Sleep in the Animal Kingdom: How Do Animals Sleep?

The phenomenon of sleep is not confined to humans. All mammals and birds, as well as in most (but not all) reptiles, amphibians, fish and even insects regularly undergo something which may be described as sleep, although sometimes this may be a profoundly different phenomenon from our usual conception of it. Some of these differences, and the sheer variability in the sleep requirements of different animals, help to provide a different perspective on what sleep is for, and indeed on the very definition of what sleep is.

Even within the human species, sleep may be regarded very differently and be subject to very different cultural, traditional, and environmental pressures. Until recent years, relatively little attention has been paid to the anthropology of sleep. But some recent research is beginning to rectify this lack, and throw some light on the similarities and contrasts in sleep practices throughout the world.

Do All Animals Sleep?

It is generally thought that sleep in some form exists across the animal kingdom, although not all scientists agree (this partly depends on the definition of sleep used). It is certainly true that sleep patterns, habits, postures, and locations vary greatly among different species.

What we think of as sleep can be observed in all mammals and birds, as well as in most (but not all) reptiles, amphibians, fish and even insects. Even such simple animals as nematodes (roundworms) and fruit flies exhibit regular sleep-like behaviors, or at least 24-hour cycles of activity and rest driven by some form of circadian timer. 

Some species of fish definitely seem to sleep regularly, while others (such as those that tend to swim in large shoals, for example, or those that for physiological reasons must swim continuously) appear not to sleep at all.

However, it can be quite difficult to detect or identify the state of sleep in some animals, particularly those with less complex brains, especially as EEG patterns of brain waves are often not comparable to those of vertebrates. Sometimes we must rely on more behavioral indicators such as minimal movement, postures typical for the species, reduced responsiveness to external stimulation, etc.

Another important criterion in the definition of sleep in lower animals is a tendency towards rebound sleep, or compensation after periods of sleep deprivation, which has been observed in all mammals and birds, and even in invertebrates.

There is even evidence of sleep disorders, broadly similar to those experienced by humans, in other animals and household pets, including insomnia, periodic limb movement disorder, narcolepsy, night terrors, bed-wetting, nocturnal seizures, etc.

Do any animals not sleep?

Almost all animals show some evidence of sleep regulation according to circadian rhythms (our body’s 24-hour clock triggered by environmental stimulus, such as sunlight) and sleep-wake homeostasis (our body’s way of telling us it’s time to sleep or stay awake) in much the same way as in humans.

Interestingly, in birds, reptiles, amphibians and fish, light can usually penetrate to some degree right through the skull into the brain to trigger these circadian rhythms, in addition to relying on photoreceptor cells in the retina like mammals.

Do animals dream? 

Warm-blooded (or endothermic) animals, like mammals and birds, which expend energy to maintain their internal body temperature, are the only ones that have been proven to exhibit distinct REM (rapid eye movement) and non-REM sleep stages. REM sleep — the stage associated with dreaming in humans — has never been observed in reptiles, for example, and from this it is thought that REM sleep may be a later evolutionary development related to warm-bloodedness.

Most mammals and birds go through cycles of non-REM and REM sleep in much the same way as humans, although the cycles of birds tend to be much shorter (for instance, non-REM sleep may be of the order of two to three minutes in each cycle, and REM sleep may only last for nine or ten seconds each cycle).

The proportion of REM sleep varies considerably between different animals. Interestingly, the platypus, a very ancient offshoot of the mammal family tree, spends almost 60% of its sleep time in REM sleep (as compared to around 20%-25% in humans). The young of most mammals tend to sleep for long periods under the watchful eye of a parent, and the duration of REM sleep in young mammals is usually substantially longer than in older animals of the species, as it is in humans. 

However, some marine mammals like dolphins and killer whales hardly sleep at all in their early weeks of life, and experience almost no REM sleep. Interestingly, those seals and whales that sleep underwater appear to experience no REM sleep, and do not seem to suffer for that in any way; other seals, that sleep sometimes in the water and sometimes on land, only appear to receive REM sleep during times when they are sleeping on land.

How Much Do Animals Sleep?

A few rules of thumb may be identified, most of them intuitive. Animals that tend to sleep in relatively safe locations (e.g. underground burrows, rather than out in the open) tend to sleep longer. Typically, herbivores sleep less than carnivores, partly because of the need for long hours foraging and eating.

