Some examples of invertebrates are mosquitoes, spiders, earthworm, jellyfish, snails, squid, bees etc.

Classifying Animals Based On Food

Like us humans, animals also need food to survive. Animals can be further classified into 3 kinds based on what they eat. 

• Carnivores: Animals that eat the meat of other animals to survive are called carnivores or carnivorous animals. For example tigers, lions, hyenas, sharks, hawks, eagles etc. 
• Herbivores: These animals eat only plants, their leaves, fruits and vegetables. Some examples of herbivorous animals are cows, horses, elephants, deer, rabbits, butterflies, silkworms etc.
• Omnivores: Animals, which eat both plants and animals are called omnivorous animals. Some examples of omnivores are human beings, wolves, raccoons, bears, dogs, rats, skunks etc.

Also explore: Read some more essay on animals with Essay on Cat , Essay On Dog and Essay On Tiger .

Essay On Animals: The Importance Of Animals

Animals are important for the environment and even our lives. They serve as our companions, our eyes and ears, our workers and even provide us with food. They are extremely vital to maintain a healthy and balanced ecosystem. 

• Animals for transportation: Since early ages, humans have used animals for transportation. Horses, camels, oxen and donkeys have pulled carts and aided in transportation for a long time. Even in today’s modern world, animals are used for transportation in some countries.
• Animals as companions: Domestic animals and pets like dogs, cats, pigs etc have served as loyal companions to humans for centuries. These days, animals like service dogs serve as help for visually impaired people, emotional support for people with special needs etc. 
• Animals for food: Humans have consumed animals and animal products like meat, fish, poultry, milk, cheese etc for ages. 
• Animals as workers: We humans often use animals for tasks like guarding, farming, hunting and protecting. For example, guard dogs, oxen for farming, hunting dogs etc.
• A balanced ecosystem: Each animal in the world has a unique place in the food chain and contributes to the ecosystem in their own way. For example, bees and birds help in pollination. Carnivorous animals keep the population of other animals in check. They are also necessary for contributing to the carbon and nitrogen cycle and decomposition. 

Humans and animals have to learn to coexist. A healthy ecosystem is dependent on relationships between different organisms, food webs and food chains. Protecting animals is important because it could have disastrous consequences on our ecosystem. Additionally, they have an equal right to survive in this world just as much as humans.

We hope you found this essay on animals interesting and helpful. Check Osmo’s essays for kids to explore more essays on a wide variety of topics. 

Frequently Asked Questions On Animals

What are animals.

Animals are multicellular, eukaryotic organisms that have special sense organs and nervous systems. They breathe in oxygen, consume organic matter and are capable of reproduction and locomotion.

How are animals classified?

Animals are classified into two main types: vertebrates and invertebrates. Vertebrates are animals with fur and a backbone. These vertebrates can be further classified into mammals, birds, reptiles, amphibians and fish. Invertebrates are animals that don’t have a backbone. 95% of the animals in the animal kingdom are invertebrates.

How are animals important for humans?

Animals are extremely important for us humans. We use them for food, transportation, companionship, as workers, for medicine etc. They are also important to maintain a healthy and balanced ecosystem.

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Essay on Animals | Animals Essay for Students and Children in English

Essay on Animals: The Earth is not just our home planet, but the home to many animals. Since the beginning of time, animals have inhabited the plant, serving as a friend and foe to humans. Humans used animals for transportation protection as well as hunting.

There are different species, such as amphibians, reptiles, mammals, insects, and birds; the population is widespread. Animals are not just fellow inhabitants but an essential part of our ecosystem. However, many of these animals face the threat of extinction due to the actions of humankind. Environmentalists and international organizations such as PETA and WWF have raised the conservation of many species.

Long Essay regarding Animals 500 Words in English

Short essay on animals 200 words in english for kids, 10 lines on animals essay in english.

• What are some of the species of the animal kingdom?
• How do animals help ecology?
• Mention some organizations for wildlife conservation.
• When is Wildlife Day commemorated?

Long and Short Essays on Animals for Students and Kids in English

There is one long essay on animals of 500 words and one short essay of 200 words on animals.

Long essay on Animals is for students of Classes 8,9 and 10 and competitive exam aspirants.

The Earth is home to many creatures. Animals have been the inhabitants of this planet, along with humans. Historically, animals were used for transportation, protection, as well as for hunting. Animals have been companions to man since time immemorial.

Animals are the kingdom while classifying their species. There are a variety of species present under this, with their presence spanning across the world. Amphibians primarily require a moist environment as they breathe and absorb through thin skins. Some amphibians include frogs, salamanders, toads, and caecilians. Mammals are vertebrates and warm-blooded. Females have mammary glands to feed their young ones and have a thick coat of fur. Mammals include carnivores, bears, rodents, etc. Reptiles are vertebrates, but lay eggs. Some of them have scales. Common reptiles are lizards, turtles, and snakes.

Insects have an exoskeleton. They have three pairs of legs, a head, thorax, and abdomen. Beetles, ants, and bees are some insects. Birds have wings, beaks, and feathers such as eagles, pigeons, crows, and sparrows. There are many species of animals that are domesticated as well, such as dogs, cats, rabbits, horses, etc.

Animals are vital to the ecosystem. Animals have different purposes when it comes to the environment. Even microorganisms help to clean our planet. Many animals aide plants to converting free nitrogen present in the air and nourishing the roots and a crucial role in sustaining ecological balance. Predatory animals keep the population in the animal population controlled. Animals that feed on plants help to control plant growth. They also provide us with the food required for our survival. Poultry, dairy, and meat serve as essentials to many cultures and their diets. Animals are one of the oldest companions of humans.

However, many species face the threat of extinction. Urbanization causes cutting down forests to meet the needs of the growing human population. Cutting down forests has led to a loss of habitat for many animals. Habitat destruction has caused damage to animal life. Lions and bears traditionally hunted for their fur, elephants for their ivory tusks, and alligators for leather sell in black markets. Torturing animals and locking them in cages affects their wellbeing. Dumping effluents into water bodies affect marine life. Global warming also affects animals, with dried up water bodies and seasonal changes that have consequences on these species. Using animals to test human-made drugs has received widespread criticism as the animals suffer irreparable damages.

The importance of conserving animals has been recognized all over the world. International organizations such as the PETA and World Wide Fund for Nature (WWF) aim to spread awareness on preservation. Countries have strict laws for animal conservation. The Indian government has many wildlife protection projects such as Project Tiger and Project Elephant, animals whose populations are decreasing drastically.

World Wildlife Day is commemorated on the 3rd of March every year. An initiative by the United Nations, 2020’s theme is “Sustaining all life on Earth” to meet sustainable development goals. It is vital to conserve animals as the Earth is there home as much as it is ours.

Students can find more English Essay Writing Topics, Ideas, Easy Tips to Write Essay Writing and many more.

Short Essay on Animals will help students of Classes 1,2,3,4,5 and 6.

Earth is the home to many animals. They are man’s companion. Animals have various species. Amphibians have thin skin through which they absorb and breathe. Frogs and toads are some examples. Mammals have a coat of fur and warm-blooded like lions, tigers, and bears. Reptiles lay eggs and cold-blooded. Some reptiles include snakes and crocodiles. Insects and birds are a part of the animal kingdom.

Animals help our environment. They provide nutrition to the soil, and they are a source of food. Predatory animals like lions and tigers help to control the animal population. They help in agriculture as well. However, animals face the threat of extinction. Man cuts down many forests to build homes and factories while animals lose their home. Hunters torture animals and kill them for leather, fur, and ivory. Caging animals and keeping them away from their habitat affects their well-being. Water bodies polluted with harmful substances affect the animals that live in water.

We need to protect animals because the Earth is not just our home; it belongs to them as well. They are the faithful companions of man. Every year we celebrate the 3rd of March as World Wildlife Day to spread the message of protecting our animals.

These ten lines are helpful for competitive exam aspirants and making speeches.

Animals have been companions to man since time immemorial.

Animals are the kingdom while classifying their species. There are a variety of species.

Amphibians primarily require a moist environment and breathe through their thin skins. Some amphibians include frogs, salamanders, toads, and caecilians.

Mammals are vertebrates and warm-blooded. Females have mammary glands to feed their young ones and have a coat of fur. Mammals include carnivores, bears, rodents, etc.

Reptiles are another species that are vertebrates but coldblooded and lay eggs, such as crocodiles and snakes. Insects and birds are also different species of animals.

Animals play a vital role in maintaining ecological balance. Predatory animals keep populations in the animal population controlled, and feed on plants help to control growth. Animals are a source of food such as poultry, dairy, and meat.

Cutting down forests has led to a loss of habitat for many animals. Lions and bears traditionally hunted for their fur, elephants for their ivory tusks, and alligators for leather sell in black markets.

Caging animals and keeping them away from their habitat affects their wellbeing. Water bodies polluted with harmful substances affect marine life.

Organizations like PETA and WWF spread awareness and work towards the conservation of animals. The Indian government has undertaken many wildlife protection projects such as Project Tiger and Project Elephant.

World Wildlife Day is commemorated on the 3rd of March every year. An initiative by the United Nations, 2020’s theme is “Sustaining all life on Earth” to meet sustainable development goals.

FAQ’s on Essay on Animals

Question 1. What are some of the species of the animal kingdom?

Answer: Some of the species of the animal kingdom include amphibians, reptiles, mammals, insects, and birds.

Question 2. How do animals help ecology?

Answer: Animals help to maintain the ecosystem. Microorganisms help to clean our planet. Many animals’ aide plants to converting free nitrogen present in the air and nourishing the roots. Predatory animals keep the population in the animal population controlled. Animals that feed on plants help to control plant growth.

Question 3. Mention some organizations for wildlife conservation?

Answer: Some organizations are PETA, WWF, or World Wide Fund for Nature and Wildlife Conservation Society.

Question 4. When is Wildlife Day commemorated?

Answer: World Wildlife Day is commemorated on the 3rd of March every year.

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The Animal Kingdom Story

Reading the compilation of short stories to children will help them develop an interest in reading and writing stories. These stories with hidden morals help in building character. 

The Animal Kingdom Story is about listening and paying heed to the wise words of elders. 

Read the story about an animal kingdom with a leopard king.

Read the Story About an Animal Kingdom

The amazing story of the animal kingdom for kids is an inspirational and fun story that young minds will appreciate. The story is about an animal kingdom where a leopard is the king. However, one day his rule is threatened by the arrival of a lion cub. What does the leopard king do? Read the story ahead to know more about it.

The Defeat of Animal Kingdom Story For Kids 

Once upon a time, there was an animal kingdom that lived in a deep and dense forest. Unlike all the other kingdoms, this one didn’t have a lion as the king but a very kind and good-hearted leopard. The animal kingdom was very happy under the rule of the leopard as he would always look after his subjects and help them in any way he could. The leopard king had a monkey as his advisor who was very wise and provided valuable advice to the king.

King of the Animal Kingdom, the Leopard.

A Lion Cub Arrives in the Forest

One day, the leopard was on his daily walks through the jungle when he noticed a lion cub near the bushes. It was very small but looked ferocious. But the cub was just a baby and didn’t know where his parents were. The lion cub was very scared and hid under the bushes to keep away from predators.

The leopard king felt bad for the cub as it was separated from its parents. However, the monkey advised the king to remove the cub from the animal kingdom in the forest . The monkey said, “We all know that lions are born to rule over the forest. If you want to remain the king of this forest, you should throw this cub away from the jungle at once.”

The leopard listened to the monkey but couldn’t throw the cub away. He then ordered his subjects to keep him in a cave nearby and raise him up. Although the monkey disagreed with the king, he didn’t say anything.

The Lion Cub Grows Up to be Powerful 

As time passed, the lion cub started to grow larger and larger. Very soon, he started developing the nature of a lion. He was proud and mighty and was constantly talking about his powers. As he grew up, he started noticing that the kingdom didn’t have a lion as a ruler. He realised that he was the strongest one in the kingdom and demanded supremacy over the others. The lion soon started to destroy the entire animal kingdom to show its power over the others.

The Lion Wants His Rule 

One day, the lion came up to the leopard and claimed a portion of the kingdom. The leopard was shocked and angry. He refused the lion immediately. He even ordered the lion to get out of the jungle. The lion went away but promised to return, seeking revenge. He soon returned to the jungle, but he was not alone; he brought a whole pride of lions with him. Together the lions destroyed the entire animal kingdom and killed the leopard as well.

The Pride of Lions.

Summary of the Animal Kingdom Story 

Children that read this story can learn a very deep and valuable moral from it. The story of the animal kingdom teaches us that we should always listen to the wise words our elders say. We must pay heed to their advice and do things accordingly. Our elders have seen the world longer than us and hence have a sense of the surroundings. So, we must always listen to what they have to say. The leopard king didn’t listen to the wise old monkey, and the kingdom had to suffer the consequences.

FAQs on The Animal Kingdom Story

1. Who was the king of the animal kingdom?

This animal kingdom in the forest was unique because it didn’t have a lion as the king. The ruler of the animal kingdom was a kind leopard.

2. Who was the advisor of the leopard king?

The leopard king chose an old monkey to be his advisor. The monkey was very wise and would help the king in matters of the jungle.

3. What advice did the monkey give to the leopard about the lion cub?

The monkey advised the leopard king to throw the lion cub out of the jungle.

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The new science of sleep: From cells to large-scale societies

Contributed equally to this work with: Omer Sharon, Eti Ben Simon

Roles Conceptualization, Visualization, Writing – original draft, Writing – review & editing

Affiliations Department of Psychology, University of California, Berkeley, California, United States of America, Helen Wills Neuroscience Institute, University of California, Berkeley, California, United States of America

Roles Visualization, Writing – original draft, Writing – review & editing

Roles Writing – original draft, Writing – review & editing

Roles Conceptualization, Supervision, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

• Omer Sharon, 
• Eti Ben Simon, 
• Vyoma D. Shah, 
• Tenzin Desel, 
• Matthew P. Walker

Published: July 8, 2024

• https://doi.org/10.1371/journal.pbio.3002684
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In the past 20 years, more remarkable revelations about sleep and its varied functions have arguably been made than in the previous 200. Building on this swell of recent findings, this essay provides a broad sampling of selected research highlights across genetic, molecular, cellular, and physiological systems within the body, networks within the brain, and large-scale social dynamics. Based on this raft of exciting new discoveries, we have come to realize that sleep, in this moment of its evolution, is very much polyfunctional (rather than monofunctional), yet polyfunctional for reasons we had never previously considered. Moreover, these new polyfunctional insights powerfully reaffirm sleep as a critical biological, and thus health-sustaining, requisite. Indeed, perhaps the only thing more impressive than the unanticipated nature of these newly emerging sleep functions is their striking divergence, from operations of molecular mechanisms inside cells to entire group societal dynamics.

