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4: Animals

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    Animals, such as lions, that eat many pounds of meat in a single meal can spend fifteen hours a day sleeping. Compare this to plant-eating animals, like giraffes, which sleep an hour or less each day (Figure 4.1). If you were to make a hasty survey, it appears predators sleep more than other animals. However, there are exceptions to that generalization and others like it (such as that smaller animals sleep less than larger ones). There are a multitude of theories about the sleep duration differences between animals, but currently, there is no clear front-runner. Even within groups of animals that are similar genetically, there are sometimes more pronounced sleep duration differences than between vastly dissimilar animals. We do know all creatures studied so far have a period of something similar to sleep. Even unicellular organisms have stretches of time, linked to the earth’s light-dark cycle, when they barely move and have a decreased reaction to stimuli.

    Since it is not possible to hook up a bug to EEG, EOG, and EMG to verify sleep physiologically, for many life-forms, we must rely on this more behavioral definition of sleep—for example, a stereotypical resting posture combined with reduced response to the external environment. And let’s be sure to add that sleep is a reversible state—one of the hallmarks of sleep, thank goodness. In the absence of polysomnography, an additional factor to add to the equation for verifying sleep is to note the strong drive the organism exhibits to return to that “sleeping” state when deprived of it. But how would you know a sleep-deprived insect is trying harder to sleep? Scientists record the baseline level of stimulation required to awaken the sleeping creature when it is left to its normal rhythm for a few days. Then the creature is kept awake during what would be its sleeping time. In this sleep-deprived condition, more intense stimulation is required to rouse the creature from sleep. Imagine your roommate awakening you by softly tapping you on the arm after you’ve had several nights of full and comfortable sleep. Compare that to the jab required if you have at long last dozed off after pulling an all-nighter. This exemplifies one aspect of sleep rebound: sleeping more deeply after being kept awake too long. The other aspect is falling asleep during what are normally waking hours.

    In lieu of physiological measurements to verify that an organism is sleeping, the behavioral definition proves helpful. Upon observation of behaviors, even insects will provide additional clues—beyond just a resting posture—to let us know they are sleeping.

    A giraffe is eating leaves from a tree.
    Figure 4.1 Delicious


    Like humans, fire ants have different stages of sleep. They start with their mouths open and antennae retracted or drooping (Figure 4.2). Then the antennae begin quivering as the fire ant moves into rapid antennal movement (RAM) sleep. Might they be dreaming? Could RAM sleep be an equivalent to REM sleep—even though much research suggests that insects have no equivalent to REM? At this point, it remains an enigma, motivating further investigations.

    A red ant.
    Figure 4.2 Red ant

    Another insect, the fruit fly, often examined when there is a need for deciphering genetics, is a favorite for sleep research. Fruit flies have many genetic and physiological similarities to humans, including getting a buzz from caffeine that keeps drowsy flies awake. As with humans, if fruit flies do not get sufficient sleep they have poor memory, have reduced learning function, and die earlier. They have also been used to demonstrate the reduction in sleep associated with starvation. If a creature is not getting enough calories, the body, much to its detriment, will reduce the time spent sleeping to instead seek food. One of the most compelling sleep-related findings in this research was the discovery of a gene that controls circadian rhythm (see chapter 3). These scientists were awarded the 2017 Nobel Prize in Physiology or Medicine for their work.1


    For ages, scientists believed that sharks do not sleep because their eyes are open when they settle into their quiet resting posture. Upon further investigation, it became apparent that sharks were sleeping, but they do not close their eyelids during sleep (Figure 4.3). Some sharks have a clear membrane covering their eyes, and others have eyelids partially covering them. The purpose of shark eyelids is not sleep related; it is to protect their eyes when fighting or attacking. Great white sharks, which do not have eyelids, have to roll their eyes back to protect their eyes when attacking. This Discovery Channel video captured a great white shark sleeping.2 You may have heard that some sharks need to move while sleeping to get oxygen across their gills, and this is true. But other sharks have spiracles that draw in water and move it over their gills so they do not need to move during sleep.

