MadSci Network: Zoology

Re: Are there animals who don't sleep or that sleep very little?

Date: Tue Aug 1 16:27:06 2000
Posted By: Juliette Faraco, Post-doc/Fellow, Psychiatry/Genetics, Stanford Center for Narcolpsy
Area of science: Zoology
ID: 964164818.Zo
The question of the process and function of sleep throught the animal kingdom is truly fascinating. Sleep occurs in animals as different as humans and flies. It's obviously important: our bodies keep track of lost minutes of sleep and then try to make them up. The need for sleep can become irresistable even in the face of death, with many car accidents each year due to drowsy drivers. When completely deprived of sleep for too long, we sicken and die. We still don't fully understand why sleep is so important - but we do know that it is a very active state. Contrary to previous assumptions, the brain is not resting- some nerve cells in our brains fire 5 to 10 times more frequently during certain sleep stages than during wakefulness.

To learn more about sleep, I highly recommend the "Sleep Syllabus" at
. Much of what I've written here is taken from that source (direct quotes interspersed with my own perspectives).

As you probably suspect, most of what we know about sleep comes from studying mammals- but sleep probably is common to most of the animal kingdom. It sort of depends on how you define sleep. In the most broad terms we tend to say that sleep must have these four behavioral characteristics 1) minimal movement; 2) a typical sleep posture (e.g., for humans, lying down; for bats, hanging upside down); 3) reduced responsiveness to external stimulation (moderate noises don't awaken you); and 4) quick reversibility of reduced responsiveness to relatively intense stimulation (distinguishing sleep from other states like death or coma). Certainly these have been observed in insects, frogs, reptiles, birds and of course mammals like us. When studying mammals or birds, we can more specifically define sleep and subdivide it into various classes by looking at brainwave patterns (EEG) and muscle activity patterns (EMG, EOG). These stages are called stage I-IV sleep and REM (rapid eye movement) sleep. Normally, animals progress through the stages of sleep in an orderly pattern of alternating REM and non-REM sleep. When you look at sleep according to these brainwave patterns instead of just the observed behaviors, things get pretty interesting! Some animals can sleep with eyes open, while moving, or with only half the brain at a time. Below, I've listed some of the wide variations in the style (posture) and the amount of total sleep, and the amount of REM sleep found among different animals.


Mammal Total Daily Sleep Time (in hours)
Giraffe 1.9
Roe deer 3.09
Asiatic elephant 3.1
Pilot whale 5.3
Human 8.0
Baboon 9.4
Domestic cat 12.5
Laboratory rat 13.0
Lion 13.5
Bats 19.9

One theory proposes that how much an animal sleeps is largely determined by its status as prey or predator-that prey animals sleep less because they might get attacked. But it's not clear that sleep increases vulnerability to predation, since the victims of predators are generally the very young, the sick, and the old, even when they are wide awake. A contrasting theory suggests that a major function of sleep is to protect animals from predation-to keep them out of harm's way when they have satisfied their need for food etc. However, this doesn't explain the unrelenting need to sleep even upon risk of death, or the pressure to retrieve "lost" sleep.

There seems to be a correlation between body size and the amount of sleep: small mammals tend to sleep more than large ones. But there are exceptions; some large animals (e.g., lions and tigers) sleep 14-16 hours per day, and
dogs and cats sleep 10-12 hours, while some small animals (e.g., moles) sleep only 8 hours. The reason for the relationship between sleep have something to do with energy conservation, which is a bigger problem for
small mammals (which carry little fat) than for larger ones. Sleep might help conserve energy, especially in smaller mammals which are in almost continuous danger of depleting their energy resources, by providing long
periods of lowered metabolic activity to save energy. However, the metabolic rate during sleep is only about 10% lower than during quiet wakefulness, so one might wonder why mammals don't simply rest, rather than sleep- there is clearly more to sleep than energy maintenance and avoiding preditors.


Humans Sleep in night, wake in the day (diurnal)
Mice Sleep in day, wake at night (noctournal)
Some rodents and insects Sleep in night and day, wake at dawn and dusk (crepuscular)
Moles/rabbits Sleep in burrows
Zebras Sleep in the open
Cows Sleep with eyes open
Horses Sleep standing
Leopards Sleep on tree limb
Bats Sleep upside down
Seals/ Hippos Sleep underwater (part of the time)
Dolphins/porpoises Sleep with one half of the brain at a time to allow breathing while swimming/sleeping
Ducks/pidgeons Sleep with one half of the brain while keeping one eye on predators
Migratory birds Sleep while flying across the ocean
Fruit flies/some fish Sleep-like rest periods with decreased movement, and altered threshold for stimuli.


We're familiar with the different states of being awake like running, eating, quietly resting, daydreaming, studying. In sleep, there are also different states. Different types of brainwave activity are characteristic of the different stages of sleep. REM sleep is a sleep state where dreaming is most common, the eyes move wildly, and the body is totally limp (loss of muscle "tone"). Not only do all mammals sleep, but as a rule, they experience cyclical alternation between non-REM and REM sleep. An outstanding exception to this pattern is the echidna (spiny anteater-a small
Australian monotreme), which is a mammal that bears its young from eggs, as birds and reptiles do. Echidnas have no REM sleep, only non-REM sleep. This is amazing since other mammals will die when totally deprived of just the
REM type of sleep. Like mammals, birds have cycles of non-REM and REM sleep, but with some differences. One of the most striking differences is that both non-REM and REM sleep episodes are quite short in birds; their non-REMsleep episodes average only about 2 1/2 minutes, and REM sleep episodes only9 seconds. In contrast, non-REM and REM episodes in humans are much longer. At the start of the night, we have non-rem sleep for about 90 minutes followed by 8-10 minutes of REM. The REM episodes get longer through the night and by early morning, the non-REM episodes are about 10 minutes followed by 90 minutes of REM. Another difference is that most birds do not "go limp"(lose muscle tone) during REM sleep as mammals do, which is understandable, since many birds sleep while standing or perching. Although birds are thought to have evolved from reptiles, REM sleep has not been
detected in reptiles and lower animals. This may be due to the fact that their brain structures are simpler. However we do know that a new neurotransmitter "HCRT" (hypocretin/ orexin) is important to sleep state control. Deficiency of HCRT causes the sleep disorder Narcolepsy in humans, and seems to particularly impact REM sleep. We know now that this neurotransmitter is present in animals as distant as frogs, and fish so it must also be in reptiles. Therefore, there are a couple of possibilities: perhaps REM sleep evolved independantly in birds and mammals, to fulfill a function essential to more complex brains. Or perhaps part of the basis of REM sleep arose early in evolution, but we only recognize it in higher animals. The field of hypocretin/orexin research is moving really fast right now. We hope that understaning the molecules involved will help us to undertand the function and process of sleep in animals as well as humans.

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