Citation: Sharon O, Ben Simon E, Shah VD, Desel T, Walker MP (2024) The new science of sleep: From cells to large-scale societies. PLoS Biol 22(7): e3002684. https://doi.org/10.1371/journal.pbio.3002684

Copyright: © 2024 Sharon et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: The author(s) received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Abbreviations: ADHD, attention deficit hyperactivity disorder; ASD, autism spectrum disorder; CSF, cerebrospinal fluid; DORA, dual orexin receptor antagonist; EEG, electroencephalogram; fMRI, functional MRI; HPA, hypothalamic-pituitary-adrenal; IRT, imagery rehearsal therapy; NREM, nonrapid eye movement; PTSD, posttraumatic stress disorder; RBD, REM behavior disorder; REM, rapid eye movement; SWA, slow-wave activity

Introduction

Sleep appears to be a universal, highly conserved state across the animal kingdom [ 1 , 2 ]. This fact would perhaps suggest a common single function of sleep that transcends phylogeny; however, proving this has been far more challenging than anticipated. Indeed, science has struggled to answer, with universal agreement, the basic question of why it is that we sleep and, to an ever greater degree in humans, why we dream. Yet, in the past 2 decades, more has arguably been uncovered about the polyfunctional nature of sleep than in the previous 200 years. Building on a wave of exciting recent discoveries, in this Essay, we provide a select collection of highlights from sleep research across genetic, molecular, cellular, whole body, whole brain, group-social, and societal levels.

This Essay is not meant to serve as a comprehensive review of sleep research nor an exhaustive cataloging of all recent discoveries. Rather, we aim to provide the reader with a sampling of representative new research areas. Towards that end, the Essay is structured into several main sections that traverse a descriptive narrative, from cells to society, each exploring different facets of sleep science. We start with recent discoveries at the level of DNA and genes, describing both genes that control sleep duration, and the newly revealed role of sleep in DNA repair. Thereafter, we ascend to physiological systems, one example of which focuses on very recent findings regarding an intimate and bidirectional link between sleep and the gut microbiome. Next, we address exciting recent work seeking to develop new technologies to augment and enhance human sleep, ranging from electrical to acoustic, kinesthetic, and thermal manipulations, all of which have marked therapeutic and intervention implications.

Having considered sleep functions within the body, we then move higher into the brain. We address novel sleep functions at the neural level, including sleep’s role in regulating the glymphatic cleansing system. Staying within the brain, we then address one of the newest emerging fields of sleep neuroscience, that of emotional wellness and mental health. Here, the latest findings move beyond sleep’s role in basic emotional regulation and, instead, signal a clear and intimate connection between sleep and complex socioemotional functions within an individual, between individuals, across large groups of individuals, and across entire societies.

Staying with the theme of sleep across groups, we then investigate sleep’s evolutionary roots across phylogenetic groups, providing a very different approach to understanding the functions of sleep. We describe new work that seeks to explain the vast, previously perplexing, and impressively large differences in sleep quantity and physiological sleep quality (Glossary) across species. From such an examination come powerful insights into the universal function(s) of sleep that only this type of approach can reveal. Finally, we move past the basic physiological state of sleep into the altered psychological state of human consciousness called dreaming. We outline both prior and the latest evidence regarding the functional importance of dreaming in service of memory enhancement, creativity, and emotional first aid, independent of the rapid eye movement (REM) sleep (Glossary) state such dreams emerge from.

Sleep quality.

Evaluated through both subjective means, which involves individual’s self-reporting their perceived quality of sleep, and objective methods, including measurements of sleep stages or quantitative electroencephalography brainwave metrics.

Rapid eye movement (REM) sleep.

Also referred to as paradoxical sleep, REM represents a sleep phase marked by a desynchronized electroencephalogram with high-frequency, low-amplitude activity (especially in the theta band), rapid movement of the eyes, muscle immobilization, and the occurrence of dreams.

Nonrapid eye movement (NREM) sleep.

Describes the sleep phase that encompasses the period between falling asleep and reaching deep sleep, yet is not REM sleep. The stages of NREM sleep are typically categorized into 3 categories: N1 (shallow sleep), N2 (light sleep), and N3 (deep sleep).

Slow-wave activity (SWA).

A characteristic electrophysiological pattern marked by slow, synchronized oscillations in the 0.5 to 4.0 Hz range. SWA reaches its peak during NREM sleep and diminishes across the night, reflecting the discharge of homeostatic sleep pressure that builds the longer an individual is awake.

Obstructive sleep apnea.

A sleep disorder characterized by recurrent impaired or absent breathing during sleep, as well as by reductions in blood oxygen saturation, caused by airway occlusion.

Sleep restriction.

A decrease (but not total absence) of sleep across the prior night or nights. Amounts typically range from 1 to 6 hours of sleep reduction. Sleep restriction is often termed chronic if it persists for more than 24 hours.

Gut dysbiosis.

An imbalance in the gut’s microbial community, potentially leading to health issues. It involves a decrease in beneficial bacteria and an increase in harmful ones, disrupting normal gut function. Restoring balance is crucial for overall health.

Disrupted sleep.

Irregular sleep patterns characterized by insufficient sleep duration, disrupted sleep cycles (such as altered sleep architecture), and/or reduced sleep quality (evaluated through measures like spectral electroencephalogram power).

Allostatic distress.

A state reflecting the cumulative physiological damage caused by chronic stress, in part stemming from prolonged activation of stress-related messengers like cortisol and adrenaline. Allostatic distress is associated with disruption of adaptive biological systems and responses, including those related to the hypothalamic-pituitary-adrenal (HPA) axis and immune function, ultimately contributing to various health issues.

Cognitive behavioral therapy for insomnia.

A scientifically supported approach to treating insomnia that involves a comprehensive psychological intervention aimed at addressing the underlying behaviors and thought patterns associated with insomnia.

Functional connectivity.

Within functional MRI, the statistical association observed between activity signals originating from 2 or more anatomically separate brain regions.

Through these exciting new discoveries, and many others like them, we have come to recognize that sleep has evolved to support polyfunctional processes for the brain and body. Moreover, such powerful new evidence reaffirms sleep as a biologically critical and health-sustaining requisite—a requisite for reasons: that are surprising in their nature.

Genes linked to short sleep need

Insufficient sleep exacts a significant toll on all cognitive and emotional brain functions and impacts all major physiological systems of the body, from the immune, cardiovascular, thermoregulatory, metabolic, and reproductive systems, to respiratory and endocrine systems ( Fig 1 ). Unsurprisingly, then, insufficient sleep also predicts all-cause mortality risk [ 1 , 3 , 4 ]. Nevertheless, there is a common claim by some that, “I’m one of those individuals who can function just fine on 5 hours of sleep or less.” While this is unlikely, based on the extent of empirical findings [ 5 – 8 ], a select collection of individuals do seem to be exceptions to the recommended 7- to 9-hour sleep requirement, on the basis of gene mutations that reduce sleep need [ 5 ]. Termed “natural short sleepers,” this small set of individuals appears to have a natural sleep requisite as low as 6 to 6.25 hours per night without showing any observable cognitive deficits assessed so far [ 5 ].

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Sleep serves a multitude of functions for humans. These functions exist at multiple physiological levels, from cells (bottom panel) to bodily systems (left panel), through to multiple brain functions and systems (right panel).

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The first genetic variant accounting for these natural short sleepers centered on a variation in DEC2 gene identified in families of naturally short-sleeping humans. Initial work focused on dizygotic twins, each of whom differed on the basis of this DEC2 gene variant (standard versus mutation). The twins had their daily sleep–wake patterns measured and also came to the sleep laboratory for full sleep physiological recordings. The data revealed that the twin with the DEC2 mutation naturally slept 30 to 60 minutes less than their noncarrier twin in both real-world and laboratory-assessed environments [ 9 ].

This was not the most interesting result, however. Following a 38-hour sleep deprivation period, the twin carrying the short-sleeping genetic variant exhibited greater resilience to sleep deprivation, defined by performance on select cognitive tasks, having only half the number of attentional failures compared to their noncarrier sibling. The final revelation emerged during the subsequent night of recovery sleep. Typically, following sleep loss, individuals sleep notably longer, indicating a buildup of sleep debt that is proportional to the amount of extended time awake. The longer the prior waking period, the longer and deeper the recovery sleep. However, the DEC2 mutant carrier did not show this normal strong sleep–rebound response, obtaining 1.5 hours less recovery sleep than their noncarrier twin. This finding once again indicates a reduced innate sleep need, here even under the pressure of prior sleep deprivation. Similar results have been observed in DEC2 mutant mice [ 5 ], with wild-type mice showing a 70% increase in NREM sleep (Glossary) following sleep deprivation, compared to a 17% increase in the DEC2 mutant mice.

How is short sleep achieved?

Using mice genetically engineered to carry the short-sleeping genes identified in humans, new findings have revealed how and why the DEC2 mutation may afford a reduced sleep need [ 7 , 8 ]. The DEC2 mutation results in an increased expression of the wake-promoting neurochemical, orexin [ 10 ]; i.e., natural short sleepers with the DEC2 variant have an amplified neurochemical wake–drive, resulting in prolonged wakefulness across the day and thus shorter sleep duration at night. However, this insight does not wholly explain the reduced homeostatic sleep need after sleep deprivation in individuals carrying the DEC2 mutation. If anything, a strong drive for wakefulness may be predicted to result in a stronger buildup of sleep homeostatic factors, such as adenosine, that would increase homeostatic sleep needs during postdeprivation recovery.

An additional explanation for the innate reduced general sleep amount (approximately 6 hours), and one that may account for a reduced homeostatic sleep need, concerns the electrical efficiency of deep-sleep brain waves. A physiological measure of deep NREM sleep quality is slow electrical brainwave activity, also known as slow-wave activity (SWA, <4 Hz; Glossary). In the aforementioned DEC2 twin studies, across all 3 nights recorded in the laboratory, the short-sleeping twin exhibited significantly greater SWA, one interpretation of which is that their deep NREM sleep was of superior electrical quality. By means of this superior SWA power, short-sleeping individuals may be able to dissipate the accumulation of sleep needs based on time awake during the day, and in doing so, reduce the total amount of time needed for sleep [ 9 ], thus increasing sleep efficiency and decreasing sleep need.

Another mutation has also been discovered in natural short sleepers. ADRB1 gene governs the beta-1 adrenergic receptor, which influences sleep–wake regulation. Much like the DEC2 mutation, those carrying the ADRB1 mutation display increased SWA during NREM sleep early in the night. Moreover, the speed of decline in SWA across the night—potentially reflecting a more efficient evacuation of accumulated sleep pressure across the waking day—was faster in those carrying this mutation. Again, this points to the possibility of superior deep-sleep electrical brainwave activity, increasing sleep efficiency and, hence, decreasing the amount of sleep needed [ 6 ].

This emerging picture of superior deep-sleep physiology in short sleepers is not, however, exclusive to NREM sleep. In several short-sleeping studies, alterations in REM sleep have also been identified, the reasons and function(s) of which are more mysterious; e.g., in the dizygotic DEC2 twin studies, following a sleep deprivation phase, the noncarrier twin spent nearly 2 additional hours in REM sleep during recovery sleep, as is typical. However, the carrier twin showed almost no change in the rebound of REM sleep [ 9 ], indicative of a reduced REM sleep need as well. Similarly, wild-type mice exhibit a 175% increase in REM sleep after sleep deprivation, yet short-sleeping DEC2 mutant mice expressed only a 74% relative increase in REM sleep [ 5 ]. ADRB1 short-sleeping mutant mice similarly do not show the same REM sleep need relative to wild-type mice under normal (nondeprived) sleeping conditions [ 6 ]. Short-sleeping gene variants, therefore, seem to require less total sleep, but also less REM sleep. There are still no clear answers as to why.

Is short sleep without true cost?

Arguably the most fundamental question in the emerging description of short sleep is that of cost—is there truly no health cost to these short-sleeping individuals? Cross-sectional analyses suggest that cognitive functions do not suffer, relative to controls without the DEC2 genetic variant, yet there have been no systematic studies assessing other known sleep-dependent brain and body functions ( Fig 1 ). Furthermore, no prospective longitudinal studies of natural short sleepers have been conducted to determine whether the health span and/or life span are similar to controls, or for twins relative to their noncarrier sibling. An assumption of no true cost, therefore, remains a hopeful one, but an assumption nevertheless. One relevant example that may temper optimism concerns work in fruit flies using the “Shaker” gene mutation that shortens sleep duration [ 11 ]. Evaluated longitudinally, the life span of these mutant flies was significantly shorter relative to wild-type flies. This would suggest that some short sleep gene variants, at least in certain species, may come with a consequence only when assessed longitudinally, in this instance, premature mortality.

Genes not only affect sleep, but the reverse is also true. During time spent awake, the double-stranded backbone of DNA accumulates breaks. This damage is specific to neurons compared with nonneural brain cells such as Schwan cells or peripheral endothelial cells [ 12 ]. However, during sleep, these double-strand breaks are repaired rapidly [ 13 ], suggesting that a lack of sleep can induce excessive mutations and potentially explain why sleep is so evolutionary conserved. These findings also support the view that sleep is especially critical for the brain with regard to the cellular function of neurons, although it is possible that neural cells in the periphery (e.g., in the enteric system) are similarly affected. Fascinatingly, the DNA damage response, in turn, can impact sleep: Expression of the DNA repair enzyme PARP1 can induce sleep [ 14 ].

