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Sleep refers to a period of rest involving unconsciousness and inactivity. Sleep may be divided into two periods, rapid eye movement (REM) sleep and non-REM sleep. During REM sleep, the eyes move rapidly under closed eyelids. Dreaming is believed to occur during REM sleep.

There is a great variation in the amount and depth of sleep among individuals. Adults may need between 3-12 hours, with an average of about 7-8 hours of uninterrupted sleep required to function maximally. Older adults may require somewhat fewer hours, and children may require 8-14 hours depending on age.

Sleep is critical for health and well-being, and even mild sleep deprivation can result in a variety of symptoms including fatigue, irritability, and reduced mental function. The effects of sleep deprivation are reversed when the normal sleep-wake cycle is resumed.

It is still unclear why the body requires sleep. Some researchers believe sleep is a relic of an evolutionary drive for humans to seek cover and rest during the dangerous dark hours. Others believe that sleep is important to allow the body to rest and rejuvenate. Some neuroscientists believe that sleep serves an important function in the brain, allowing structures like the hippocampus to "play back" recent experiences and encode them in permanent memory, a process called consolidation. All of these views may be partially correct.

Further reading: "The Remains of the Day"