t should surprise no one to learn that sleep is somewhat different in different animals-sleep varies phylogenetically. Indeed, it would be truly amazing if all animals slept the same, because animals differ so drastically in their anatomy, physiology, environment, and modes of adaptation to their environment. Variations in the ways that animals sleep are intrinsically interesting from a naturalistic point of view. Also, correlating sleep with other characteristics of a species can provide clues to the adaptive functions of sleep.

Although examinations of variations in sleep cannot by themselves yield a definitive answer to the question of "why we sleep," such studies have provided a substantial bank of data and some promising theories. It is also important to note that the importance of sleep in the animal kingdom is underscored by an amazing number of accommodations and circumstances during which it occurs.

As might be expected, the most thoroughly studied animals have been mammals. The sleep of over 90 species of mammals has been studied by the careful observation of their behavior and/or by monitoring the electrical changes in their brain activity. The following table lists just a few of the many species whose sleep time has been documented.

MammalTotal Daily Sleep Time (in hours)
Roe deer3.09
Asiatic elephant 3.1
Pilot whale 5.3
Baboon 9.4
Domestic cat 12.5
Laboratory rat raffe13.0
Lion 13.5
Eastern chipmunk 15.8
Little brown bat 19.9

Sleep habits, sleep places, and sleep postures vary greatly. Some mammals (e.g., moles and rabbits) sleep in burrows whereas others (e.g., zebras) sleep in the open. Some mammals (e.g., cattle) can sleep with their eyes open. Others (e.g., seals and hippopotami) spend part of their sleep under water. Some mammals (e.g., gorillas) settle into nests to sleep, whereas others (e.g., dolphins and porpoises) sleep while swimming. Furthermore, dolphins and porpoises can sleep with one half of their brain at a time, while the other half is awake so as to permit these air-breathing mammals to come to the surface to breathe periodically (Figure 1).


EEG patterns in right (R) and left (L) brain hemispheres in a porpoise. A: bilateral desynchronization. B: bilateral intermediate synchronization. C, D: unilateral delta waves.

The fox sleeps curled up; the leopard straddles a tree limb; and the bat sleeps hanging upside down. Under certain conditions, a select group of mammals have even been observed to be able to sleep in the classroom habitat. Some mammals sleep primarily at night (e.g., humans), while others sleep mostly during the day (e.g., rats). Horses spend the major portion of every 24-hour period standing, part of the time in NREM, by utilizing passive locking mechanisms in their limbs. In spite of such variations, all mammals sleep. The universality of sleep in mammals, despite all these variations, is evidence that sleep is indispensable.

Not only do all mammals sleep, but as a rule, they experience cyclical alternation between NREM 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 NREM sleep. There is as yet no good explanation for this. The echidna's egg-laying or its low body temperature may have something to do with its lack of REM, but this seems unlikely since birds have REM, and so do other mammals with low body temperatures.


Sleep in Birds

Like mammals, birds have cycles of NREM and REM sleep, but with some differences. One of the most striking differences is that both NREM and REM sleep episodes are quite short in birds; their NREM sleep episodes average only about 2 1/2 minutes, and REM sleep episodes only 9 seconds. Also, most birds do not lose muscle tone during REM sleep as consistently as mammals do, which is understandable, since many birds sleep while standing or perching. Some birds, such as pheasants and partridges, sleep on the ground. Water fowl can sleep while swimming. Some parrots sleep hanging down. It is suspected that some birds sleep aloft during long transoceanic flights. Once again, the importance of sleep in the animal kingdom is underscored by the amazing number of accommodations and circumstances during which it occurs.

Sleep in Reptiles, Amphibians and Fish

Studying sleep in reptiles, amphibians and fish or in invertebrates (e.g., worms and insects) brings us face to face with the problem of how we define sleep. Sleep's presence has traditionally been judged by behavioral criteria, as noted in the previous section, by: 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; and 4) quick reversibility of reduced responsiveness with relatively intense stimulation (which distinguishes sleep from other states like death, anesthesia, and coma). On the basis of these criteria, we can conclude that "sleep" is present throughout the animal kingdom. It has been identified in alligators, turtles, lizards, frogs, salamanders, bees, wasps, flies, dragon flies, grasshoppers, butterflies, scorpions, the primitive invertebrate sea hare and even, according to one unsubstantiated report, in adolescent turtles of the mutant ninja variety.

