The phenomenon of sleep is not confined to humans. All mammals and birds, as well as in most (but not all) reptiles, amphibians, fish and even insects regularly undergo something which may be described as sleep, although sometimes this may be a profoundly different phenomenon from our usual conception of it. Some of these differences, and the sheer variability in the sleep requirements of different animals, help to provide a different perspective on what sleep is for, and indeed on the very definition of what sleep is.
Even within the human species, sleep may be regarded very differently and be subject to very different cultural, traditional and environmental pressures. Until recent years, relatively little attention has been paid to the anthropology of sleep, but some recent research is beginning to rectify this lack, and to throw some light on the similarities and contrasts in sleep practices throughout the world.