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A state that suspends the voluntary exercise of bodily functions and consciousness.

A healthy adult sleeps an average of 7.5 hours each night and most people (approximately 95 percent) sleep between 6.5 and 8.5 hours. Tracking brain waves with the aid of electroencephalographs (EEGs), researchers have identified six stages of sleep (including a pre-sleep stage), each characterized by distinctive brain-wave frequencies. Stage 0 is the prelude to sleep, which is characterized by low amplitude and fast frequency alpha waves in the brain. At this stage, a person becomes relaxed, drowsy, and closes their eyes. Stages 1 through 4 are sometimes characterized as NREM (non-rapid eye movement) sleep. In Stage 1, the eyes begin to roll and rhythmic alpha waves give way to irregular theta waves that are lower in amplitude and slower in frequency as the person loses responsiveness to stimuli, experiences, fleeting thoughts, and images. In Stage 2, electroencephalogram tracings show fast frequency bursts of brain activity called sleep spindles, marked by muscle tension and accompanied by a gradual decline in heart rate, respiration, and temperature. Stages 3 and 4 normally occur 30 to 45 minutes after falling asleep. In Stage 3, there are fewer sleep spindles, but high amplitude and low frequency delta waves appear. When these begin to occur more than 50 percent of the time, the fourth stage of sleep has been entered. Delta waves demarcate the deepest levels of sleep, when heart rate, respiration, temperature, and blood flow to the brain are reduced and growth hormones are secreted. A person roused from Stage 4 sleep will be groggy and confused. Altogether, it takes about a half hour to pass through these four stages of sleep.

Rapid eye movement (REM sleep), which makes up approximately 20 percent of sleep time, is interspersed with NREM sleep every 30 to 40 minutes throughout the night. It is during REM sleep that dreams are experienced. In this state, the same fast frequency, low-amplitude beta waves that characterize waking states occur, and a person's physiological signs—heart rate, breathing, and blood pressure—also resemble those in a waking state. However, muscle tone decreases to the point of paralysis, with sudden twitches, especially in the face and hands. REM periods may last from 15 minutes at the beginning of a sleep cycle to one hour at the end of it. Most people complete four to six complete sleep cycles each night, with each cycle lasting about 90 minutes. These cycles vary in composition, however; early in the night most of the time is spent in Stage 3 and 4 sleep, with Stage 2 and REM sleep predominating later on. Sleep patterns also vary in the course of a person's life. On the average, an infant sleeps about 16 hours a day, in contrast to a 70-year-old who sleeps only about six hours. While REM sleep comprises about half of total sleep at birth, it eventually decreases to only 25 percent. Sleeping patterns also vary greatly among individuals, and even among different cultures (in terms of napping, for example).

Two theories of sleep, the repair and the adaptive theories, attempt to explain why sleep occurs. In the repair theory, sleep serves a biological need, replenishing key areas of the brain or body which are depleted during the day. The adaptive theory suggests that sleep as a function evolved over time because it prevented early humans from wasting energy and exposing themselves to nocturnal predators, thus aiding in survival. REM sleep in particular has been thought to serve special functions. Research subjects whose REM sleep was interrupted made up for the loss by spending extra time in the REM stage on successive nights. It has also been suggested that REM sleep aids the activity of neurons that use the neurotransmitter norepinephrine, thus maintaining waking alertness. Persons deprived of REM sleep have shown poorer retention of skills learned during the day, leading to the hypothesis that REM sleep helps in assimilating daytime learning experiences.

As with many other physiological processes, sleep is linked to a 24-hour circadian rhythm and affected by signals such as light and dark. The effects of disrupting the sleep-wake cycle can be seen in jet lag, which is characterized by fatigue, irritability, lack of alertness, and sleeping problems. A person affected by jet lag feels like sleeping at the wrong times of day. It has been found that the body maintains a circadian sleep-wake rhythm even in the absence of external cues like lightness and darkness, although research subjects deprived of such cues eventually adopt a 25-hour "day." The "internal clock" that maintains this pattern is a section of the brain called the supra chiasmatic nucleus (SCN), located in the hypothalamus.

Various disorders interfere with sleep. The most common is insomnia, the inability to fall asleep or stay asleep. Nearly one-third of all Americans are affected by some degree of insomnia. Often associated with mental distress, insomnia is treated with medication, psychotherapy, relaxation techniques, or a combination of these methods. The medications most commonly prescribed are benzodiazepines (Valium, Halcyon, Restoril) and barbiturates. While they alleviate insomnia in the short run, these drugs interfere with normal sleep patterns, and can lead to increased tolerance and dependence. Researchers and clinicians have had success treating insomnia with the hormone melatonin, a naturally occurring substance related to sleep onset and secreted by the pineal gland. Melatonin supplements first became available in American health food stores in 1993 and have become increasingly popular as a sleep aid, although their use has caused some controversy in medical circles.

Narcolepsy, a disorder characterized by sudden and uncontrollable occurrences of sleep, afflicts 100,000 people in the United States. This condition is genetically linked, and may be curable in the future. Individuals affected by narcolepsy abruptly enter REM sleep states during the daytime, collapsing and remaining immobile for a period of time after awakening. Napping and stimulants have both been used to treat this condition. Another disorder associated with sleep is sudden infant death syndrome (SIDS), in which a healthy baby stops breathing during sleep, fails to awaken, and suffocates. While the exact cause of SIDS is unknown, researchers are attempting to identify and save at-risk infants by studying the relationship between the disorder and sleeping patterns. In sleep apnea, a person repeatedly stops breathing while asleep but awakes each time. The disrupted sleep that results from these multiple awakenings leaves the sleeper fatigued and sleepy during the daytime. Night terrors are non-REM dream experiences from which the sleeper never fully awakes and which he or she does not recall upon awakening. This condition mostly occurs in children and can be treated with hypnosis or medication in severe cases.

See also Sleep Disorders.

Additional topics

Psychology EncyclopediaPsychological Dictionary: Perception: early Greek theories to Zombie