Sleep - What Sleep Is, How Sleep Works, Why Do We Sleep, How Sleep Can Go Wrong


Sleep in some form exists across the animal kingdom, although sleep patterns, habits, postures, and locations vary greatly
Sleep in some form exists across the animal kingdom, although sleep patterns, habits, postures, and locations vary greatly
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 (round worms) and fruit flies exhibit regular sleep-like behaviours, 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 behavioural 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 be observed in all mammals and birds, and even in invertebrates.

Almost all animals show some evidence of sleep regulation according to circadian rhythms and sleep-wake homeostasis in much the same way as in humans (see the section on How Sleep Works). 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 photoreceptive cells in the retina like mammals. There is even evidence of sleep disorders, broadly similar to those experienced by humans, in other animals and household pets, including insomnia, periodic limb moment disorder, narcolepsy, night terrors, bed-wetting, nocturnal seizures, etc.

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.

Average Daily Sleep Period

0.5/1.9/4.6 hours (varied reports)
2.9 hours
3.1 hours
3.5 hours
3.8 hours
4 hours
5.3 hours
Gray Seal
6.2 hours
7.8 hours
8 hours
8.4 hours
9.7 hours
Red Fox
9.8 hours
10.1 hours
10.3 hours
Bottle-nosed Dolphin
10.4 hours
10.8 hours
Rhesus Monkey
11.8 hours
House Mouse
12.5 hours
12.5 hours
12.6 hours
13.5 hours
14 hours
14.9 hours
15 hours
15.8 hours
Tree Shrew
15.8 hours
14.4/16 hours
18 hours
18 hours
Giant Armadillo
18.1 hours
Little Brown Bat
19.9 hours

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 sleep less than smaller ones, although their individual sleep cycles tend to be of longer duration.

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 and non-REM sleep stages. REM sleep 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). 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.

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.

Some birds and aquatic mammals, like the bottlenose dolphin, have evolved to be able to sleep with one side of their brain only (unihemispheric sleep)
Some birds and aquatic mammals, like the bottlenose dolphin, have evolved to be able to sleep with one side of their brain only (unihemispheric 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 unihemispherically, 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.

A wide variety of postures may be adopted for sleeping, ranging from curled up position of cats, dogs and many other mammals, to the leopard’s nonchalant straddling of a tree branch, to the upside-down hanging technique of bats, to sleeping while swimming or floating. The flamingo even manages to sleep while balancing on one leg. Most land animals lie down to sleep, as their muscles go limp, although horses and some other herbivorous ungulates can lock their legs and sleep standing up, apart from the short periods they require for REM sleep. A huge variety of different locations are also employed for sleep, such as in nests or underground burrows, in open spaces, in the relative safety of trees, in the water, etc, etc.

While many animals are diurnal (active in the daytime) like humans, nocturnal animals, including bats, opossums, armadillos, raccoons, rats, wolves, badgers, etc, have adapted their sleep schedules to sleep primarily at night. 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. Metaturnal animals, like cats, rabbits and mice, have evolved to sleep partly during the day and partly at night. Crepuscular animals, such as deer, moose, ocelots and skunks, are active mainly during the twilight hours of dawn and dusk. Cathemeral animals, including lions and several primates, are characterized by sporadic, irregular and random intervals of activity during the night or day, an activity pattern that is neither diurnal, nocturnal nor crepuscular. 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).

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 is conserved. However, it differs fundamentally from sleep, which 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.