On the other hand, top-of-the-food-chain animals like lions and tigers, which have little fear of predators and often consume huge meals at one sitting, can afford to spend much of their day sleeping. 

Generally speaking, those species which have greater total sleep times tend to also have higher core body temperatures and higher metabolic rates. Also, as a general rule (at least among herbivores), larger animals sleepless than smaller ones, although their individual sleep cycles tend to be of longer duration.

Animal	Average Daily Sleep Period
Giraffe	0.5/1.9/4.6 hours (varied reports)
Horse	2.9 hours
Donkey	3.1 hours
Elephant	3.5 hours
Sheep	3.8 hours
Cow	4 hours
Goat	5.3 hours
Gray Seal	6.2 hours
Pig	7.8 hours
Human	8 hours
Rabbit	8.4 hours
Chimpanzee	9.7 hours
Red Fox	9.8 hours
Dog	10.1 hours
Baboon	10.3 hours
Bottle-nosed Dolphin	10.4 hours
Duck	10.8 hours
Rhesus Monkey	11.8 hours
House Mouse	12.5 hours
Cat	12.5 hours
Rat	12.6 hours
Lion	13.5 hours
Platypus	14 hours
Squirrel	14.9 hours
Chipmunk	15 hours
Tiger	15.8 hours
Tree Shrew	15.8 hours
Sloth	14.4/16 hours
Python	18 hours
Opossum	18 hours
Giant Armadillo	18.1 hours
Little Brown Bat	19.9 hours

When Do Animals Sleep?

Specific species of animals fall into various time partitioning behaviors that dictate when they sleep: 

Nocturnal: Characterized by being active during the night and sleeping during the day. Nocturnal animals tend to have highly developed sense of hearing and smell, as well as specifically adapted eyesight. Interestingly, the circadian clock mechanism of nocturnal animals is identical to that of diurnal animals, but there is a kind of switch, outside the circadian clock itself, which changes the way nocturnal animals utilize the clock’s signals. 

Examples: Bats, opossums, armadillos, raccoons, rats, wolves, and badgers.

Diurnal: Characterized by activity during the daytime with a period of sleeping or inactivity at night. Examples:Humans, dogs, birds, bats, lizards, honeybees, and squirrels.

Crepuscular: These animals are active primarily during twilight hours of dusk or dawn. However, some crepuscular animals may also be active during bright moonlit nights or during an overcast day. Examples include deer, moose, ocelots and skunks.

Examples: Deer, moose, ocelots and skunks.

• Matutinal: Animals active only before sunrise.
• Vespertine: Animals only active during sunset. 

Cathemeral or Metaturnal: Refers to animals that are neither fully nocturnal, diurnal, nor crepuscular, but rather have sporadic, irregular and random intervals of activity during the night or day. Metaturnal animals have evolved to sleep partly during the day and partly at night.

Examples: Cats, lions, many primates, rabbits, and mice.

How Mammals Sleep

In practice, the vast majority of mammals have adapted to polyphasic sleep (multiple sleep periods within any 24-hour period), with humans among the 15% or so of naturally monophasic sleepers (sleeping just one long period each day).

Humans, of course, generally sleep lying down. Monkeys, however, sleep sitting upright, a defense mechanism to help protect against predators. Larger primates, like apes, orangutans and gorillas, however, tend to sleep lying down on nesting platforms in trees. Not only do the platforms keep them safe from predators, it allows them enough security that they can sleep deeper and benefit from more restorative REM sleep. 

How Reptiles & Amphibians Sleep

You may not be able to tell a reptile is sleeping by just looking at it. Lizards and snakes, for example, may not actually close their eyes when it’s time for slumbering. But researchers have found that they do experience sleep cycles, though they last only about a minute and a half compared to about an hour and a half for humans. They also go through 350 of these cycles per night as opposed to just four or five for humans. 

A study published in Science magazine identified typical features of slow-wave sleep and REM sleep on EEG scans of the brains of Australian dragons, indicating that the brainstem circuits responsible for slow-wave and REMS sleep have been part of sleep dynamics in reptiles for hundreds of millions of years. But since reptiles don’t have cerebrums, the slow wave brain activity occurs in the anterior dorsal ventricular ridge in Australian dragons. 

Amphibians (salamanders, toads, and frogs) have periods where they are inactive, but are still able to react quickly to potentially threatening stimuli in this state. 

