sleep, resting state in which an individual becomes relatively quiescent and relatively unaware of the environment. During sleep, which is in part a period of rest and relaxation, most physiological functions such as body temperature, blood pressure, and rate of breathing and heartbeat decrease. However, sleep is also a time of repair and growth, and some tissues, e.g., epithelium, proliferate more rapidly during sleep. Sleep also aids in the strengthening of memories, and researchers have discovered that an increased circulation of fluids in the brain during sleep removes waste products from the brain.

In humans, sleep occurs in cyclical patterns; in each cycle of 11⁄2 to 2 hr, the sleeper moves through four stages of sleep, from Stage 1 to Stage 4, and back again to Stage 1. In the first stage, low-frequency, low-amplitude theta waves characterize brain activity. The stage usually lasts only several minutes, before the individual drifts into Stage 2 sleep, and the brain moves into low-frequency, high-amplitude waves. Stage 3 signals an increase of low-frequency, high-amplitude delta waves, and at Stage 4 sleep these delta waves account for more than half of all brain wave activity (see electroencephalography). Rapid-eye-movement (REM) sleep occurs during Stage 1 sleep at the end of each cycle, and people woken up at this time usually report that they have been dreaming. Dream deprivation or sleep deprivation results in detrimental changes in personality, perceptual processes, and intellectual functioning. There is some evidence that emotional and environmental deprivation disrupts the sleep patterns of young children, which in turn inhibits the secretion of growth hormone, normally secreted maximally during sleep.

The amount of sleep needed depends on both the individual and the environment: For instance, worrying, critical individuals tend to need both more sleep and more dream sleep than easygoing ones, and stress and worry during the day result in an increase in REM sleep. It has been hypothesized that while deeper stages of sleep are physically restorative, REM sleep is psychically restorative. REM sleep is also believed to integrate new information in the brain and to reactivate the sleeping brain without waking the sleeper. There is evidence that the hypothalamus and thalamus of the brain initiate sleep and that part of the midbrain acts as an arousal system. See also dream; insomnia; narcolepsy; sleep apnea.

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