Oneirology

Sleep is a naturally recurring state of relatively suspended sensory and motor activity, characterized from the exterior by total or partial unconsciousness and the inactivity of nearly all voluntary muscles. It is distinguished from the exterior from quiet wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than hibernation or coma. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals that have been studied (such as some species of fish, birds, ants, and fruit flies), regular sleep is essential for survival.

The purposes and mechanisms of sleep are only partially clear from the exterior and are the subject of intense research.