Sleep in animals

Sleep is a biological requirement for all animals that have a brain, except for ones which have only a rudememtary brain. Therefore basal species don't sleep, since they don't have brains. 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.