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Quiet sleep

An alternate term for non-REM sleep. In the quiet phase of sleep most physiological activities are very stable and regular, and many are reduced compared to wakefulness.

Rapid-eye-movement (REM) sleep

A resting state with little consciousness of the environment, high cortical activity, vivid dreams, and periods of rapid eye movements. Nearly all skeletal muscles are paralyzed during this state, which prevents an individual from acting out dreams. Also called paradoxical or dreaming sleep.

Restless legs syndrome (RLS)

A condition characterized by achy or unpleasant feelings in the legs associated with a need to move. Most prominent at night, making it hard to fall asleep or stay asleep.

Seasonal affective disorder (SAD)

Sadness and depression that is brought on by a lack of exposure to sunlight. SAD usually appears in the fall or winter and subsides in the spring.

Shift work

A general term to describe a job that requires an individual to work other than the standard working hours of mid-morning to late afternoon, Monday through Friday. For instance, shift work may involve working from midnight until 7:00 a.m.

Shift work sleep disorder

A chronic condition that is directly related to a shift work schedule. Two primary symptoms are insomnia during times when a person is trying to sleep and excessive sleepiness when a person needs to be awake and alert.


A rest or short nap, usually after the midday meal.

Sleep apnea

A cessation of breathing during sleep. In adults episodes last at least 10 seconds; in children they last at least the duration of two breath cycles. See also central sleep apnea and obstructive sleep apnea.

Sleep debt

An individual’s accumulated sleep loss from insufficient sleep, regardless of cause.

Sleep drive

See homeostatic sleep drive.

Sleep efficiency

The proportion of sleep in the period available for sleep; that is, the ratio of total sleep time to time in bed.

Sleep inertia

The grogginess that occurs immediately after awakening, which can adversely affect cognitive and psychomotor abilities. Overcoming sleep inertia can take minutes or even an hour or more. This depends on multiple factors, possibly including the stage of sleep from which a person awakes and the duration of prior sleep.

Sleep latency

How long it takes a person to fall asleep from the onset of the potential sleep period; for instance, time measured from turning the lights out to the onset of sleep.

Sleep onset

The transition from waking to sleep, normally into non-REM N1 sleep.

Sleep paralysis

Brief episodes of immobility when falling asleep or upon awakening. The paralysis typically affects all muscles except those needed for breathing. This is probably the occurrence during wakefulness of the paralysis that normally occurs during REM sleep.

Sleep spindle

Spindle-shaped burst waves on the EEG occurring at 12–14 Hz for over half a second. Sleep spindles are one of the identifying features of N2 sleep.

Sleep-wake homeostat

See Homeostatic sleep drive

Slow-wave sleep (SWS)

See N3 sleep (Stage 3 or slow-wave sleep [SWS])

Suprachiasmatic nucleus (SCN) [also referred to as circadian clock, circadian pacemaker, or internal biological clock]

The internal circadian pacemaker is a small group of nerve cells located in the hypothalamus that controls the circadian cycles and influences many physiological and behavioral rhythms occurring over a 24-hour period, including the sleep/wake cycle.

Sympathetic nervous system

One of two subdivisions of the autonomic nervous system; it intensifies certain body activities and releases stress hormones in response to perceived or real dangers. These subdivisions usually work in opposition (like a brake and an accelerator in a car). Thus, for example, slowing the heart can be achieved by simultaneously decreasing sympathetic activity (reduced acceleration) and by increasing parasympathetic activity (increased braking).

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