Every animal that has been carefully studied seems to need some form of sleep,…

Every animal that has been carefully studied seems to need some form of sleep, from elephants to fruit flies. Yet the ways in which animals sleep vary enormously, and some of them seem almost impossible to humans.

Take dolphins. As mammals, they must come to the surface regularly to breathe, which makes long periods of deep unconsciousness dangerous. Their solution is to sleep with only half of their brain at a time. While one half rests, the other stays alert enough to control breathing and watch for danger. One eye usually stays open, too. After a while, the two halves swap roles.

Some birds can do something similar. Frigatebirds, which fly over the ocean for weeks without landing, were long suspected of sleeping in the air, but this was only confirmed in 2016, when scientists attached small devices to the birds' heads to record their brain activity. The birds did sleep while flying, often with one half of the brain at a time, but only in short bursts lasting a few seconds. In total, they slept for less than an hour a day in the air, compared with more than twelve hours a day on land.

The amount of sleep animals need also varies widely. Some bats sleep around twenty hours a day, waking mainly at dusk to hunt insects. At the other extreme, a study of wild African elephants found that they slept only about two hours a day, mostly standing up, and sometimes went without sleep for nearly two days while moving long distances. Giraffes also sleep very little, usually in short naps.

Why such differences exist is not fully understood. Diet appears to play a role: animals that eat grass and leaves, which contain relatively little energy, must spend many hours feeding and cannot afford long periods of sleep. The risk of being attacked matters too. Animals that sleep in safe places, such as bats in caves, can sleep longer than those that sleep in the open.

What all these examples share is that no animal has managed to give up sleep completely. Even creatures under enormous pressure to stay awake have found ways of sleeping in pieces. This suggests that sleep performs some function so important that evolution has never been able to remove it, although scientists are still debating exactly what that function is.