Sleep is a natural state of bodily rest observed in humans and other animals. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than hibernation or coma. It is common to all mammals and birds, and is also seen in 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 partly clear and are the subject of intense research.
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