The gut microbiome: A sleep interface?

Sleep, it was logically believed, primarily serves the sleeping organism itself. This view has changed, or at least been revised, in a model of symbiosis. Within us lives a diverse community of microorganisms, particularly in our gut, collectively known as the gut microbiota. The gut microbiota is composed of several billion bacteria, viruses, fungi, and additional microbes [ 15 , 16 ] and is known to influence a broad swathe of host physiology and behavior [ 17 ]. Dysfunction of the microbiome is now linked to numerous disorders and conditions, including obesity, type 2 diabetes, cardiometabolic diseases, nonalcoholic liver disease, and several immune disorders, as well as neurological disorders such as autism spectrum disorder, Alzheimer’s disease, depression, multiple sclerosis, Parkinson’s disease, and stroke [ 18 – 20 ]. Seminal work by Toth and Krueger [ 21 , 22 ] first linked sleep and the microbiota in the 1980s. Although, this field of research is still in its embryonic stages, a plethora of recent work is now providing exciting, and many surprising, new insights to add to those made by Toth and Krueger many decades ago. This is of particular interest as it could further promote the way we think about the established link between sleep and immunity [ 23 , 24 ], as the microbiome is fundamental for the development, training, and operation of the host’s immune system [ 25 , 26 ]. Most alluring, this relationship between the microbiota and sleep is bidirectional, opening up the possibility that modifying the gut microbiota may be a new tool for improving human sleep.

How sleep impacts the gut microbiota

Chronic sleep disruption alters the configuration of the gut microbiota in several deleterious ways. A pioneering study in mice investigated the effects of 4 weeks of repeated sleep interruptions. The mice were gently handled every 2 minutes to trigger awakening, mimicking the frequency of interruptions as a model of obstructive sleep apnea (Glossary) in humans [ 27 ]. After 9 days, the amount of Firmicutes bacteria in the gut, which are associated with the fermentation processes involved in energy extraction, increased. Conversely, Bacteroidetes species decreased, which is notable as they serve anti-inflammatory functions. As predicted, the mice had increased markers of inflammation and infection, including the number of macrophages and neutrophils. In tandem with these microbiota changes caused by a lack of sleep came an increase in food intake [ 27 ]. This resulted in escalating amounts of visceral fat, even though total body weight remained constant [ 27 , 28 ], suggesting an impact on how the body partitions energy when sleep loss alters the microbiome. Encouragingly, these changes subsided within 2 weeks of restoring healthy sleep.

Similar causal evidence in humans has since emerged, although with some inconsistencies. Two consecutive nights of sleep restriction (approximately 4 hours per night; Glossary) moderately increased the ratio of Firmicutes to Bacteroidetes in humans [ 29 ], similar to the results observed in the mice [ 27 ]. By contrast, in a study that looked at 1 week of similar 4-hour per night sleep restriction, the authors failed to detect a change in microbiota composition [ 30 ]. Increasing the severity of sleep restriction to 2 hours each night for 3 consecutive nights did, however, significantly reduce the diversity of microbiota in the gut, leading to dysbiosis (an imbalance in the microbial communities living in the gastrointestinal tract; Glossary). This was especially true for a decrease in Ruminococcaceae, which normally contributes to the production of short-chain fatty acids (e.g., butyrate) [ 31 ]. Short-chain fatty acids help improve gut outer barrier integrity and metabolism and regulate immune function and blood pressure [ 32 ]. Yet, the changes in the diversity of the microbiome were not accompanied by changes in gut permeability, at least when assessed using urine samples [ 31 ].

While most of society’s sleep debt is brought about by sleep restriction, there are circumstances in which total sleep deprivation is common and necessary, including in medicine, and for those working as emergency responders, in the military, in aviation, and in law enforcement. When individuals are acutely sleep-deprived for 40 hours [ 33 ], a dose-dependent escalation of gut dysbiosis unfolds, the severity of which increases the longer without sleep an individual goes. Replicating earlier studies in mice, the progressive dysbiosis is paralleled by increases in circulating inflammatory markers, including the pro-inflammatory cytokines IL-1, IL-6, and TNFα. In addition to securing sufficient sleep, new findings point to sleep regularity as an independent emerging factor in protecting gut health [ 34 ]. In experiments in rats, circadian rhythm disturbances triggered by an 8-hour circadian shift every 3 days can lead to imbalances in gut microbiota composition and rhythms [ 35 ]. In humans, greater objectively measured night-to-night variability in sleep duration, together with increased time awake after sleep onset and lower sleep efficiency, are associated with lower microbiome diversity [ 36 ]. Thus, irregular sleep patterns, especially if coupled with poor-quality sleep, interfere with stable profiles of gut microbiota, one consequence of which is poor metabolic health [ 37 ].

A clever study has added new insight into the link between dysbiosis and inflammation caused by insufficient sleep by using a combination of species [ 33 ]. If the microbiota of sleep-deprived humans is transplanted into well-rested, non-sleep-deprived mice, those mice experienced a significant increase in inflammation relative to mice who received a transplant from well-rested humans. In addition, these pro-inflammatory effects caused by lack of sleep extended into the brain, with levels of pro-inflammatory cytokines IL-1 and IL-6 increasing in the medial prefrontal cortex and dorsal hippocampus, while levels of the anti-inflammatory cytokine IL-10 decreased. These findings confirm at least one of the directions of effect, such that changes in the gut microbiota caused by a lack of sleep represent an explanatory path leading to systemic inflammation [ 33 ]. In addition to changes in circulating markers of inflammation, there was increased expression of Iba-1protein, an index of microglia activity (the brain’s primary immune cells) in the medial frontal cortex and hippocampus following transplantation. These findings suggest that the cognitive effects of sleep deprivation could, in part, be mediated by brain inflammation caused by the sleep-loss-induced changes in gut microbiota composition. It also provides a possible biological mechanism—changes in glial inflammatory activity—that might explain how and why chronic gut dysbiosis and brain disorders are related.

How the microbiota impacts sleep

Like so many other core physiological consequences, the idea that a lack of sleep impairs the microbiome is perhaps to be expected, but the idea that the microbiome could conversely impact sleep is more novel. The first pioneering investigation into this topic involved a 4-week antibiotic regimen in mice to deplete their gut microbiota [ 38 ]. Following the antibiotic course, the mice experienced a 100,000-fold reduction in gut bacteria. However, the causal manipulation of the microbiome led to a significant impairment in their brain’s ability to generate normative sleep in several ways. First, the mice aberrantly flip-flopped back and forth between NREM and REM sleep, indicative of unstable sleep-state regulation. The wake phase also suffered after the microbiome had been depleted. The mice could not sustain robust wakefulness across this period, experiencing excessive wake-time sleepiness. Added to this were uncharacteristic intrusions of NREM and REM sleep during the wake phase when the mice should otherwise be alert; the latter stage also pervaded into the sleep phase. Even the electrical brainwave quality of REM sleep was abnormally slowed in the microbiome-depleted mice. While preliminary, and despite the potential impact of antibiotics treatment on sleep patterns, these findings offer promising therapeutic potential. If microbiota composition can alter sleep, microbiome-specific interventions to restore and improve sleep may be possible.

The microbiota, sleep, and disease

Given that experimental sleep loss impairs the gut microbiota, disorders showing sleep disruption would be expected to show co-occurring impairments in the composition of the microbiota. Insomnia is one such confirmatory example. Both acute (lasting days to weeks) and chronic insomnia (lasting months to years) have now been linked to significant gut dysbiosis and a decrease in bacteria that produce short-chain fatty acids. Indeed, individuals with these conditions showed increases in circulating levels of the pro-inflammatory cytokine IL-1β, suggesting the increases in inflammatory response observed in sleep disruption in the lab are the everyday reality of individuals with insomnia [ 39 ]. Collectively, these cross-sectional observations reinforce experimental data indicating that disrupted sleep (Glossary) robustly compromises the gut microbiome.

Longitudinal studies tracking several hundred patients over a 6-year period have since found similar impairments. No matter whether patients were recently diagnosed with insomnia or had been experiencing insomnia for many months or years, all went on to show gut dysbiosis, relative to healthy individuals who slept well [ 31 , 40 ]. This included a reduction in Ruminococcaceae bacteria, notable for their varied functions, including regulating the gut barrier integrity that normally shields an organism from pathogens. Notably, patients who went on to recover from their insomnia ultimately became indistinguishable from healthy individuals in their microbiome composition.

Possible mechanisms

Since sleep impacts the microbiome, and the microbiome alters sleep, how do these distant systems converse? We would tender several candidates. First, a lack of sleep skews eating behavior, increasing food intake, biasing preference for higher caloric foods, and driving up consumption of simple and complex carbohydrates [ 41 ]. This altered eating behavior could, by itself, alter the gut microbiota by increasing the level of energy-extracting bacteria, which are responsible for digesting 10% to 30% of the nutrients that the digestive system cannot digest on its own [ 42 ]. Since the relationship among different species of bacteria is often competitive, this increase in energy-extracting bacteria occurs at the expense of bacteria that regulate other functions, such as combating inflammation [ 27 ]. These changes in microbiota may then lead to even greater sleep impairment, further slanting eating behavior, and instigating a vicious cycle [ 43 ].

A second direct pathway is the vagus nerve, which connects the brain to the gut’s intrinsic nervous system, called the enteric system. If rats have their vagus nerve severed, they are not affected by microbiome-related inflammation caused by sleep deprivation [ 44 ]. This indicates that the gut microbiome and sleep communicate, in part, in almost real time by way of the vagus nerve.

A third indirect pathway involves allostatic distress (Glossary). Sleep disruption increases the activity of the sympathetic nervous system and the hypothalamic adrenal pathway, increasing heart rate, decreasing heart rate variability, and increasing stress-related chemicals including catecholamines and cortisol [ 45 ]. Arousal-related catecholamines, primarily norepinephrine, and overactivation of the sympathetic nervous system can stimulate the growth of pathogenic bacteria such as Escherichia coli [ 46 ]. Aberrant sympathovagal drive, paired with catecholamines and glucocorticoids, may then change the microbiota habitat by increasing gut motility [ 47 ] and relevant iron availability [ 48 ].

Therapeutic implications

With the multitude of pathways on offer, if an unhealthy microbiome impairs sleep, it follows that improving microbiome health may represent a novel therapeutic tool for improving sleep. While no causal interventions yet exist in humans, a recent study in mice offers early clues. Mice received a 4-week treatment of Lactobacillus fermentum PS150, a “psychobiotic” bacterium strain previously shown to reduce stress in rats [ 49 ]. At the end of the 4-week supplementation with L . fermentum , the mice were placed into the standard anxiogenic challenge of a new environment that reliably triggers sleep disruption [ 50 ]. The control mice displayed the typical reduction in NREM sleep caused by the anxiogenic challenge. By contrast, the mice who received the microbiome supplementation showed sleep resilience, suffering no such sleep impairment. While not a direct demonstration, it nevertheless hints at a functional pathway wherein improving gut microbiota may improve sleep. If correct, it may usher in a new concept of “physiobiotics,” here facilitating the physiological process of sleep (i.e., somnobiotics), beyond the psychobiotic field.

Therapeutic enhancement of human sleep

Throughout most industrialized nations, almost 1 out of every 3 individuals sleeps less than the recommended 7 to 9 hours of sleep per night [ 51 , 52 ]. Current pharmacological sleep aids have limitations and adverse effects [ 53 ] and the number of qualified individuals available to provide the behavioral alternative treatment of cognitive behavioral therapy for insomnia (Glossary) is limited, relative to the demand [ 54 ]. Thus, a need exists for new approaches that are cost-effective, low friction (i.e., interventions requiring minimal user effort or resources), have high compliance, and are scalable at a societal level. Emerging research developments, including electrical and acoustic brain stimulation, kinesthetic methods, and thermal manipulations, are beginning to show promise ( Fig 2 ).

Several different noninvasive methods have been developed for artificially augmenting human sleep. (A) Electrical brain stimulation, including when it is time-locked to the upcoming peaks of individual deep NREM sleep brain waves, can enhance the power of those slow waves in a mechanism similar to the external assistance, or pushing, of a swing. ( B ) A similar outcome can also be achieved by slowly rocking a bed at frequencies close to the slowest oscillations of deep NREM sleep (purple, approximately 0.25 Hz), leading to an increase in the amount of deep sleep, and helping with a faster sleep onset, relative to a stationary bed (gray). ( C ) Thermal stimulation of specific regions of the body represents another method for artificially improving human sleep. Normally, the mechanism instigating human sleep (sleep onset) involves an increase in skin peripheral temperature of vascular regions such as the hands and feet (yellow dashed line). As the blood rises to the surface away from the inner body, core body temperature decreases, and the coincidence of these 2 changes provides a thermal signal triggering sleep onset (red dashed line, left-side panel). Thereafter, further decreasing core body temperature is associated with increasing amounts of deep NREM sleep. By artificially accelerating these transitions, mostly by experimentally warming the hands and feet, core body temperature decreases more rapidly, therefore reducing the time it takes those individuals to fall asleep (right-side panel), with further such thermal intervention subsequently increasing the amount of deep NREM sleep and reducing the amount of nighttime awakenings (i.e., increasing sleep stability and the consolidated nature of sleep).

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Electrical sleep stimulation

Slow brain waves are the hallmark of deep NREM sleep and quickly became a natural candidate for approaches using noninvasive electrical brain stimulation. Early findings demonstrated that applying 1 Hz transcranial direct current stimulation during NREM sleep over the frontal lobe (a main epicenter of NREM SWA) can boost naturally occurring slow waves by up to 60% in healthy adults [ 55 ]. The sleep enhancement was meaningful, with individuals consolidating memories during this enhanced sleep in a superior manner, therefore forgetting less the next day [ 55 ]. A number of replication studies have since been published, though with some exceptions [ 56 ]. There has, however, been an unexpected change in sleep in the story of slow brainwave stimulation. In numerous electrical stimulation studies, the boost in SWA came with a secondary enhancement of sleep spindle activity, as was observed with kinesthetic rocking. This would indicate that, independent of the stimulation method (movement or electricity), when SWA is boosted, increases in faster-frequency bursts of sleep spindles also follow.