    A shark laying on the sandy ocean floor.
    Figure 4.3 Shark


    As late as 2016, many believed that reptiles have NREM sleep but not REM sleep. However, when researchers at the Max Planck Institute for Brain Research in Frankfurt began to study the Australian dragon—a type of lizard that is a popular pet in Germany—a surprising story emerged (Figure 4.4). The scientists set out to study visually guided behaviors and were continuously recording the lizards’ brain activity using electrodes. They did this for several days at a time, also using infrared cameras to record nighttime behavior. Although sleep was not the focus of the study, they found the lizards had the typical NREM slow-wave sleep activity—and also REM brain activity, combined with tiny twitches in the eyelids during the REM phases. Part of the significance of this finding is related to how REM sleep is also found in birds and mammals, creatures that evolved separately and much later than reptiles. Until this Australian dragon research, the conventional wisdom was that evolutionary pressure along two isolated evolutionary pathways (those of birds and mammals) resulted in the emergence of REM sleep—people thought REM sleep did not exist before birds and mammals. Now it is clear that REM sleep existed much earlier in the evolutionary line and was likely handed down to both birds and mammals.

    A mature eastern water dragon.
    Figure 4.4 Australian dragon

    REM sleep in other reptiles remains to be examined, but reptilian behavioral sleep patterns have been observed for ages. The beloved honu (green sea turtle of Hawaiʻi) sleeps in the ocean for several hours, usually close to the surface, holding its breath (Figure 4.5). Near the shore, they doze several feet under water, cozy under the edge of a coral reef.

    A sea turtle swimming in the ocean.
    Figure 4.5 Honu

    It can be difficult to determine if snakes are sleeping or simply not moving. Since the thin membrane covering their eyes is clear, they appear to sleep with their eyes open.

    Some geckos do actually sleep with their eyes open, but they constrict their pupils down to protect the retina. Other geckos are fortunate enough to have eyelids they close, just as humans do when sleeping.


    Birds have evolved to sleep with one hemisphere of their brain at a time, keeping an eye on things while they snooze. This leaves the awake half of the brain alert and able to process information coming from its associated eye, which remains open during sleep. Ducks and many other birds use this skill not just for their own self-preservation but also that of their community. Sleeping in a row, sentinels on each end of the row keep one eye open, sleeping with only one hemisphere of their brain. The birds in between the sentinels enjoy a completely restful night of shut-eye, with both hemispheres sleeping at once (Figure 4.6). At either end, each sentinel bird’s open eye faces out, so after a period of sleep in this position, the bird arises and turns around to face the opposite direction, opening the closed eye, and letting the previously active brain hemisphere and eye get some sleep. Even though they are processing information with only half of their brain, it takes less than a fifth of a second for the guard birds to react to a predator. Although REM sleep is normal for birds, it seems both hemispheres must be engaged in sleeping to generate REM. Consequently, these vigilant guardians on the end of the row are only able to get NREM sleep when on duty.

    Ornamental ducks lined up in a row on a log.
    Figure 4.6 Safe sleeping with sentinels

    Humans have a rather subtle version of sleeping with one hemisphere for the sake of vigilance. Have you ever noticed you sleep a little lighter on the first night staying over at a friend or family member’s home? And then if you stay with them for a few more nights, you notice your sleep feels more satisfying. During NREM in new surroundings, half of our brain will have a lighter version of deep sleep; the other half will have its normal restorative depth. This allows us to keep watch, ever so slightly, in our less-than-familiar setting until we have settled in for a few more nights and feel entirely comfortable.

    Birds have another fascinating sleep-related adaptation. Due to their need to migrate thousands of miles over the ocean, they have evolved to safely fly nonstop for hours and hours, seemingly without sleep . . . or are they sleeping while flying? Yes, they are, and it is a unique sleep pattern. Frigate birds (ʻiwa in Hawaiian) fly for months straight and so will sleep about ten seconds at once, in flight, getting less than half an hour of sleep each day.

    Some birds, such as the white-crowned sparrow, have even attracted attention from groups such as the US Department of Defense. This sparrow can stay awake for two weeks at a time during its migratory period and apparently not suffer the usual deleterious consequences of sleep deprivation. During these phases, the bird also remains capable of proficiently responding to stimuli. The US military, with its history of pressuring troops to use various forms of stimulants such as amphetamines (with deadly consequences), is highly motivated to determine a way to keep people such as pilots awake for long stretches at a time without not compromising their judgment or damaging their health. At this point, the science suggests it is not possible. Let’s hope for the sake of the troops, their families, and the world community that there are also researchers actively investigating revolutionary and innovative solutions to minimize the need to put people in harm’s way.