How Marine Animals Sleep

Ever wonder how whales, porpoises, and dolphins swim, especially since they need to come to the surface from time to time to breathe? Mother Nature has equipped these animals with unihemispheric sleep, the ability to sleep in one hemisphere of the brain while the other hemisphere is awake. These animals still move, see through one eye, and breathe while they are sleeping. 

Sometimes dolphins sleep floating on the top of the water. Other times they may sleep swimming in a circle. Sperm whales, on the other hand, do not sleep uni hemispherically. Instead, they sleep in an upright position.

How Birds Sleep

Some birds (including chickens, blackbirds, mallards, some sparrows) and aquatic mammals (including some dolphins, whales, sea-lions and seals) can even sleep with just one half of their brain, while the other half (complete with one open eye) remains alert for predators, a phenomenon known as unihemispheric sleep. 

Only non-REM sleep occurs uni hemispherically, and for this reason it is sometimes referred to as unihemispheric slow-wave sleep. This can be a great advantage in areas of high predation, for birds that spend long periods in migration, and for marine mammals that need to surface regularly to breathe.

Also, birds do not tend to lose muscle tone and go limp during sleep like most mammals, which allows them to sleep while standing or perched in a tree. Many birds can function quite well on hundreds of “microsleeps” of just a few seconds each, and their sleep times can be reduced by as much as 70% during periods of migration with little or no ill effects and no apparent rebound sleep.

How Fish Sleep

Most sharks must keep water moving through their gills. Thus, they don’t actually enter REM sleep or even close their eyes. But they do have active and restful periods. Nurse sharks, however, have spiracles that force water across their gills which allows them to rest stationary.

How Insects Sleep

The activity and passivity of insects appears to be governed by circadian rhythms but for the most part they don’t seem to have a homeostasis sleep need nor do they appear to experience REM sleep. 

But this doesn’t apply to all insects. In fact, one study found that fruit flies appear to sleep and when deprived of sleep, tend to suffer cognitive disabilities. Cockroaches also go through a period of rest during which they fold down their antennae and become less responsive to stimuli. And it is believed that honeybees experience REM sleep and dream.

How Does Hibernation Work? 

Hibernation (or the hot/dry weather equivalent, estivation), although superficially similar to sleep, is actually a quite different phenomenon. Hibernation is a long-term state of inactivity in which an animal’s body temperature and metabolism is depressed, and food and energy are conserved.

Animals hibernate to conserve food when food options are limited. However, it differs fundamentally from sleep. Unlike hibernation, sleep must be more short-term, repeated in 24-hour cycles and easily reversible (i.e. the animal must be able to wake up in response to stimuli). 

The brain waves of animals in hibernation closely resemble their wakeful brain wave patterns, although somewhat suppressed, whereas sleep is primarily characterized by changes and cycles in brain activity. Although hibernation may reduce the need for sleep to some limited extent, it does not in any way replace or remove it, and animals newly emerged from hibernation still need the more functional and restorative benefits of sleep. Some animals may even interrupt their hibernation once or twice during the winter in order to sleep.

Animals that hibernate include bears and bats as well as skunks, bees, snakes, chipmunks, and groundhogs. Like hibernation, torpor is a survival tactic of some animals. But instead of enabling them to conserve food when options are limited, animals go into torpor to survive harsh winter weather. During this period, the animal appears dead though they are very much alive.

However, their body temperature, breathing rate, heart rate, and metabolism drop. While inactive, they are able to enter a deeper sleep that allows them to conserve energy to survive the winter. Animals that go into torpor sleep include birds, some marsupial species, mice, and bats.

Summary

Many studies of the sleep patterns of animals have been carried out in an attempt to try and understand in what way the all-too-apparent costs and dangers of sleep (in evolutionary and survival terms) are outweighed by other adaptive values. 

Care should be taken with these measurements, however, because the majority are based on animals kept in captivity (and therefore subject to very different, and typically fewer, environmental pressures), and it seems likely that this would substantially overstate sleep times. For example, one specific study has shown that brown-throated three-toed sloths sleep about 70% of the day in captivity, but only about 40% in the wild.  

As an aside, it is notoriously difficult to estimate the natural sleep patterns of humans “in the wild” and, although experiments and studies have been tried, the results are inconclusive and debatable.