We cannot yet conclude, however, that sleep in all these animals reflects the same physiological processes and fulfills the same functional goals as the sleep of mammals and birds. Because mammals and birds have similar EEG, EOG and EMG activity when they sleep, we have some confidence that their similar sleep behaviors are produced by similar physiological processes. Reptiles show subcortical electrical activity indicative of NREM sleep. However, animals with more primitive nervous systems do not have the brain structures which can generate the same electrophysiological patterns of brain activity that we use to define sleep in mammals, so the analogy to mammalian and avian sleep is uncertain and we do not know if they "sleep."

A particularly fascinating problem is posed by the failure of most studies to reveal REM sleep in reptiles. Birds, which clearly have REM sleep, are believed to have evolved from reptiles. Therefore, one important implication is that REM sleep may have evolved independently in birds and mammals to satisfy some function common to both, but not to reptiles. Birds and mammals regulate their body temperatures differently than reptiles. Additionally, birds and mammals can increase their metabolic rates to maintain a constant body temperature in the cold, but reptiles cannot. Therefore, some sleep researchers have speculated that REM sleep is related in some way to temperature regulation. In addition, because we are not certain what the function of sleep is in mammals, birds, reptiles and other species, we cannot be sure that sleep serves the same function in all animals. Perhaps this is why there are so many different theories of sleep (see Part IX, Functions of Sleep).

Sleep and Predator-Prey Status

One prominent theory proposes that how much an animal sleeps is largely determined by its status as prey or predator-that prey animals sleep less because sleep makes them vulnerable. This theory is difficult to evaluate because predator-prey status is not generally as obvious as in the case of the lion and zebra, or the wolf and caribou. Some species are both predator and prey, and judgments of their vulnerability are often subjective. Also, it is far from clear that sleep significantly increases vulnerability to predation. The victims of predators are generally the very young, the sick, and the old, even when they are wide awake. In fact, another 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, procreation, etc. This is consistent with the fact that many prey animals (i.e., rodents) sleep 12 or more hours per day, which is as much or more than many predators. This theory, however, fails to explain both the unrelenting need to sleep when we have many "important" things to do and the even greater pressure to retrieve "lost" sleep.

Sleep, Body Size and Energy Expenditure

There is a well-established, but modest relationship in mammals between daily sleep and body size: small mammals tend to sleep more than large ones. For example, bats sleep about 20 hours a day; chipmunks about 15; laboratory rats about 13. On the the other hand, elephants sleep about 4 hours a day; pilot whales about 5; donkeys about 3; and giraffes about 2. There are exceptions to the rule; 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 reasons for the relationship between sleep and size are not entirely clear, but it is thought to have something to do with energy conservation, which is a much greater problem for smaller mammals than for larger mammals. For example, the energy expense for locomotion is greater in smaller mammals. Because a smaller proportion of their weight is comprised of fat, smaller mammals have a more limited capacity for energy storage. Smaller mammals lose heat energy at a greater rate than larger mammals because their surface area is proportionately larger to their mass. In addition, larger mammals generally have more fat insulation and thicker fur than smaller mammals; they are better protected against the loss of heat.

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 which results in a decrease in energy expenditure. 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. If they only rested, they would not suffer the decrease in vigilance that is part of the sleep state. On the other hand, the lowering of vigilance may be an essential and necessary feature of sleep because it limits an animal's reactivity to the environment and thereby keeps it from being perpetually active, with the resultant high cost to its energy reserves. A third possibility is that, by mechanisms as yet unknown, sleep helps to regulate energy expenditure by maintaining specific temperatures which conserve energy resources.

Sleep and Life Span

There is a tendency-again, not without exceptions-for mammalian species that sleep the most to have short life spans. This does not mean that sleep shortens life. On the contrary, long-lived mammals may have some characteristic that enables them to get by with less sleep. This could be a clue to the mystery of sleep.

As noted in the following section which examines the developmental course of sleep, young mammals tend to have more REM sleep than older mammals. This relationship is echoed in species differences. Species which are born relatively immature (e.g., rat, cat) tend to have more REM sleep than species born with fairly mature regulatory systems (e.g., guinea pig, horse).

Most of our knowledge about sleep has been obtained in the past 20 years. We have learned a great deal, but not enough to answer many important questions. In the meantime, it has become very clear that we cannot achieve an overall understanding of the behavior, adaptation, and the biology of animals, including humans, without knowing a great deal about their sleep.

Proceed to Part C.

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