    A frigate bird flying in the sky.
    Figure 4.7 Frigate bird


    Imagine swimming through the ocean with half of your brain asleep, or catching z’s while dangling like a ripe mango from a tree (Figures 4.8 and 4.9). The unihemispheric sleep of dolphins allows them to swim and communicate—during sleep—with other dolphins. Up in the trees, bats, with their unique wing structure, are unable to create the rapid vertical takeoff mastered by birds. The best way for a sleeping bat to escape a hungry raccoon lumbering toward its roost is for the bat to drop from the tree and take flight midair. In the sea, dozens of sea otters come together and wrap themselves in seaweed, creating a sea otter raft for safety in numbers and to keep from drifting away while snoozing. Occasionally, they even hold hands (Figure 4.10). In open grassy areas, cows, horses, zebras, and elephants can sleep standing up, able to quickly flee if attacked. They have a “stay apparatus” that allows them to essentially lock their legs, minimizing muscular effort to remain standing. At times, these big mammals also lie down to sleep in order to complete their sleep architecture.

    A pod of dolphins in the ocean.
    Figure 4.8 Dolphins
    A bat sleeping upside down.
    Figure 4.9 Bat
    Sea otters floating on their backs in the ocean wrapped in seaweed.
    Figure 4.10 Sea otters

    Noticing the range of adaptations and behaviors that make it possible for animals to sleep and survive, we see that sleep has persisted even in the face of environments where it seems it would have been simpler to just eliminate it from the mix. As Alan Rechtschaffen, a sleep science trailblazer, has said, “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.”3 Then how can we resist the question, “Why is sleep so vital?” Let’s further investigate animal sleep and get some answers.

    Although they live and sleep in the water, whales, seals, and dolphins are mammals and so must breathe in the air. If they fell fully asleep underwater, they would drown, so like some birds, only half of their brain sleeps at a time. When one hemisphere is sleeping, the other hemisphere guides the animal to the surface and activates the body to take a breath. The visual system of the awake hemisphere is vigilant for danger and stays connected to other animals in its group, such as companions or offspring. These elaborate evolutionary adaptations suggest that sleep must provide a crucial function, since sleep does not make it to the bargaining table when evolutionary pressure looks for behaviors to remove. At first blush, it seems it would be easier to evolve to not sleep than to evolve the mechanisms necessary to sleep while swimming. In other words, sleep is indispensable!

    Getting back to unihemispheric sleep in aquatic mammals, there are exceptions: seals have bihemispheric sleep underwater (they hold their breath) and sperm whales sleep with both hemispheres too, seemingly dangling in the water, tail down, until they awaken to swim to the surface to take a breath (Figure 4.11).

    Sperm whales hang vertically while sleeping in the ocean.
    Figure 4.11 Sleeping sperm whales

    Dolphins and some whales do not show obvious signs of REM sleep, but scientists speculate that they may experience transient REM sleep or REM brain activity in deeper structures than the cortex. This motivation to not rule out dolphin and whale REM sleep is partially due to observed muscle twitching, penile erections, and eyelid movements during dolphin sleep. These behaviors are associated with REM sleep in land mammals but also occur during waking states, so REM sleep in dolphins remains an area of active research. Dolphins are so highly evolved that maybe we will see they have a unique form for REM that provides additional survival benefits beyond those given to us humans during REM sleep.

    Fur seals do clearly have REM sleep, but they add a unique variation to its predictability. On land, the fur seal sleeps with both hemispheres at once and goes through REM and NREM stages, similar to most mammals (Figure 4.12). However, when a fur seal sleeps in the water, its sleep is similar to a dolphin’s: it sleeps with one hemisphere at a time, and NREM is the only obvious sleep stage. Because fur seals spend weeks at a time in the sea, they go for long stretches without REM. But why would a seal have two different patterns of sleep, depending on whether or not it was sleeping in the water?

    A Hawaiian monk seal sleeping in the water.
    Figure 4.12 Hawaiian monk seal

    A current theory about REM is it increases brain metabolism and warms the brain and brainstem, balancing out the lower metabolic rate and brain temperature of NREM. When fur seals, dolphins, and whales sleep in the water, one hemisphere at a time, and exclusively in NREM, the theory is they would not need REM to warm up the brain, since half of the brain is always awake and warm. Then when the fur seal is sleeping on land, it reverts to the typical land mammal pattern of bilateral NREM interspersed with REM. We humans feel much more alert if we wake up shortly after or even during an REM period, as opposed to when our alarms go off in the middle of deep NREM sleep, when our brains are cool and sluggish.

