Sleep

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.

Sleep and Dreaming

A, R, Siegel JM.  2000.  Sleep and Dreaming. Principles of Neuroscience. :936-947.