Although SWA has classically been linked to the enhancement of fact-based memory (i.e., textbook-like memory), so too have sleep spindles [ 57 ]. New closed-loop monitoring methods of electrical brain stimulation (i.e., a control system wherein the deviation signal of sleep is measured in real time and used to control the action of ongoing brain stimulation to fine-tune and perfect) have selectively targeted sleep-spindle frequencies [ 58 ]. Artificial sleep-spindle enhancements also led to superior next-day memory recall of previously learned facts [ 59 ], although interestingly, they did not improve motor-skill motor memories, which are also known to be improved by naturally occurring sleep spindles [ 58 ].

One of the most reliable and striking physiological changes as we age is a pernicious erosion of sleep, with a disproportionately large decline in deep NREM sleep [ 60 ]. This change is further exacerbated in those with dementia [ 61 ]. Considering the cognitive hallmark of aging linked to memory failure, and the fact that deep NREM sleep aids overnight memory consolidation, the direct health and disease applicability of electrical brain stimulation in older individuals and those with dementia has become a target. To date, electrical brain stimulation has, with some degree of consistency, improved the quantity and quality of deep NREM SWA in older adults, and those with dementia [ 62 – 64 ]. While large-scale randomized controlled trials are still required, transcranial stimulation is seen as promising since it is inexpensive and somewhat pragmatic and, therefore, scalable.

At least 2 applications have emerged. First is the use of electrical brain stimulation for facilitating healthy aging and potentially reducing the cognitive burden of dementia and/or enhancing glymphatic brain clearance of amyloid and tau proteins (see the section on “ To sleep, perchance to cleans the brain ”). Second, and lofty in speculation, is the question of whether such technology could offer a future in which we shift from a model of late-life treatment of aging and age-related disorders to a model of midlife prevention. It is during the fourth decade of human life that the decline in deep NREM sleep begins [ 65 ]. Starting a regimen of sleep augmentation at this time (e.g., as we commonly do with calcium supplementation to prevent osteoporosis) could help bend the arrow of age-related ill-health and dementia risk down on itself by maintaining life-long quality sleep.

Acoustic sleep stimulation

Electrical brain stimulation still requires some degree of proactive motivation from an individual (applying the device each night, charging it, etc.). However, an alternative, low-to-no friction method for sleep enhancement is acoustic stimulation. When sounds are played without respect to ongoing slow waves, SWA is increased but memory retention is not improved [ 66 ]. A more sophisticated auditory stimulation approach has since emerged. Slow waves are detected in real time, with specialized algorithms predicting the timing peak of the next slow wave. At this time, a short sound is delivered to arrive at the peak of the next slow wave. This timed acoustic stimulation approach enhanced the expression of slow waves for some seconds after, and upon awakening, participants’ memory was 2-fold better compared with the unstimulated sleep nights [ 67 ]. A recent meta-analysis [ 68 ] has confirmed reliable and moderate effect sizes of acoustic sleep stimulation and the associated memory benefits.

Kinesthetic sleep stimulation

In the human historical record, there is ample reference to rocking a small infant to invite sleep with alacrity. Several recent reports, in humans and nonhuman species, provide physiological data that support this long-known parental wisdom of kinesthetic sleep stimulation [ 69 ]. When healthy adults sleep on a bed suspended from the ceiling during a nap period, and the bed is then rocked laterally at an even, slow frequency of 0.25 Hz, sleep is enhanced [ 69 ]. Seeking to mimic the frequency of the very slowest NREM sleep slow waves, this 0.25 Hz stimulation had participants falling asleep significantly faster, spending less time in the shallowest stage of NREM sleep, entering a deeper stage of NREM sleep sooner, and obtaining more of that deeper sleep, relative to when they slept without the rocking motion. Deconstructing the sleeping brainwaves, the rocking method boosted the amount of ultraslow NREM sleep waves (0.5 to 1 Hz) and increased another physiological bursting oscillation often paired with these slow waves, called sleep spindles (10 to 15 Hz).

Recently, these findings were replicated across a whole night’s sleep [ 70 ], with the study further showing that these rocking-induced benefits also had functional effects. Participants performed almost 10% better on a memory test after sleeping on the rocking bed compared to when sleeping on the stationary bed (not dissimilar to a full grade increase on an exam). Similarly, mice that were rocked gently by having their cage placed on a moving platform fell asleep faster, and spent more time in NREM sleep, although without changes in brainwaves. Elegantly, when the same experiment was performed on mice lacking sensitivity to linear movement, they did not experience any changes in their sleep patterns, confirming that it is the kinesthetic movement that augmented the sleep benefit [ 71 ]. By employing a vibrating pad, set at a specific frequency, even fruit flies can be lured into slumber [ 72 ]. Interestingly, with each repetition of the rocking procedure, the flies fell asleep more rapidly. However, this enhancement occurred only when the frequency remained unchanged; any slight alteration prompted the flies to reacquaint themselves with the new rhythm.

These latter findings suggest that the process of getting used to sensory stimulation helps in reducing arousal levels and, thus, promoting sleep [ 72 ]. More generally, the idea that a slow rocking kinesthetic improves sleep has already spurred the development of at least 1 commercial appliance at the time of writing this article. The device—essentially a set of 4 sturdy motor-driven movement pads—is placed under the feet of the bed. The pads instigate a rocking motion at the aforementioned slow frequency with the hope of sleep improvement (Enseven LLC, Arizona, United States of America).

Thermal sleep stimulation

If you isolate an individual from time and context cues, they will unwittingly report the greatest natural urge to sleep precisely when their core body temperature begins to plummet [ 73 ]. Temperature, therefore, offers 1 novel and newly harnessed pathway for enhancing human sleep [ 74 , 75 ]. The main evidence for this comes from pioneering work carried out by a team of sleep scientists led by Eus van Someren [ 76 ]. The team ingeniously developed a thermal bodysuit filled with tubes, much like veins, capable of selectively perfusing water of different temperatures to any specific part of the body. To artificially accelerate a drop in core body temperature, the scientists first focused on increasing the temperature of the peripheral extremities (hands, feet, arms, legs). When these peripheral areas are warmed, blood rises to the skin’s surface. As a result, warm blood from the inner core of the body is encouraged outward, allowing the rapid expulsion of core body heat, dropping central body temperature, and thereby inducing sleep. By controlling the temperature of the perfused water, they effectively accelerated the natural temperature drop that facilitates sleep (i.e., peripheral body warming to cause core body cooling; Fig 2 ). As a result, they had participants falling asleep approximately 25% faster than was normal for them. As they continued to mimic the body’s natural thermal sleep change further into the night, more sleep benefits unfolded. By continuing to cool the body into the first half of the night using the same suit, the scientists reduced the amount of time awake, thus increasing the amount of time spent in stable sleep, and the electrical quality of deep NREM sleep also increased [ 76 ].

Elderly individuals are one population that struggles with sleep and thermoregulation. Van Someren and colleagues have since targeted these older adult populations [ 77 ]. Before the body-cooling therapy, older adults in the study had more than a 50% probability of waking up in the last half of the night. After applying the thermal cooling manipulation throughout the night, the number decreased to less than 5% likelihood, and deep NREM sleep also increased.

Of course, thermal suits are not scalable owing to high cost and low compliance. However, baths and showers are a simpler, cheaper, and accessible alternative. Upon exiting the warm bath or shower, heat is again expelled faster and more efficiently from the body than without either of these thermal manipulations, leading to a drop in core body temperature [ 78 ]. A collection of studies utilizing warm baths or showers before bed [ 78 – 81 ] have, on average, resulted in individuals falling asleep between 10% and 30% faster, having fewer awakenings at night, and increasing the amount of NREM sleep by 50 additional minutes, relative to nights without prior hot bath or shower interventions. There may be a cost though. Some studies have reported a co-occurring decrease in REM sleep following hot baths or showers, either due to the NREM increase or to a change in body temperature shifting away from that which is optimal for REM sleep. Notably, manipulating REM sleep using temperature has been achieved by changing the ambient temperature in the room of the sleeper, rather than skin temperature. Absent sheet bedding, when the ambient temperature is close to thermal neutrality for endotherms (which, for humans, is between 29°C and 31°C, or 84°F and 88°F), REM sleep is maximal [ 82 , 83 ]. In rodents, when the ambient temperature is increased from 22°C to 29°C, moving more toward the thermal neutral zone, REM sleep more than doubles [ 84 , 85 ]. Nevertheless, the effect follows an inverted U-shape function, with REM sleep decreasing back down if the temperature is increased to 36°C [ 84 , 85 ]. Consumer technology groups have taken note. Smart home thermostats for ambient room temperature, and thermal-modulating mattresses controlling the temperature below the covers, all offer scalable approaches to altering human body temperature during sleep, although no formal peer-reviewed articles have been published to date.

Novel pharmacological sleep aid

Another development for enhancing REM sleep has emerged from the pharmacological arena. Over the past decade, drugs targeting receptors for orexin (also known as hypocretin) have emerged. Orexin is a neuropeptide that stimulates wakefulness and food intake [ 86 , 87 ]. These drugs, known as dual orexin receptor antagonists (DORAs), block both orexin receptors (OX1 and OX2), thereby inhibiting wakefulness and promoting sleep. Unlike previously developed hypnotic drugs, such as benzodiazepines and Z-drugs, which predominantly augment NREM sleep in a sedative-hypnotic manner, DORAs promote a different sleep signature. The 3 dominant DORAs (suveraxant, lemborexant, and daridorexant) not only enhance sleep by reducing sleep onset latency, wakefulness after sleep onset, and total sleep time [ 88 ], but these medications also reduce the time to the first appearance of REM sleep and increase the total amount of time spent in REM (suverxant [ 89 ], lemborexant [ 90 ], Deoraxant: N/A). Surprisingly, only 1 study (looking at suverxant) has published electroencephalogram (EEG) spectral data that offers insight into the effect of the drugs on sleep oscillations [ 91 ]. No significant changes to electrical EEG activity in REM or NREM sleep were observed at any of the wide-ranging doses of the drug used (even after 28 days of use). While preliminary, such data suggest that the DORAs consolidate and lengthen sleep without altering its fundamental oscillatory characteristics of cortical activity. Notably, when administered to older adults with suspected Alzheimer’s disease, suverxant increased sleep duration by 73 minutes per night (28 minutes more than placebo) [ 92 ]. In addition, recent findings in a small group of unimpaired middle-aged adults indicated that suverxant use decreased amyloid-β levels overnight by 10% to 20% in the cerebrospinal fluid (CSF) [ 93 ], the consequences of which we discuss in the next section.

To sleep, perchance, to cleanse the brain?

The body’s cleansing system, or lymphatic system, was first described in the 17th century by Olaus Rudbeck and Thomas Bartholin [ 94 ], yet the existence of any such cleansing system within the brain was not discovered until 1984, when Patricia Grady and Marshall L Rennals replaced the CSF of anesthetized cats and dogs with a tracer solution that could be tracked in brain slices under the microscope [ 95 ]. Still, it was only in 2013 that a team of researchers led by Maiken Nedergaard published a landmark set of discoveries that associated this cleansing system with sleep and postulated that it may explain why animals (or metazoans) with nervous systems require sleep.

The glymphatic system of the brain is made up of a matrix of glial cells that are nonneuronal in nature and utilize a set of water channels called aquaporins on their end feet [ 96 ]. Glial cells combine to form a space around the brain’s vasculature, called the perivascular space, in which CSF flows [ 97 – 100 ]. The glymphatic system services the removal of metabolic detritus, solutes, and toxins from the brain, specifically from the interstitial space between neural cells [ 98 ].

Sleep and the glymphatic system

Nedergaard and colleagues’ discovery, together with the contributions of many others [ 96 , 101 ], has established that the pulsing, cleansing glymphatic mechanism is not always switched on in high-flow volume across the 24-hour period. Instead, it is during sleep, and particularly during NREM sleep, that the glymphatic system shifts into full tempo. A seminal study in mice utilized CSF tracers to measure the CSF pulsing flux throughout the brain. When the mice were awake, CSF flow was minimal; however, when the mice entered NREM sleep, CSF flow increased considerably [ 97 ]. Strikingly, the extracellular space between the brain’s cells and structures (interstitial space) increased by 60%. As a result, there was markedly greater CSF flow coursing through the interstitial space, enhancing the exchange of waste products between the CSF and brain cells. Two notable waste products removed are amyloid-β and tau proteins, the excess accumulation of which is the hallmark of Alzheimer’s disease, and which we will return to in the section on “Disease implications” [ 102 ].

In humans, various studies have demonstrated that sleep has a causal role in removing waste products from the brain. Depriving individuals of sleep for an entire night, or even just selectively reducing the amount of deep NREM sleep (while holding a constant total sleep time), results in a next-day increase in amyloid-β and tau. This has been measured by markers in the circulating bloodstream [ 103 ], within the CSF (assessed using lumbar puncture) [ 104 ], and directly in the brain using amyloid-β- and tau-sensitive PET scans [ 105 ].

Sleep-dependent mechanism

Why is the sleep state essential for glymphatic clearance? First, the high levels of brain noradrenaline that dominate during arousal drop during sleep. Within the brain, one structural consequence is that the interstitial space expands [ 97 ], allowing for better-flowing conditions. Second, cardiorespiratory oscillations change markedly during NREM sleep. Both cardiac and respiration cycles slow down, respiration becomes deeper, and the temporal coupling between the two increases [ 106 ]. These pulses drive the mechanical contraction and dilation of the blood vessels, which, in turn, results in a corresponding and respective expansion and shrinkage of the space surrounding the vessels in which CSF resides [ 100 ]. Indeed, these cleansing fluctuations are 2 to 5 times larger in NREM sleep relative to the waking state [ 107 ]. Third, recent studies show that neural activity itself might influence CSF flow locally [ 108 ]. When neural activity decreases, the demand for fresh oxygenated blood decreases as well, which translates to narrower surrounding blood vessels and wider perivascular spaces that fill with CSF [ 108 ]. During NREM sleep, brain activity shows synchronous rhythmic SWA spanning vast brain areas, as opposed to the faster and desynchronized brain activity observed during wakefulness. The newly discovered involvement of brain activity affecting CSF flow could explain how spatially coordinated and rhythmic neural activity during NREM sleep, as opposed to the erratic and spatially diverse metabolic demands during wakefulness, supports efficient cleansing by synchronously widening the vascular space across larger brain territories.

