Sleep - Sleep in Non-humans

Sleep in Non-humans

Neurological sleep states can be difficult to detect in some animals. In these cases, sleep may be defined using behavioral characteristics such as minimal movement, postures typical for the species, and reduced responsiveness to external stimulation. Sleep is quickly reversible, as opposed to hibernation or coma, and sleep deprivation is followed by longer or deeper rebound sleep. Herbivores, who require a long waking period to gather and consume their diet, typically sleep less each day than similarly sized carnivores, who might well consume several days' supply of meat in a sitting.

Horses and other herbivorous ungulates can sleep while standing, but must necessarily lie down for REM sleep (which causes muscular atony) for short periods. Giraffes, for example, only need to lie down for REM sleep for a few minutes at a time. Bats sleep while hanging upside down. Some aquatic mammals and some birds can sleep with one half of the brain while the other half is awake, so-called unihemispheric slow-wave sleep. Birds and mammals have cycles of non-REM and REM sleep (as described above for humans), though birds' cycles are much shorter and they do not lose muscle tone (go limp) to the extent that most mammals do.

Many mammals sleep for a large proportion of each 24-hour period when they are very young. However, killer whales and some other dolphins do not sleep during the first month of life. Such differences may be explained by the ability of land-mammal newborns to be easily protected by parents while sleeping, while marine animals must, even while very young, be more continuously vigilant for predators.