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Sleep in animals
Sleep appears to be a biological requirement for all animals except for basal species with no brain or only a rudimentary brain. It has been observed in mammals, birds, reptiles, amphibians, fish, and, in some form, in insects. The internal circadian clock promotes sleep at night for diurnal organisms (such as humans) and in the day for nocturnal organisms (such as rats). 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.
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