The majority of mechanistic data illustrating the sleep-dependent operation of the glymphatic system has been in mouse models. However, a recent seminal study in humans employing a novel functional MRI (fMRI) marker to measure the strength of the CSF flow signal has provided the first hints of the same mechanistic system at work. As participants went into NREM sleep inside the MRI scanner, a significant increase in CSF flow was observed at the fourth ventricle, a large CSF cavern deep in the brain. Interestingly, this surge in CSF flow was preceded by a coupled increase in whole-brain oxygenated cerebral blood flow, which was, in turn, preceded by the electrical SWA that is prevalent in NREM sleep [ 109 ]. Thus, a physiomechanical rhythm creates a corresponding pulse and flow of CSF fluid, thereby representing a sleep-dependent pathway that supports the glymphatic sanitary service.

Disease implications

Impaired glymphatic clearance has been described and/or proposed in a collection of neurological disorders, including Alzheimer’s disease, traumatic brain injury, and Parkinson’s disease, as well as in psychiatric disorders [ 110 ]. Of note, every one of these conditions has well-established impairments in sleep. Of these, the most studied is the relationship between impaired sleep, Alzheimer’s disease, and the glymphatic system [ 89 ].

Hour-to-hour fluctuations in amyloid-β levels across the 24-hour period correlate strongly with the sleep–wake cycle in both mice and humans, rising during the wake phase when sleep is absent, and declining during the sleep phase when sleep occurs. However, mouse models of Alzheimer’s disease, in which sleep is impaired, do not show such diurnal fluctuations [ 111 ], suggesting that appropriate waste clearance is not taking place due to deficient sleep. Relatedly, mice whose sleep is pharmacologically suppressed for 9 hours experience a 2-fold increase in tau levels [ 112 ] and a 17% increase in amyloid-β levels within the brain [ 111 ]. In humans, the lower average duration of sleep across the life span, together with the disorders of sleep apnea and insomnia, are all associated with increased amyloid-β levels in later life and/or with a higher risk of developing early cognitive decline and Alzheimer’s disease. Moreover, there is a progressive linear impairment of fMRI-measured CSF flow—a proxy for aspects of glymphatic activity—in later life, with the severity of impairment increasing with the transition in older adults from health, to those showing signs of mild cognitive impairment, and, finally, to those with Alzheimer’s disease [ 113 ]. Showing bidirectionality, treating sleep apnea in midlife delays the onset of cognitive decline by over a decade [ 60 , 114 ].

While these data offer an explanatory mechanism for the well-known link between insufficient sleep and Alzheimer’s disease, they also raise the question of sleep as therapy. If the decline in deep NREM sleep, which begins as early as the fourth decade of human life [ 65 ], can be prevented, one could conceivably be able to decrease Alzheimer’s disease risk.

Sleep and emotional health

Any parent knows that poor sleep in a child the night before leads to poor emotional reactivity the following day. The same, it turns out, holds true for adults. Insufficient sleep quantity, quality, and select NREM and REM sleep abnormalities are associated with emotional dysregulation, anxiety, aggression, and worse mood (effect-size range g = 0.39 to 0.94) [ 115 – 118 ]. Recent neuroimaging studies have further revealed a unique neural mechanism accounting for these alterations in mental health caused by a lack of sleep [ 119 – 121 ]. Most intriguing, the sleep manipulations used to produce these affective changes in healthy adults mimic those expressed in specific psychiatric and neurodevelopmental disorders, including major and bipolar depression, anxiety, schizophrenia, autism spectrum disorder (ASD), and attention deficit hyperactivity disorder (ADHD) [ 122 – 126 ]. Indeed, no major psychiatric disorder has been studied to date in which sleep is normal [ 124 ].

Sleep loss and emotional health

Three key domains of affective brain function become compromised when sleep becomes short or of poor quality: mood and emotional baseline; (mis)perception of other people’s emotions; and an individual’s outward emotional expressivity to other people.

Concerning the basic tenor of an individual’s emotional baseline, complete or partial sleep restriction worsens mood states and increases emotional reactivity. Consequently, negative feelings of anxiety, agitation, hostility, anger, and restlessness [ 116 , 117 , 127 , 128 ] and, to a lesser degree, impulsivity [ 129 – 131 ], are increased. However, the adverse effect of a lack of sleep on blunting positive emotions is even greater than that of amplifying negative mood. Almost all dimensions of positive mental health diminish with insufficient sleep, including feelings of happiness, excitement, energy, motivation, and the general ability to gain pleasure from normally pleasurable experiences (anhedonia) [ 115 , 117 , 132 , 133 ]. Sufficient research enabled 2 very recent meta-analyses to be performed that quantify how sleep compromises mental health. A large effect size was found for the blunting of positive affect by sleep loss (g = −0.94, n = 25 studies), while increases in negative mood and increases in anxiety were also robust, although less pronounced (g = 0.45, n = 55 studies; and g = 0.39, n = 34 studies, respectively [ 115 , 116 ]). Interestingly, recent data indicate an important role of sleep regularity in protecting better mood and emotional health [ 134 ]. For example, increased variability of sleep duration (as measured across a week) predicts lower satisfaction with life, greater depressive symptoms, and increased anxiety [ 135 ]. Similarly, variability in sleep timing from day to day precedes poor mood, and worsening mood the following week, and does so independently of age, sex, level of physical activity, and sleep duration [ 136 ]. These findings collectively support the realization that, in addition to sleep duration and quality, the consistency of sleep can also be linked to numerous mental health outcomes.

Beyond dulling pleasure while increasing states of negativity, sleep loss also impacts the intensity with which these emotions are experienced [ 137 ]. When facing a modest cognitive challenge (such as counting backward in steps of 2), sleep-deprived participants will rate it as more stressful than those who had a night of sleep [ 132 ]. This suggests that sleep loss changes the internal cutoff or emotional threshold the brain uses to determine our transition into emotional distress. As a result, sleep restriction, poor sleep quality, and irregular sleep have all been linked to heightened subjective stress [ 137 – 139 ], an association that is only exaggerated in children with ADHD or ASD [ 140 ]. Notably, sensitivity to stress is known as the “lowest common denominator” that promotes vulnerability, or exacerbates symptoms of almost all mental illnesses, the majority of which include sleep loss or insomnia as part of their diagnostic criteria [ 141 – 143 ].

The underlying mechanisms explaining these changes in our innate emotional balance have been linked to aberrative physiological changes to the brain and body. Within the brain, sleep loss increases limbic reactivity and decreases functional connectivity (Glossary) between the medial prefrontal cortex and limbic structures, thereby diminishing emotion regulation capabilities ( Fig 3A ) [ 133 , 144 – 147 ]. Notably, the neural circuit connecting the amygdala to the anterior cingulate cortex has recently been shown to protect against mood disruption triggered by one night of sleep deprivation in both healthy individuals and those with depression [ 145 ]. Such findings indicate that changes to amygdala connectivity following a lack of sleep have a significant role in shaping both emotion and mood regulation without sleep. These changes in connectivity can be viewed more generally as confirmatory to the synaptic homeostasis hypothesis [ 148 ], suggesting that one function of sleep may be to rebalance or downscale synaptic strength that is potentiated during the day.

(A) Within an individual, sleep loss (pink) triggers a sharp reduction in positive mood and, to a lesser extent, an increase in negative mood (left-side panel). The emotional intensity felt by sleep-deprived individuals is also amplified by lack of sleep. However, there is a paradoxical decrease in the outward emotional expressivity triggered by sleep deprivation (right-side panel). These affective changes are further reflected in the brain. Here, sleep loss increases activity in the limbic network involved in emotional processing (red, left-side brain) yet reduces activity in the mPFC (blue, right-side brain). In addition, functional connectivity between the mPFC and amygdala is also reduced by sleep loss (dashed blue line), which is a communication pathway that normally regulates emotion. ( B ) Interindividual affective processes and behaviors are also altered by sleep loss. For example, sleep loss increases feelings of loneliness within the sleep-deprived individual and lowers feelings of empathy towards others (left panel). This asocial phenotype within an individual is further reflected in the reduced desire to interact with other, rested individuals. This effect is bidirectional. Rested individuals, unknowing of the sleep-deprived state of their conspecific, nevertheless show a similar reduction in the desire to interact with underslept others (right panel). ( C ) Across larger societal scales, insufficient sleep impairs prosocial behavior observed in large groups of individuals. For example, underslept groups express a reduced overall trend of helping behaviors and reduced motivation of typical societal civic duties, such as volunteering or voting (left panel). One underlying mechanism accounting for these collective asocial consequences is impaired activity in the social cognition brain network of underslept individuals (right panel), which is relevant as this network normally supports the ability to understand the state of others (i.e., theory of mind), and also promotes prosocial helping and cooperation. ACC, anterior cingulate cortex; mPFC, medial prefrontal cortex; TPJ, temporal parietal junction.

https://doi.org/10.1371/journal.pbio.3002684.g003

Potentially related to the aforementioned changes in limbic brain connectivity, or contributing to them, are increases in autonomic pupillary reactivity, increased skin conductance, up-regulated cortisol release, and increased blood pressure following lack of sleep [ 149 – 154 ]. Such a collection of changes suggests an explanatory biological framework of skewed brain–body sympathetic drive in response to emotionally inciting events, although, paradoxically, when quiescent, a new study has shown an opposite pendulum swing to excess parasympathetic drive under sleep-loss conditions [ 155 ]. Similarly, habitual short sleep (<7 hours) has recently been linked with reduced amygdala reactivity compared to normal sleep duration (7 to 9 hours), potentially indicating long-term desensitization of limbic reactivity following chronic insufficient sleep [ 156 ].

In addition to internal emotional feelings, contemporary work has established that emotional perception becomes skewed when sleep is insufficient. As a result, individuals can perceive a distorted view of incoming emotional signals from others and from the world.

Following reductions in either sleep quantity or quality, participants pay greater attention to and react faster to negative emotional stimuli (relative to neutral or positive stimuli) [ 129 , 157 – 159 ]. More than a change in the rose tint of an individual’s emotional perception is the recent discovery of emotional misperception. Here, sleep-deprived individuals fail to discriminate accurately between the different gradings of emotional facial expressions [ 160 , 161 ], with sleep loss biasing individuals to perceive greater threat signals relative to safety [ 162 ]; i.e., sleep-deprived individuals will more commonly mistake friend for foe [ 163 ], consistent with the proposal that the underslept brain loses the appropriate “tuning curve” of accurate emotional stimulus discrimination [ 45 ]. Nevertheless, when using short clips of individuals enacting varied emotional expressions, rather than still images, sleep loss does not significantly impair emotion recognition [ 164 ]. One explanation for this is that richer, more dynamic stimuli might sufficiently heighten attentional focus, motivation, or levels of arousal to compensate for the otherwise observed discrimination impairments triggered by lack of sleep.

The third domain of emotional function altered by a lack of sleep is one uncovered only recently, perhaps in part because it was so paradoxical. Contrary to the prediction one would make based on the internal sensation of amplified negative emotions, combined with the physiological sympathetic and limbic reactivity, the outward emotional expressiveness of underslept individuals is stunted ( Fig 3A ). This has been demonstrated across the emotive level of vocal expressiveness and facial expressivity [ 165 – 169 ]; e.g., sleep loss reduces pitch variations when individuals speak, making their voice sound more monotonic or “flattened” [ 165 , 166 ]. Therefore, sleep-deprived individuals experience increased emotional sensitivity themselves yet suffer a paradoxical outward reduction in expressivity (of that amplified emotional state).

There are many implications for these discoveries. One of the most powerful ways that human beings communicate nonverbally is through emotional behavioral expressions (e.g., voice, face, movement) [ 170 ]. Consider a sleep-deprived patient in a hospital not being fully communicative of their pain state and, thus, not being given appropriate pain treatment by medical staff (particularly relevant as sleep-deprived individuals feel noxious stimuli as more painful relative to when they are well rested [ 171 – 173 ]). Indeed, this very absence of signaled outward expression may explain why sleep-deprived participants are routinely viewed as less desirable to interact with, propagating the impact of sleep loss into the social domain (as we discuss below).

Benefits of sleep for emotional health

Sleep loss and sleep restriction lead to clear detrimental effects on our emotional well-being. The latest work has inverted the question: What is it about sleep, when we do get it, that beneficially improves mental health? Initial findings highlighted the role of REM sleep in the support of emotional processes [ 174 – 177 ] and in providing a form of overnight therapy, dissipating the subjective intensity of emotion when individuals are reexposed to an emotionally challenging event from the previous day [ 178 , 179 ].

However, the most recent findings have offered a revision of this REM sleep focus, establishing a role of NREM SWA in offering complementary effects on the mood state of anxiety, more than moment-to-moment emotional reactivity. More specifically, the amount of time spent in deep NREM sleep, as well as the electrical brainwave quality of that deep sleep indexed in SWA, service an overnight amelioration of anxiety in healthy adults, returning it to baseline levels. The greater the amount and quality of NREM SWA, the less anxious the individual felt the next day. When sleep was absent, however, anxiety progressively increased across the night and into the next day [ 128 ]. Interestingly, the underlying neural mechanism associated with this deep-sleep anxiolytic effect was somewhat similar to the effects of REM sleep. Both the amount and the quality of SWA predicted the extent of medial prefrontal cortex reengagement the next day, a region essential for the down-regulation of anxiety, and which is impaired in those with anxiety disorders (who also have co-occurring deficiencies in NREM sleep) [ 180 – 182 ]. Sleep, and the unique biological states of REM and deep NREM, may therefore explain the prophetic wisdom of American entrepreneur, Joseph E Cossman, who once declared, “The best bridge between despair and hope is a good night’s sleep.”

