ABSTRACT
Sleep can be defined in behavioral terms as a normal, recurring, reversible state of loss of awareness with inability to perceive and respond to the external environment. Voluntary motor activity largely ceases and a quiescent posture, specific to each species, is adopted. Sleep is present in mammals and birds, probably in reptiles, amphibians, and fish, and likely in at least some invertebrate species such as fruit flies. It may be present in only one cerebral hemisphere at a time in dolphins, porpoises, whales, and some species of birds, presumably as a defense against predators. Sleep is generated by the brain but is associated with profound changes in physiology elsewhere in the body. Contrary to early belief, the neurophysiology of sleep involves active and dynamic changes in neural functioning and is far from a passive process of absence of wakefulness.
