Sleep

Sleep is a recurring natural state involving reduced activity of the brain, movement of muscles, and altered consciousness. It differs from coma or anesthesia because a sleeping person can be easily awakened. It occurs in humans, mammals, birds, and many other organisms.

Sleep is regulated by two interdependent mechanisms: the homeostatic sleep drive that accumulates during waking hours, and the circadian rhythm tied to light and darkness. The hypothalamus in the brain acts as the sleep-timing center, while the hormone melatonin rises at night.

Stages of sleep

Sleep is divided into two major types: REM sleep (rapid eye movement) and non-REM sleep. Non-REM sleep has distinct stages, ranging from light sleep to deep sleep with increasingly slowed brain waves. REM sleep is associated with vivid dreams and rapid movement of facial muscles. A night consists of recurring cycles, each lasting roughly an hour; later cycles are richer in REM sleep.

Recognized functions

Research links sleep to memory consolidation, energy restoration of brain cells, physical growth in children, and immune system function. It has also been suggested that during sleep the brain clears chemical waste accumulated during the day. Chronic sleep deprivation affects memory, alertness, mood, and blood pressure.

Sleep duration requirements

Sleep needs vary by age. Infants sleep many hours distributed throughout the day, while adults typically need approximately seven to nine hours at night. Older adults often experience fragmented sleep and more nighttime wakefulness.

Sleep disorders

Many conditions affect sleep. Insomnia involves difficulty falling or staying asleep. Sleep apnea causes brief pauses in breathing during the night, disrupting sleep. Narcolepsy is sudden sleep onset during the day. Other conditions include sleepwalking and nightmares.

Culture and history

Different societies have approached sleep in varying ways; for example, daytime napping (such as siesta) is customary in many warm climates. Islamic tradition contains references to sleep and prayer times, including early sleep and night waking for prayer. The use of artificial lighting and screens has been linked to changes in sleep patterns in modern societies.