Sleep-dependent prosocial control?

Humans are a social species, psychologically and biologically requiring social connectedness. Collectively, as a species, survival necessitates such social, interindividual cooperation [ 183 ]. Indeed, without prosocial cooperation and helping, the advent of modern societies would not have occurred.

Sleep is a fundamental prosocial glue that binds human beings and entire societies together. The impact of sleep, and a lack thereof, has now been elicited from the level of a single individual’s social proclivity (e.g., social approach, social withdrawal, and loneliness) through to the prosocial interactions between humans (including the complicated processes of empathetic understanding and cooperative helping), and all the way up to the en masse coordination of societal behaviors ( Fig 3 ).

Sleep loss and the (a)social individual

Within an individual, a lack of sleep leads to feelings of social disconnection and loneliness. Insufficient sleep, including that caused by insomnia, poor sleep quality, difficulty falling asleep, and greater daytime sleepiness, are all associated with greater loneliness and a reduced desire to interact with others [ 184 – 187 ]. Moreover, sleep loss changes the way individuals evaluate their own social experiences, reducing a sense of connectedness and related positive affect and reducing the desire to interact further [ 188 ]. In longitudinal studies, initial poor sleep quality (including sleep fragmentation) and lower sleep satisfaction are predictive of higher levels of loneliness 2 to 7 years later [ 189 , 190 ], while preexisting loneliness is predictive of worsened subjective sleep quality, highlighting the bidirectional link between sleep and social isolation [ 186 ].

By contrast, superior sleep quality, including an ability to fall asleep more quickly with fewer nighttime awakenings, is associated with a higher likelihood of daytime active socializing [ 191 ]. This relationship is especially true regarding prior NREM slow wave sleep (SWS), with greater amounts and quality of SWS resulting in increased amounts of real-world social interactions the following day [ 192 ]. Offering bidirectional evidence once again, the social isolation of mice triggered a significant decrease in sleep amount, most notably reductions in the electrical quality of deep NREM sleep [ 193 , 194 ]. Thus, insufficient sleep, specifically reduced amount and electrical quality of NREM, can lead to a behavioral phenotype of social withdrawal and loneliness, while loneliness and social isolation instigate impairments in sleep quantity and NREM quality—a self-perpetuating cycle. Yet, REM sleep also appears highly relevant. Recent work has established a causal role for REM sleep in the consolidation of social memory [ 195 ], such that REM-specific suppression of hippocampal neural circuits in sleeping mice lowered the typical preference for novel social interaction the next day [ 196 ]. Similar impairments in social novelty preference were also recently observed following sleep disruption in adolescent mice, an effect that was linked to impaired reward-related dopaminergic activity when meeting a new conspecific [ 197 ]. Together, such findings indicate that sleep disruption of numerous kinds and stages leads to a phenotype of social withdrawal and disengagement driven by sleep-dependent neural circuits that otherwise sustain adaptive prosocial behavior.

Sleep and interpersonal social interaction

In addition to changes within an individual, interactions between individuals are also dependent on sleep ( Fig 3B ) [ 119 , 198 , 199 ]. Among romantic partners, poor sleep quality is associated with greater conflict the following day, higher levels of aggression, and lower marital satisfaction [ 200 , 201 ]. In children and teens, poor sleep quality predicts increased hyperactivity, more conduct problems, more disagreements with peers, more violent behavior, and a greater propensity for bullying [ 202 – 204 ]. Similar outcomes are observed in children with ASD, in whom short sleep duration and poor sleep quality are also related to difficulties in social interactions and fewer prosocial behaviors [ 205 , 206 ]. Notably, improving sleep in individuals with ASD can alleviate their symptoms, increase social communication skills, improve appropriate emotional reactivity, and decrease maladaptive and repetitive behaviors [ 207 ].

Further leading to the interindividual erosion of social bonds by a lack of sleep, underslept individuals are rated as less interesting or desirable to interact with by well-rested individuals, even when those well-rested individuals know nothing about the sleep status of the people they are rating [ 208 , 209 ]. Sleep-deprived individuals are further rated as lonelier, less attractive, less charismatic, more anxious, and more unhealthy-looking by independent judges who are similarly blind to the sleep status of those individuals they are rating [ 184 , 210 ]. This suggests that sleep deprivation curates a form of individuals who are socially repulsing (in the literal sense of the word) to the rest of society.

Our workplaces also suffer the deleterious impact of sleep loss on social functioning. A lack of sleep decreases the extent of helping behavior among colleagues in the workplace [ 211 , 212 ] and raises levels of overall hostility between employees. Morality suffers, too. Underslept employees show a significantly higher probability of unethical behaviors, such as blaming someone else for their own mistakes, or dishonestly taking credit for someone else’s work [ 213 ]. The social disconnection between individuals that ensues from a lack of sleep has also been identified within the hospital setting, to ill effect. Doctors who have insufficiently slept when working a night shift are significantly less empathetic to their patients’ pain and, as a result of this deficient empathy, prescribe fewer analgesic medicines to help alleviate patients’ pain, relative to doctors working a day shift [ 214 ].

A new development has added a peculiar feature to our understanding of sleep’s influence on interindividual dynamics. When a well-rested individual interacts with an underslept individual, the nonverbal signals of loneliness emitted by the sleep-deprived participant can be “transmitted” to the well-rested individual, making the well-rested person feel lonelier themselves [ 184 ]. Such virus-like propagation from sleep-deprived to well-rested conspecifics intimates that the ill effects of sleep loss can spread to nearby social circles and further aggravate loneliness, leading to a wider-reaching impact of insufficient sleep on social withdrawal.

Sleep and society

Moving beyond interpersonal interactions, new developments point to an influence of sleep loss in altering the unique societal forces that shape human communities. Humans help each other—helping is a fundamental aspect of social humanity and one that is eroded by a lack of sleep [ 215 ]. For example, decreasing sleep simply by 1 hour diminishes helping acts of civic engagement, such as signing petitions and volunteering [ 216 ], and reduces the likelihood of voting across multiple different nations [ 216 , 217 ]. Insufficient sleep, be it total deprivation or simply modest night-to-night fluctuations in sleep quality, also leads individuals to withdraw their normal proclivity to help others [ 215 ] ( Fig 3C ). One study examined over 3 million charitable donations made in the USA in the past decade. The loss of 1 hour of sleep opportunity, using the manipulation of the change to Daylight Saving Time, substantially decreased altruistic helping across all states that undergo a clock transition [ 215 ]. This same dent in compassionate gift-giving was not seen in regions of the country that did not change their clocks and, thus, whose sleep was uncompromised.

How a lack of sleep produces this potent impact on human sociability appears to be driven, in part, by alterations in brain networks that compute and make complex social choices. The social cognition network, which involves regions of the medial prefrontal cortex, mid and superior temporal sulcus, temporal–parietal junction, and the precuneus [ 218 – 220 ], helps support social computation and, consequently, decisions on appropriate prosocial actions [ 221 – 223 ]. Two recent studies have shown that a lack of sleep impairs the activity and social responsivity of this network [ 184 , 215 ]. Furthermore, the magnitude of impairment predicted a greater withdrawal of choices to help others [ 215 ], suggesting a neural basis for asociality when sleep gets short. Such an effect of sleep on the higher-order complex social computations of the brain remains even when taking into account changes in negative mood and motivation. Moreover, sleep loss could stunt the altruistic helping nature of the individuals in a manner that discounted close social bonds, such that participants who had had insufficient sleep withdrew their help to others regardless of whether those in need were strangers or people they personally knew, such as close friends or family members. These results suggest that sleep loss can trigger a phenotype of asocial behavior with a broad and indiscriminate impact.

Parenthetically, data have indicated a steady decline in empathy behavior and civic participation in the USA over several recent decades [ 224 , 225 ] that is paralleled by declines in sleep quality and aspects of sleep quantity across the same time period [ 226 , 227 ]. Reductions in sleep quantity and quality in industrialized nations may thus be a previously unconsidered factor contributing to some asocial trends.

What is in a dream?

Each night, individuals experience a state of altered consciousness known as dreaming. At times, they are notably disorientated, losing track of time, place, and person. They experience hallucinations, perceiving things that are not present, and show signs of being delusional, believing things that are clearly not possible. Added to this are large emotional pendulum swings, vacillating between extreme positive and intense negative emotions. Finally, upon awakening, they endure a degree of amnesia, forgetting large segments of the bizarre journey that has just happened, if not the entire experience. If this was not peculiar enough, almost all of this experience unfolds without any volitional control. This is the state of dreaming, and since the record of human species began, dreams have been a noted part of it [ 228 ]. However, only recently have sleep scientists begun to understand some fundamental aspects of dreaming, including how human brains dream and if other species show similar neural instantiation of the dream state; what, if any, function(s) dreams serve (above and beyond the state of sleep they come from), leading to the development of dream therapies to restore these benefits; how to “mind read” the dreams of others using fMRI; and if and how individuals volitionally control their dreams (known as lucid dreaming).

How the brain dreams

Depending on the definition, dreaming occurs in almost every sleep stage. However, prototypical dreams—those that most people would label as such—principally occur during REM sleep. As a result, neuroimaging studies were initially focused on REM sleep to uncover the objective neural underpinnings that explain dreaming [ 229 – 234 ]. A canonical signature emerged ( Fig 4A ). First, both primary and higher-order visual regions became strongly active as the brain entered REM sleep, aligning with the vivid visual nature of dreams. Second, areas involved in motor functions such as motor and premotor cortices, the cerebellum, and the basal ganglia also became activated, consistent with the perception of first-person actioned movements. Third, limbic brain areas responsible for emotional processing, including the amygdala, hippocampal formation, and anterior cingulate cortices, display heightened activity, potentially accounting for the intense emotional tone dreams commonly take [ 235 – 237 ]. More interesting, however, were large regions of the brain that showed a converse decrease in activity during this otherwise highly active brain state of REM sleep, including the prefrontal cortex, the functions of which include volitional control and deliberative decision-making. Without knowing the experience of the individual, or even their state, should one look at this stereotypical pattern of brain activity and predict what subjective experience the individual was having, it would be a reasonable description of dreaming: perception of visual elements, motor action, emotionally laden, layered with autobiographical memories, yet disorganized, illogical, and without volitional control.

(A) Brain activation during REM sleep—one of the principal stages associated with vivid dreaming. Relative to brain activity when an individual is either awake or in non-REM sleep, there is increased activation of visual, sensorimotor, and affective pathways during REM sleep (golden clusters). Additional regions then come online and are activated when individuals experience lucid REM sleep (red clusters; relative to nonlucid REM sleep). These include regions of the anterior prefrontal cortex involved in volitional executive decisions and actions, and the precuneus, involved in self-referential processing. ( B ) Incorporation of recent waking events into dreams unfolds in a 2-peak reliable pattern over time. The first temporal peak of waking incorporations occurs on the first 2 nights and then fades. However, these same prior waking experiences reemerge as a second peak 5–7 days later. This temporal pattern of waking life incorporation is known as the dream lag effect. ( C ) IRT is a behavioral intervention method for treating and dissipating nightmares. IRT includes the waking rehearsal of alternatives to nightmare scenarios, developed between the patient and their therapist. These more neutral or positive alternatives to the nightmare scenario are rehearsed daily by the patient for up to 2 weeks. As a result, the nightmares become significantly less distressing. A recent study added an additional methodological step. During the daytime rehearsal of the nightmare alternative, an auditory tone (here, a piano chord) was played every 10 seconds in the background. Then, as the patient slept and went into REM sleep—the stage most commonly associated with nightmares—the same piano chord was played at a level that did not wake the patient up. The goal was to reactivate the memory of the alternative scenario as the sleeping brain is processing. As a result, patients experienced an even larger decrease in the distressing nature of the nightmare, relative to standard IRT. IRT, imagery rehearsal therapy; REM, rapid eye movement.

https://doi.org/10.1371/journal.pbio.3002684.g004

This neuroimaging signature was a key first step in answering a more fundamental question of whether other species dream. Of course, animals cannot provide verbal dream reports, making proof of the dream process difficult to ascertain for anyone other than humans. Evidence from the study of animal models of REM behavior disorder (RBD) provides added clues. Human RBD is a disorder characterized by dream enactment behaviors caused by the loss of muscle paralysis that accompanies the dreaming state of REM sleep [ 238 ]. Commonly violent, these movements may endanger the patient and their bed partners and, as recently affirmed, can reliably anticipate Parkinson’s disease [ 239 ]. Genetic, surgical, and pharmacological manipulation of the neural mechanisms that otherwise instigate muscle paralysis in rodents and cats results in remarkably similar behavioral repertoires during REM sleep, suggesting the possibility of dreams in animals (although there remains debate as to the acceptance of this premise [ 240 , 241 ]).

However, knowing the neural brain signature of human REM sleep that is associated with dreaming, scientists have started to ask whether a similar, objective neural signature could be found in other species and are getting closer to positive proof. The first fMRI-measured indications were recently published in Columba livia (pigeons). The researchers imaged the pigeon brains during the waking state and then again during REM sleep. What emerged was a strikingly similar pattern of brain activity to that associated with the experience of dreaming in humans; increased neural activity in visual regions, areas of the motor cortex, and subcortex, together with regions that regulate emotional affective states [ 242 ]. Of course, this finding does not prove that pigeons dream, but it does suggest that pigeons, despite their evolutionary divergence from mammals, nevertheless express a REM sleep neural pattern resembling that of humans, who do dream. Since neuroimaging methods in humans are fast becoming capable of deconstructing and then visually reconstructing the subjective waking experience of humans [ 243 – 245 ], a similar visual reconstruction of pigeon REM sleep brain activity may soon be on the horizon.

