sleep

sleep The body’s rest cycle. Sleep is triggered by a complex group of hormones that respond to cues from the body itself and the environment. About 80 percent of sleep is dreamless and is known as nonrapid eye movement (NREM) sleep. During NREM sleep, the breathing and heart rate are slow and regular, the blood pressure is low, and the sleeper is relatively still. NREM sleep is divided into four stages of increasing depth of sleep: Level 1 sleep is a transition period between sleep and wakefulness; Level 2 sleep features significant slowing of heartbeat and breathing and makes up about 50 percent of all sleep; and Levels 3 and 4 (delta) sleep are marked by very slow respiration and heartbeat. Level 4 sleep leads to rapid eye movement (REM) sleep, also known as Level 5 sleep. Dreams occur during three to five periods of REM sleep each night. REM sleep occurs at intervals of 1 to 2 hours and is variable in length. REM sleep is characterized by irregular breathing and heart rate and involuntary muscle jerks. Most adults need around 8 hours of sleep on a regular schedule to function well, although some require less and others more. Children, particularly teenagers, often need 9 or 10 hours for optimal functioning. See also NREM sleep; REM sleep.