The body’s rest cycle. Sleep is triggered by a complex group of hormones that are active in the main, and that respond to cues from the body itself and the environment. About 80 percent of sleep is dreamless, and is known as non-rapid 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 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. Dreams occur during three to five periods of REM sleep each night. REM sleep occurs at intervals of one to two hours, and is variable in length. REM sleep is characterized by irregular breathing and heart rate, and involuntary muscle jerks. 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
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A physiologic state of relative unconsciousness and inaction of the voluntary muscles, the need for which recurs periodically. The stages of s. have been variously defined in terms of depth (light, deep), EEG characteristics (delta waves, synchronization), physiologic characteristics (REM, NREM), and presumed anatomic level (pontine, mesencephalic, rhombencephalic, Rolandic, etc.). [A.S. slaep]
- electric s. a condition of convulsions and unconsciousness induced by the passage of an electric current through the brain.
- electrotherapeutic s. electrotherapeutic s. therapy.
- hypnotic s. SYN: hypnosis.
- light s. SYN: dysnystaxis.
- rapid eye movement s., REM s. that state of deep s. in which rapid eye movements, alert EEG pattern, and dreaming occur; several central and autonomic functions are distinctive during this state.
- s. terror SYN: night terrors.
- winter s. SYN: hibernation.

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sleep 'slēp n
1) the natural periodic suspension of consciousness during which the powers of the body are restored compare REM sleep, slow-wave sleep
2) a state resembling sleep: as
a) DEATH (1) <put a pet cat to \sleep>
b) a state marked by a diminution of feeling followed by tingling <her foot went to \sleep>
sleep vi, slept 'slept; sleep·ing to rest in a state of sleep

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a state of natural unconsciousness, during which the brain's activity is not apparent (apart from the continued maintenance of basic bodily functions, such as breathing) but can be detected by means of an electroencephalogram (EEG). Different stages of sleep are recognized by different EEG wave patterns. Drowsiness is marked by short irregular waves; as sleep deepens the waves become slower, larger, and more irregular. This slow-wave sleep is periodically interrupted by episodes of paradoxical, or rem (rapid-eye-movement), sleep, when the EEG pattern is similar to that of an awake and alert person. Dreaming occurs during REM sleep. The two states of sleep alternate in cycles of from 30 to 90 minutes, REM sleep constituting about a quarter of the total sleeping time.

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(slēp) a period of rest for the body and mind, during which volition and consciousness are in partial or complete abeyance and the bodily functions partially suspended. Sleep has also been described as a behavioral state marked by a characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli. Sleep is divisible into two types: NREM (non–rapid eye movement) s. and REM (rapid eye movement) s.

Medical dictionary. 2011.

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