Not only does the brain show surges in stereotypical activity during human REM sleep, but so too do the peripheral nervous system, respiratory system, and vascular system [ 246 ]. A surprising recent discovery was made in cephalopods, specifically octopi, who have no central structured brain. During their offline state of sleep, the researchers systematically observed very clear, cycling swells of nervous system activity that repeated in bouts (approximately 1 minute) that are now believed to potentially be primitive REM sleep [ 247 , 248 ]. Also matching human REM sleep dreaming, it was harder to provoke the octopi to respond during these REM-like, fast-breathing, neural activation cycles relative to when awake [ 247 ]. Uniquely, these sleeping surges involved rapid pulsating body movements and synchronized changes in skin patterning. While this too does not prove dreaming, it is of note that octopi typically alter their skin patterning for camouflage during situations of threat and mating, both themes (threat and sex) that are present in human dreams [ 249 , 250 ].

Why brains dream

The “why” of human dreaming has been one of the most contentious, and fiendishly difficult, questions to answer scientifically. Although subtle in distinction, to establish a function of dreaming, one has to determine that any such benefit is not simply that of the underlying biological sleep state from whence those dreams came (e.g., REM sleep) but is instead specific to the dream itself. Using this framework, 2 main functions of dreaming (above and beyond sleep or REM sleep) have so far emerged: associative memory and creativity, and emotional processing and mood recalibration.

Memory, association, and creativity.

Although sleep is documented to boost learning and memory, only recently has dreaming been understood in terms of information processing independent of sleep. The benefits linked to dreaming are arguably even more powerful than the simple strengthening of individual facts that takes place during NREM sleep. Dreams can help interconnect large amounts of information, such that an individual wakes with a revised mind-wide web of associations capable of creatively divining solutions to problems previously faced while awake.

One recent study affirming this memory benefit assessed how efficiently individuals were able to weave together different memory components of a virtual maze they had been initially exposed to [ 251 ]. Those participants who obtained sleep after the learning phase were far better at assimilating the individual maze elements into a coherent whole, denoted by participants navigating their way through the maze faster, relative to a group that did not sleep during this time. This alone was not proof that dreaming itself was necessary. For that, researchers obtained dream reports throughout the sleep phase of those in the sleep group. Participants who slept and reported dreaming about the maze demonstrated a 10-fold improvement in navigation upon awakening, relative to those participants who slept, and still dreamt, but did not dream about the maze itself. It was not, therefore, enough to sleep or even to dream. Individuals had to dream about the waking problem itself in order to gain the associative memory benefit helping them to navigate the maze.

The process by which the dreaming brain accomplishes information assimilation, abstraction, and creativity is not completed in a single night. When studying dream content and waking life events systematically, information that individuals experience is most strongly integrated into their dreams over the first 2 nights, after which that information reprocessing appears to fade [ 252 , 253 ] ( Fig 4B ). Yet, these waking events then unexpectedly but very reliably resurface again 5 to 7 nights later—a phenomenon known as the “dream lag” effect [ 254 ]. These findings suggest that the conscious act of dreaming, and perhaps its memory function, evolves in 2 distinct temporal waves. The end product of these processes is arguably the difference between knowledge (learning individual facts, largely the role of NREM sleep [ 255 ]) and wisdom (knowing what they mean when we put them together, the role of conscious dreaming). Indeed, there is no shortage of science-related anecdotes of dream-instigated creativity. Examples include the dreams of Otto Loewi, which inspired experiments that led to his Nobel Prize–winning discovery of neurochemical transmission [ 256 ], and the equally impressive dream-inspired creative insights gifting Dmitri Mendeleev the elemental, universal (in the literal sense), conception of the periodic table of elements. Little wonder the advice is never to “stay awake on a problem.”

Emotional processing.

Posttraumatic stress disorder (PTSD), which reflects an inability of the brain to process and ultimately overcome a mentally damaging event, epitomizes the disability that occurs when the brain’s otherwise normal ability to resolve and move past difficult, painful experiences becomes impaired. Reactive depression to a specific event, such as bereavement or divorce, offers another clinical example of challenging mental health resolution. Dreaming appears to be one mechanism through which such emotional restitution (or overnight therapy) is accomplished [ 175 ]. Although Sigmund Freud arguably opined some version of this dream benefit [ 257 ], it was seminal works by Rosalind Cartwright and her colleagues in the 1990s that provided initial, scientifically credible evidence. Cartwright studied individuals with reactive depression, assessing their sleep and dream content, and tracking their clinical progress over time. Patients with depression exhibit significantly fewer dream reports relative to controls [ 258 , 259 ], and the more severe the patient rated their depression, the fewer dream reports they mustered [ 260 ]. Yet, it was what the patients were dreaming about, more than simply if they dreamt, that predicted recovery. Patients who ultimately overcame their depression, accomplishing remission a year later, were dreaming expressly about the trigger of their depression (i.e., the content of their dream), relative to those who were dreaming, yet not about the inciting experience as much [ 261 , 262 ]. Adding to this evidence, recent data have confirmed that the negative and positive emotions nested within the dream content predict next-day waking mood changes [ 263 ], with negative dreams increasing negative mood and vice versa. Thus, more than just the state of REM sleep, indeed more than the act of dreaming, it seems that the content of one’s dreams, and their emotionality, offers a form of nocturnal emotional first aid [ 264 ].

Understanding how dreams are curated and used by the brain has led to the development of new dream therapies, such as imagery rehearsal therapy (IRT), specifically targeting the most distressing dreams common in nightmare disorder and prevalent in PTSD [ 265 ]. IRT is a cognitive-behavioral technique aimed at reducing nightmares by modifying the content of distressing dreams. Individuals learn to “rescript” and mentally rehearse revised versions of their nightmares while awake, which helps transform the narrative and reduce the frequency and intensity of distressing dreams ( Fig 4C ). For example, having been in a serious car accident, someone might say they have a terrible repeating nightmare where they are unable to steer their car out of the way of incoming traffic, the brakes stop working, and then BANG … they wake up utterly distraught. The therapist will ask them to imagine alternative endings to their nightmare (the imagery part). So, in our example, perhaps they now reimagine a different ending where they realize they can reach down and gently use the handbrake to slow the car down. Next is the rehearsal part, where patients would rehearse this less distressing alternate ending daily for a couple of weeks in the hope of modifying and updating nightmare memories. Indeed, in a recent study using IRT, patients with nightmare disorder experienced a significant reduction in their nightmare frequency [ 266 ].

However, the therapeutic potential of IRT was even greater. In a second group of patients, the researchers asked participants to rehearse their alternative endings while listening to a pleasing piano chord played every 10 seconds in the background. Then, over the ensuing 2 weeks of the study, that same chord was played to participants at sub-awakening volume whenever they entered REM sleep at night. The purpose was to trigger the memory of those rehearsed alternative endings when nightmares often manifest. This dual manipulation strategy resulted in an even greater reduction in nightmare frequency. Indeed, they experienced an 80% greater relative reduction in nightmare frequency compared with the group that received IRT alone. Moreover, these sound-paired participants also reported a 2-fold greater increase in the number of positive emotions in their dreams, relative to the control group. Even more remarkable, the added benefits of the combined IRT and sound protocol were still significant in a 3-month follow-up, despite the fact participants were no longer receiving any cues during the night. Such advances illuminate a path forward in altering dream content to better facilitate the innate “overnight therapy” of dreams long after awakening.

Peering into dreams

Until recently, an individual’s dreams have been their own: a private experience that one decides if and when to reveal to others. Using advanced fMRI scanning methods, new data suggest this may no longer be the case [ 267 ]. While awake inside the MRI scanner, participants viewed many different objects across category themes (faces, cars, houses, etc.), with scientists then training a machine-learning model on this waking “ground truth” data of brain activity. The participants then performed a second fMRI scanning session. Now they were allowed to fall asleep, and the researchers obtained dream reports from these scanned sleeping periods. With high statistical probability, and using only the brain activity, blind to what the individuals had been dreaming of, the fMRI scans were able to predict what the individuals had experienced in their dreams with 70% accuracy. More specifically, the scientists could predict the category themes the individuals were dreaming about; e.g., they could decode that the dream included a car, predicting the form of the dream someone else was having. However, they could still not predict the unique content of the dreams (e.g., the make or model of the car), that level of detailed knowledge remains private—the purview of the dream owners themselves, at least for now. Based on recent EEG findings focused on specific regional brainwave frequencies, that time may be closer than once believed [ 268 ]. These developments could give rise to significant privacy concerns, considering that EEG is increasingly applicable to the home environment, and in the future, perhaps might even be as commonplace as contemporary smart wristbands [ 269 ].

Although most people do not have volitional control over their dreams within dreams, some individuals do. Such volitional awareness of, and controlling choices and actions, is called lucid dreaming. The challenge has been to empirically prove such a seemingly unprovable assertion, considering that the lucid dreamer in question is asleep, and thus unable to communicate with scientists, paralyzed by the dream state. Yet, scientists have overcome this challenge by harnessing one of the few muscle groups spared from REM sleep paralysis; the extraocular eye muscles. It is, therefore, possible to train lucid dreamers in a pre-agreed-upon pattern of what is essentially eye movement Morse code. Using this, it enabled the participants to signal to scientists the moment when they gain lucid control of their dream and further signal what they claim to be doing as they were lucid dream [ 270 ].

One elegant experiment dispelling any doubt of lucidity combined this eye movement communication method with neuroimaging [ 271 ]. After signaling the initial state of lucidity when asleep inside the scanner, the participants provided a pre-agreed eye movement signal that they were about to deliberately clench their left hand in their dream, and another signal that they were then clenching their right hand. When compared to the waking “ground truth” brain activity of actual physical hand movement, the brain activity occurring during the claimed lucid dream hand movement was an unmistakable match. Thus, scientists obtained objective proof of the subjective claim of lucid dreaming.

Researchers have now gone further. Using these communication methods, they have had back-and-forth, real-time dialogues with lucid dreamers, in objectively convincing ways. After a participant indicated that they had gained lucidity, the experimenter posed simple mathematical questions to the dreamer (e.g., 8 minus 6) using either speakers or visual flashing codes that participants had previously learned [ 272 ]. The dreamer then responded using eye movements while still asleep in the lucid state (confirmed by EEG), providing their deliberated answer. The participants were able to respond correctly, with a level of accuracy far above chance. This finding offers further ratification of the lucid dreaming claim and indicates the ability of lucid dreamers to comprehend information in a volitionally conscious way, offering deliberate logical answers under willful control. While rudimentary, such evidence may open up the opportunity for sleep-dependent, and dream-dependent, interventions, boosting the innate benefits of dreaming to new realms.

Lessons from the diversity of sleep across species

Every organism that has been carefully studied to date sleeps. From vertebrates to boneless species such as jellyfish [ 273 ], octopuses [ 274 ], and even worms [ 275 ], sleep appears to be evolutionarily ancient, strongly conserved, and near universal. Yet, there is confusing controversy within this narrative of consistency. Being as strongly preserved across evolution as it is, one would assume that the amount of sleep, and how that sleep is structured, would similarly be consistent across species or, at the very least, more similar than different. The opposite is true: The only thing more axiomatic and surprising than the homogeneity of sleep across species is the heterogeneity of how sleep is expressed across and even within species. This is mirrored by the nearly equal numbers of hypotheses attempting to explain these differences. In this section, we outline long-standing evidence and several new discoveries seeking to decipher this perplexing heterogeneity ( Fig 5 ).

Sleep duration varies markedly across the entire animal kingdom, from invertebrates to mammals. In most invertebrates and fish, sleep is defined behaviorally (e.g., for fire ants [ 276 ] and Port Jackson sharks [ 277 ]). Physiological evidence of NREM sleep can be found in a few amphibian species (e.g., the common frog [ 278 ]), as well as in reptiles [ 279 ]. In birds and mammals, evidence of sleep includes physiological recordings of both REM and NREM sleep, often measured in the lab [ 280 – 282 ]. NREM, nonrapid eye movement; REM, rapid eye movement.

https://doi.org/10.1371/journal.pbio.3002684.g005

Sleep variability

In mammals, sleep duration can range from as little as 2 hours in elephants [ 283 ], to 20 hours in the little brown bat [ 282 ]—a 10-fold difference in sleep duration. Indeed, some [ 284 ] have pointed out how idiosyncratic sleep duration is by highlighting clever comparisons [ 284 ]. One example is the ground squirrel, which sleeps for 15.9 hours on average, while the degu, which is ranked within the same taxonomic order, sleeps for an average of just 7.7 hours. By contrast, animals from different taxonomic orders, such as the guinea pig and the baboon, sleep for an identical amount of time (9.4 hours).

Recent data have added a new dimension of sleep variability within the same species. Elephant seals sleep for a little over 2 hours when making their months-long trips at sea, yet once they return to the land, they will defy that trend and consistently sleep for 10 hours or more each day ( Fig 5 ) [ 285 ]. Similarly, male pectoral sandpipers reduce their sleep amount 2-fold when females are present and fertile, relative to their sleep during nonbreeding periods. This adaptive act of ecologically pressured sleep reduction comes with a benefit. Those birds that slept the least during the breeding period gave rise to the most offspring [ 286 ]. Therefore, just when a fixed sleep duration label for a given species had been assumed from short-term, in-laboratory evaluations, it turned out that sleep duration within the same animal is far from fixed when tracked longitudinally in the wild.

Beyond sleep duration, species also differ in how their brains obtain sleep. During their trans-oceanic migrations, birds can switch off an entire hemisphere of their brain, putting it into a deep sleep while the other half of the brain remains wide awake. This feature, termed “unihemispheric sleep,” allows the migrating birds to have an open eye (linked to the respective waking hemisphere) turned to face the direction of flight [ 280 ]. The other eye, connected to the sleeping hemisphere, is closed shut. Ducks use this same sleep adaptation in response to a different evolutionary pressure. When ducks are lined up in a row, those individuals at the far ends of the flock sleep unihemispherically, directing the open waking eye to the vulnerable side of the flock to monitor predatory threats, while the eye (and corresponding opposite hemisphere) facing inwards to the safety of the flock is closed due to sleep. Those ducks seated within the flock have the luxury of sleeping with both hemispheres, since 360° perimeter threat detection is covered by the 2 sentinel ducks sitting at each end of the flock [ 287 ]. Many aquatic mammals show similar unihemispheric sleep patterns, driven by the equally different evolutionary pressure of needing to continue surface breathing. Nevertheless, they still manage to accomplish their sleep needs without drowning, one-half of the brain at a time.

