Animals and Sleep

Mammals sleep like humans for the most part – the sleep can be divided into light sleep, deep sleep, and REM sleep. (There is some suggestion that dolphins don’t have REM, at least the way humans do.) Most mammals sleep in a polyphasic manner. Perhaps there is more sleep during certain periods of the day or night – diurnal animals sleep at night in general - but the overall pattern in polyphasic. Primates, including humans, sleep uniphasically.

Evolutionary biologists speculate that predation and fear of predators has influenced the development of sleep patterns across species. Carnivores tend to sleep more than herbivores. The range of mammalian sleep length is considerable, though, with armadillos and opossums sleeping 18 hours a day and horses and giraffes sleeping less than 3 hours per day. Horses are known as work animals that take a lot of what humans ask from them, but it is true that horses need little sleep.

Some mammals live in the water but come to the surface to breath. Think dolphins and seals. How do they sleep? If they are asleep can they continue to be the water and come up for air? Evolution has figured out a way to make it work. Nature has given some animals the ability to sleep in one brain hemisphere at a time. The other hemisphere is awake. That allows the animal to continue to move and access air during the night. Scientists have found some dolphins sleep while swimming in a circle – there is a decline in behavior that require higher level navigation and cognition. But another study of dolphins has found unihemispheric sleep can be so optimized it allows dolphins to accomplish complex tasks requiring what we would call vigilance in humans.

Other quirks in marine mammals.

Newborn whales – still learning how to live - go weeks without sleep and so do their mothers. These are temporary situations but still extraordinary.

Blind Indus dolphins have another situation. They sleep in small spurts. Really small spurts, like a few seconds at a time. Microsleep. Added up over the course of a day, it adds up to 7 hours of sleep per day.

Reptiles and birds

Birds have REM, but typically very short periods. Some species of birds can also sleep unihemispherically and this may them continue functioning in long flights. Scientists still don’t understand much in this area, but it would be analogous to dolphins using unihemispheric sleep to keep breathing while sleep.

It used to be thought that reptiles did not have REM. REM was considered a hallmark of more highly evolved animals. Recent examination has found reptile sleep is so different from mammalian sleep that it makes little sense to say there is no REM.

The mechanism that produces sleep inside the brain is largely located in the cerebrum. Reptiles don’t have cerebrums' their sleep is different from ours. As cold-blooded animals, reptiles don’t have internal regulation of body temperature, and in human REM, the body temperature is not regulated tightly (it used to be thought that there was no thermal regulation during REM at all.)

Are There Animals That Do Not Sleep?

No. All animals have something like sleep, even insects. The lower animals with little or no brains sleep differently from humans, but they exhibit periods of inactivity when they are less responsive to external stimuli.

In fact, research with fruit flies has shown some of the same biochemical action in them as happens in human brains during sleep. The commonality points to how ancient sleep is in evolutionary terms. Sleep is universal.

OK, but do some animals skip the "rebound sleep" that humans experience after a sleep loss?

Some insects and fish do not seem to experience rebound sleep after being forced deprivation. It could be that the sleep subsequent to the deprivation is more intense in these animals, the way it is in humans, but there is no way to measure that. Available evidence suggests that mammals all have to get rebound sleep after deprivation - sleep that is either longer or more intense or both.

Do animals ever escape the negative consequence of sleep deprivation?

Well, if the deprivation is long enough it can kill animals. This is certainly true for rats, which have been used extensively in sleep deprivation studies. Some insects also appear to die due to prolonged deprivation although insects are so different from us it is difficult to know what the cause of death is. It is also difficult to tell whether animals suffer from the equivalent of the cognitive impairment that humans have, and whether animals experience the lack of sleep manifests as sleepiness or fatigue the way humans make this distinction.

The Sleepdex book is now available on

Click here