Sleep:The body'srestcycle.
Sleepis triggered by a complex group of hormones that are activeinthe main, and that respond to cues from the body itself and theenvironment. About 80 percent of sleep is dreamless, and is known as non-rapideyemovement (NREM) sleep.
During NREM sleep, the breathing and heart rate are slow and regular, theblood pressureis low, and the sleeper is relatively still.NREM sleepis 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 Level 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.
Dreamsoccur during three to five periods of REM sleep each night.REM sleepoccurs at intervals of one to two hours, and is variable in length. REM sleep is characterized by irregular breathing and heart rate, andinvoluntarymusclejerks.
Most adults need around eight hours of sleep on a regular schedule to function well, although some require less, and others more. (It has been said that men need an hour less sleep than women.) Children, particularly teenagers, often need nine or ten hours for optimal functioning.