Satisfying one’s full sleep need while keeping one-half of the brain awake to continue waking activities would seem like an envious ability that humans do not possess. Or so it was thought. Recent reports have shown that humans do, in fact, perform a version of unihemispheric sleep, albeit one that does not afford us the ability to continue functioning as if one hemisphere is completely awake. Have you ever had the experience of feeling as though you did not sleep well in a strange new environment [ 288 ]? A study has shown that when individuals sleep in a new environment, one hemisphere of the brain sleeps in a more shallow state of deep NREM sleep than the other. As with the ducks, the interpretation is that the human brain adapted to a change in how it sleeps when the potential for threat increases. While not fully awake, half of the brain in the proposed threat detection mode of lighter sleep is more responsive to sensory stimuli [ 289 ].

Theories of sleep variability

Building on these findings, new theories have been set forth regarding the explanatory functions of how sleep developed across the tree of life. Specifically, since REM sleep appears to be exclusive to warm-blooded animals, it has been proposed that thermoregulation was the original evolutionary force behind the development of REM sleep as a novel state designed to maintain homeothermy while still accomplishing sleep [ 290 , 291 ]. Consistent with this view, brain temperature drops in homeotherms (including humans) as they go deeper into NREM sleep. However, this trend reverses during REM sleep, when brain temperature reliably increases. Thus, REM sleep is thought to have emerged due to the evolutionary need for a reheat cycle during sleep; otherwise, the cognitively slowed state of brain functioning and the impeded autonomic function often necessary upon waking up quickly would be unacceptable for survival and fitness. Thus, the evolutionary reason that REM sleep emerged in warm-blooded mammals and birds (the classes that show reliable REM sleep) was to protect against excessive central brain hypothermia that would otherwise occur by experiencing NREM sleep alone [ 291 ]. However, the recent discovery of a “proto” version of REM sleep in cold-blooded reptiles [ 279 , 292 , 293 ], zebrafish [ 294 ], and marine invertebrates (including cuttlefish and octopuses [ 247 , 248 ]) has challenged the REM sleep thermoregulation hypothesis. Instead, these new findings suggest that the features of REM sleep, from muscle paralysis to rapid eye movements and increased cortical activity, developed earlier than previously thought. Therefore, the evolutionary reason for developing REM sleep must have preceded the need for temperature regulation. As with all good new discoveries, these data have only led to more interesting, as yet unanswered, questions about the function of REM sleep. The initial evolutionary reason for the emergence of paradoxical sleep, or REM sleep, therefore, still remains a paradox.

Even theories that sought to explain the variability in sleep duration across species, above and beyond the individual stages, remain controversial. A prominent and simple first theory was that brain size is the explanatory factor. The brain is, after all, a disproportionately demanding metabolic organ, and this size-related cost may, therefore, explain differences in sleep amount. Not so. The variability in sleep amount across species is not explained by brain size nor is it explained by cognitive ability [ 295 ]. Modified theories focused next on metabolic rate. One may logically predict that the more metabolically active a species, the more sleep (i.e., energy savings) it requires. Yet, metabolic rate only marginally accounts for variability in sleep amount, and even there, the association is in the opposite direction. Species with a high metabolic rate sleep less than those with a lower rate [ 284 ]. The current explanation is that more metabolically active species must spend more time awake foraging for food to satisfy their greater caloric demand and, thus, sleep less [ 2 , 291 ]. Here, too, as in the case of the flocking ducks, predatory risk further shapes sleep variability. Analysis of sleep duration across 58 species of mammals indicated that indeed those exposed to higher predatory risk sleep less, even when correcting for the size of the animal [ 295 ]. But since predation risk and being a herbivore tend to correlate, which factor is the more important is still an open question. One especially interesting species is the omnivore baboon ( Papio anubis ), which is subject to nightly predation risk imposed by leopards and lions. To mitigate the risk, and similar to the ducks, the baboons sleep in groups and also alternate sleep locations, 2 modifications that tend to compromise their sleep amount due to, what seems like, the first night effect (described for humans above) and awakenings caused by fellow baboons [ 296 ]. Despite the survival benefit of sleeping close to conspecifics, the price, in the form of insufficient sleep, has been observed across many mammals sleeping in the wild [ 295 ].

These are just some of the many examples of theories seeking to explain the heterogeneity of sleep among species. What all of these examples (from unihemispheric sleep to a 10-fold difference in sleep needs) have taught us is perhaps obvious, but worth reiteration: If sleep were dispensable and, thus, optional, or even if certain stages of sleep were desirable but not required, evolutionary pressures would have led to sleep, or a specific stage of sleep, being forfeited a long time ago. Nevertheless, the fact that sleep has consistently persevered throughout the evolution process, and done so in ingenious ways (within an individual, and across groups of individuals), serves to reinforce the conclusion that sleep is a critical necessity, serving numerous functions within and across species. This may not be so surprising, considering we have long recognized the polyfunctional nature of wakefulness.

Conclusions

This collection of recent discoveries not only affirms the role of sleep as a biological life-sustaining necessity but also extends the polyfunctional nature of sleep and the conscious state of dreaming in unexpected ways. These include DNA repair, immune function governance, effects on the gut microbiota, brain cleansing, controlling and enhancing complex social and emotional functioning, novel means of memory optimization, and the co-opting of divergent creativity. Moreover, through a growing understanding of basic sleep physiology, mechanism, and function, a plethora of new technologies are emerging that are capable of manipulating and enhancing human sleep physiology. As a result, there is a distinct possibility in the future that humans will be able to therapeutically manipulate sleep in precise ways for the treatment of specific diseases and disorders. These may range from regulating the gut microbiota to managing the mental health of an individual, slowing brain aging and its pathologies, aiding in trauma resolution, and even facilitating prosocial engagement in the face of a growing loneliness epidemic [ 297 – 299 ].

Apart from this, a different discussion theme emerges from the new wave of research discoveries, i.e., the elemental “why” of sleep, and more specifically, how researchers conceptualize the question, aside from any answers they arrive at. Wakefulness, the antithesis of sleep, becomes a meaningful lens through which to explore a claimed sleep-dependent benefit/process. In this framework of questioning, the guiding principle has been to search for a sleep function that cannot be served by wakeful rest. The query, therefore, becomes, for any functions thought to be ascribed to sleep, is there any evidence that this function can be supported by wakefulness, and if not, why not?

Another nonmutually exclusive framework for answering the question of sleep’s why is in its absence. Classical methods of total and selective sleep deprivation were scientifically limited based on the confounds of the deprivation methods used, such as stress, or the fact that selective deprivation of a sleep stage also meant that total sleep time was defacto reduced. Now, however, there are much more sophisticated methods that obviate many of these concerns and offer stronger causal affirmations. A good example is the method of sub-awakening auditory tones. Using this method, the individual is selectively deprived of deep NREM sleep in a specific brain area by the tones that lift them into lighter NREM sleep without waking them, and so total NREM sleep duration is preserved [ 300 ]. Another example is using implanted electrodes in animal models. Here, the electrodes are used to selectively disrupt neural events, such as forward memory-sequence replay during NREM sleep (the replaying of the order of memory-cell firing that was coded during initial spatial learning while the animal was awake), thereby demonstrating a causal dependence on a physiological sleep mechanism for memory consolidation [ 301 ]. As new methodological advances grow in their nuance and ability to selectively excise other stages of sleep, or even specific electrical brain-wave oscillations, the dissection of the “why” of sleep dependency will become ever more concrete. And “dependent” not only in the sense of sleep versus wake, but also of one sleep state relative to another, or even to the extent of one specific brain region’s experiences of a sleep-oscillation state relative to other brain regions.

More generally, the revelations brought forth by these new, highly diverse functions of sleep do not negate the possibility that one consensus and common function of sleep nevertheless exists across species. There may very well be a singular (original and/or common) function of sleep that transcends taxonomy. Moreover, the notions that a single common function of sleep exists, while additional multiple functions of sleep have later evolved across time, are not mutually exclusive or antagonistic.

As the polyfunctional view of sleep grows, another fruitful framework is that of the interdependence and interconnectedness of different sleep functions that achieve benefits to the organism greater than the sum of each part. Here again, it is something that has long been accepted regarding many of the functions of wakefulness. For example, without sleep’s interconnected support, the ensuing free radical damage caused by sleep deficiency may increase inflammation, which, in turn, leads to sickness behavior, which consequently triggers social withdrawal and loneliness in the sleep-deprived individual. Another possible example would be the emotional and social changes in behavior caused by sleep loss that impair the immune system, which leads to worse gut microbiome health and, through afferent vagal signaling, alters mood and emotional states, each of which only further disrupts sleep, leading to an interconnected negative spiral. For sleep and its functions, cinematically speaking, it is “Everything, Everywhere, All At Once.”

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Humans Could Learn a Lot From Anxious Cows

We love to focus on personality types, attachment styles, and diagnoses. but we’re part of a herd too..

I like to talk to my therapy clients about anxious cows. Among a group of peacefully grazing cows, the mere whiff of a nervous herd member can get the other cows all stirred up. Ears perked and tails twitching, they’ll seek out a familiar cow friend, maybe one who seems a little more chill, and the pair will start licking each other’s heads, reducing heart rates with a nice, juicy tongue massage.

We are not so different from cows. We all have ways of stirring each other up and calming each other down. One member of a team at work worries about a deadline, and suddenly you’re all a little on edge (then headed for a soothing happy hour drink). Your partner is upset about a neighbor’s noisy renovation project, and before you know it, you are too.

I find that telling my clients about cows—or elephants, or even bugs—can help. I live and work on Capitol Hill in Washington, where you’ll find The Body Keeps the Score in every lending library, but I’ve never seen a copy of Frans de Waal’s Chimpanzee Politics . Maybe that book should be ubiquitous. In my experience, when you let the animal world loose into the therapy room, people relax a little. They begin to see how a dreaded trip home or conflict at the office is a brilliant opportunity to observe anxiety among a group of animals—to metaphorically pull out a naturalist’s notebook and record patterns.

Much of the therapy world is disconnected from the natural world. We are focused on personality types, attachment styles, and diagnoses backed by the Diagnostic and Statistical Manual (diagnoses we would never give a cow because they are just … all cows). But our emphasis on human uniqueness, while well intentioned, has backfired into a pattern of labeling a lot of adaptation as dysfunction. We turn to treatments that focus on the individual, instead of looking at how our behaviors are part of a group dynamic. We have become less concerned with our place in the grand story of life.

Humans are not unknowable unicorns. We are products of evolution. Our behavior is influenced by the processes that govern the natural world. Our families and communities are natural systems trying their best to survive and thrive. When I was writing my latest book, True to You , it was important to me to use examples from nature to help people think about human relationships. Because we can learn something about ourselves when we study other natural systems, whether it’s a prairie dog town, a termite mound, or a troop of mushrooms.

When a client shamed herself for getting too competitive with her colleagues, I suggested she read about elephant hierarchies at the watering hole. When a manager wondered why he couldn’t seem to inspire some of his team members, I pointed out that 25 percent of ants in a colony barely work at all. Maybe the answer to why you or your child is handling a situation in a particular way isn’t buried deep in a stack of psychology research or in a therapist’s TikTok dance. Maybe it’s that you are creatures trying your best to survive out there—just like every other creature on this planet.

I bring the natural world into the therapy room not to excuse behaviors but to help my clients get curious about how they, and the people in their herd, end up acting the way they do. When people see behaviors as adaptive, rather than dysfunctional, they have a better chance of shifting out of self-blame. They also stop trying to change others. Instead, they get interested in how the patterns play out, and in how they can change their part in the automatic functioning of the group. They start asking themselves, How can I learn to regulate my own anxiety when there’s not another person around to metaphorically lick my head? and How can I avoid letting my fellow cows stir me up so much?

Because that is what makes humans unique: the ability to step outside what’s automatic and activate our own best thinking. The capacity to not always have to go along with the group. To calmly speak into your phone, “Well, Mom, I think about that a little differently.”

Of course, learning to operate differently takes a lot of observation. We can learn something about how to observe our fellow humans (and ourselves) from researchers who study the natural world. Here are some books that I frequently recommend to my therapy clients.

If you’re overwhelmed by conflict and drama in your relationships, there’s no better book than de Waal’s Chimpanzee Politics . After you’ve met a 30-year-old chimp who acts like a child to get sympathy, and a female who tricks two warring males into grooming each other, you’ll never experience Thanksgiving with your family the same way again.

If you’re trying to build community or want to feel more connected to existing friends, Caitlin O’Connell’s Wild Rituals will have you stealing ideas from the elephant families she has studied for decades. When I learned that zebras greet each other with playful nips, it made me consider how my friendships might benefit from an elaborate handshake or a ridiculous curtsy.

I learned about the anxious cow-licking from Ashley Ward’s The Social Lives of Animals , a great read for those who tend to be too hard on themselves and need a comforting laugh. You’ll learn that cockroaches who live isolated childhoods often struggle to find love, and how locusts will chew the ass off the locust in front of them to keep the swarm moving in the same direction. (I’ll let you decide the area of your life in which this metaphor is most useful.)

If you’re lying awake at night worried about the future of America, Thomas Seeley’s Honeybee Democracy will teach you how honeybees wiggle their tiny bee butts to make important decisions about the future of the hive. Who doesn’t love a story with a dance-off?

No book can replace the value of getting out in nature. Even 10 minutes outside can be enough to reduce some stress and improve your mood. Feeling connected to the natural world also keeps us mindful of the global challenges we face and the part we can play in evolving ourselves out of these messes. So get outside. Notice which way a sunflower turns or what starts a squabble among the neighborhood birds. Head to the farm and watch the anxiety ebb and flow.

I like to ask my therapy clients, “What will keep you curious about your own functioning?” Although curiosity isn’t unique to humans, it is certainly our superpower. Getting interested in life in all forms, allowing ourselves to be delighted, inspired, and a little convicted, is a strategy I’d encourage you to try. Chances are you’ll teach your therapist something in return.