Sleep

The natural state of lowered consciousness and reduced metabolism. There are 2 types of sleep: REM (rapid eye movement) and NREM (nonrapid eye movement) sleep, which alternate in cycles. NREM sleep consists of 4 stages of progressively greater “depth”, with slowing of brain activity. In REM sleep, the brain becomes more active; the eyes move rapidly and dreaming occurs. Sleep is a fundamental human need, as shown by the effects of sleep deprivation, although its purpose is not understood in detail. The need for sleep varies from person to person and decreases with age. Sleep disorders include difficulty in falling or remaining asleep (see insomnia); difficulty in staying awake (see narcolepsy); disruption of sleep by jetlag and bed-wetting, night terrors, or sleepwalking. (See also sleep apnoea.)