Sleep in animals

Sleep is a biological requirement for all animals that have a brain, except for ones which have only a rudimentary brain. Therefore basal species do not sleep, since they do not have brains. It has been observed in mammals, birds, reptiles, amphibians, fish, and, in some form, in arthropods. Most animals feature an internal circadian clock dictating a healthy sleep schedule; Diurnal organisms, such as humans, prefer to sleep at night; Nocturnal organisms, such as rats, prefer to sleep in the day; Crepuscular organisms, such as felidae, prefer to sleep for periods during both. More specific sleep patterns vary widely among species, with some foregoing sleep for extended periods and some engaging in unihemispheric sleep, in which one brain hemisphere sleeps while the other remains awake.

Sleep as a phenomenon appears to have very old evolutionary roots. Unicellular organisms do not necessarily "sleep", although many of them have pronounced circadian rhythms.