Sleep is a natural state of bodily rest observed throughout the animal kingdom. 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 which have been studied — such as fish, birds, ants, and fruit-flies — regular sleep is essential for survival. However, its purposes are only partly clear and are the subject of intense research. ... Physiology · Functions · Dreaming · Effect of food and ...

Read about how to get a healthy nights sleep, how much sleep is enough, and why skimping on sleep can lead to poor performance, health, and moods.

Bath and Body

sleep n. A natural periodic state of rest for the mind and body, in which the eyes usually close and consciousness is completely or partially lost,

Fact sheet on normal sleep and sleep disorders developed by the National Institute of Neurological Disorders and Stroke (NINDS).