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.