Sleep is vital; it improves the functions of many mental and physical systems, “a kind of nightly tune-up for the body” (260). Without sleep, creatures from fruit flies to humans will die, yet no one knows why sleep is so important that animals must spend hours each day paralyzed, unconscious, and vulnerable to predators. 

Sleep contains a number of stages. The first is falling asleep; the second is a light slumber; the third is a deep unconsciousness. The fourth is Rapid Eye Movement (REM) sleep, during which dreams occur and mental activity is as great as during wakefulness, but the muscles of movement are paralyzed, except for the eyes, which dart about as if watching a movie. These stages repeat in four or five cycles of about 90 minutes each. With each cycle, the REM phase gets progressively longer, for a total of one-fourth of a sleep session. 

People turn over or shift 30 to 40 times a night; young adults wake up briefly several times per sleep session, while the elderly can awaken dozens of times. Dreams may simply be the random firings of neurons during the brain’s housecleaning processes. 

Sleepiness is caused by one of several of the body’s biological clocks.