    If REM sleep did not provide an essential benefit, then it seems the fur seal would continue its unihemispheric NREM sleep when snoozing on land. However, it has evolved to incorporate REM sleep whenever it returns to its terrestrial home. With the myriad REM sleep–associated benefits—including emotional healing, cardiovascular system regulation, and more—it is tempting to believe REM sleep would be incorporated into a creature’s sleep cycle if at all possible.

    Let’s look way back in time to compare variations in mammalian sleep. During the early stages of mammalian evolution, monotremes branched off from placentals and marsupials. Monotremes (e.g., platypuses) are egg-laying mammals. This is in contrast to placentals (e.g., humans), which carry the fetus in the uterus until a relatively late developmental stage, and marsupials (e.g., kangaroos), which give birth before the animal is completely developed, so after birth, it is usually carried in a pouch on the mother’s body (Figure 4.13). Although for decades, scientists believed monotremes do not experience REM sleep, there are now studies showing that platypuses not only have REM sleep but have a higher rate of it than placentals or marsupials.5 During REM, the platypus has rapid eye movements and twitches its bill. Its REM EEG is similar in many ways to newborn placental mammals, which have high rates of REM too. The brainstem EEG of a platypus shows that REM occurs at the same time as cerebrocortical slow-wave sleep, explaining why early investigators may have miscategorized its sleep pattern.

    A monotreme platypus, a marsupial kangaroo and a placental pregnant woman.
    Figure 4.13 Monotreme, placental, and marsupial mammals


    A common myth is that hibernation, which can last a few hours or as long as several months, is the same as sleep. However, although hibernation has evolved from sleep, there are fundamental differences between the two. In fact, some animals will bring themselves out of deep hibernation in order to get sleep and then return back to hibernation after a satisfying snooze. Also, sleep is easily and rapidly awoken from, but it takes an hour (or more, depending on the animal) to rouse from hibernation. And what about the function of hibernation compared to sleep? A fundamental purpose of hibernation is to save energy. If sleep and hibernation had energy conservation as a common goal, it would not make sense to expend energy to warm up the body during hibernation in order to create conditions necessary for true sleep. Yet some animals, even in frosty conditions, do exactly that.

    Ground squirrel body temperature can remain close to zero degrees Celsius, the temperature at which water freezes, during hibernation, which could last the entire winter (Figure 4.14). However, once a week, for around twenty-four hours, they bring themselves out of hibernation. It takes a while, and a lot of energy, for them to speed up their metabolism and activate their brain. When they are hibernating, their EEG is practically a flat line, as though they were not alive. Remember learning about dendrites in chapter 2? During the first day of hibernation, ground squirrels lose about one-fourth of their dendrites. Then, within hours of coming out of hibernation, the dendrites are restored. Other physiological functions, such as urine production, come to almost a halt during hibernation as well. But after rousing from their week of hibernation, during that twenty-four-hour period at their normal body temperature, they eat, pass waste, and sleep before returning to another week of hibernation.

    Belding’s ground squirrel.
    Figure 4.14 Ground squirrel

    If I say “hibernating animal,” what creature comes to your mind? If it is a bear, you are in good company, as this is the typical response (Figure 4.15). You may find it surprising that some scientists argue that bears are not true hibernators; others suggest that theirs is just a different form of hibernation. A bear’s body temperature will drop only a few degrees, even when outside temperatures are below freezing. This closer-to-normal body temperature allows bears to generate NREM and REM sleep during hibernation. They also stay in their state of torpor for the entire winter, not bothering to invest energy in the weekly rousing practiced by the ground squirrel. Lastly—and this is a significant difference from the ground squirrel, as intrepid hikers will tell you—a hibernating bear can be roused quickly and easily.

    A bear sleeping in a den.
    Figure 4.15 Bear

    1 “The 2017 Nobel Prize in Physiology or Medicine—Press Release,” Nobel Prize, accessed May 28, 2021,

    2 Discovery, “Great White Naps for First Time on Camera,” YouTube video, 2:33, June 28, 2016,

    3 E. Mignot, “Why We Sleep: The Temporal Organization of Recovery,” PLoS Biology 6, no. 4 (April 2008),

    4 Ocean Conservation Research: Sound Science Serving the Sea, last modified May 2021,

    5 J. M. Siegel et al., “Monotremes and the Evolution of Rapid Eye Movement Sleep,” Philosophical Transactions of the Royal Society B: Biological Sciences 353, no. 1372 (July 1998): 1147–57,

    This page titled 4: Animals is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Sheryl Shook via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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