Glossary: Sleep Disorders

Content provided by the Faculty of the Harvard Medical School

Glossary

advanced sleep phase syndrome: A daily sleep/wake rhythm in which the onset of sleep and the time of awakening are earlier than desired; the person progressively wakes up earlier and wants to retire earlier each day.

apnea: See sleep apnea.

cataplexy: Sudden paralysis of some or all muscles brought on by laughter, anger, or strong emotions; a hallmark of narcolepsy.

central sleep apnea: Sleep apnea caused when respiratory control centers in the brain fail to activate breathing muscles.

circadian rhythm: The innate biological clock that regulates sleep and waking and controls the daily ups and downs of physiologic processes, including body temperature, blood pressure, and the release of hormones.

continuous positive airway pressure (CPAP): A treatment for sleep apnea in which a continuous stream of air is delivered through a mask worn over the nose to keep the sleeper's airway open.

deep sleep: See slow-wave sleep.

delayed sleep phase syndrome: A daily sleep/wake rhythm in which the onset of sleep and wake times are later than desired; the person tends to go to bed later and get up later each day.

electroencephalogram (EEG): A recording of brain waves obtained by attaching flat metal discs (electrodes) to the scalp; it shows changes in brain wave voltage and frequency (in cycles per second).

hypnagogic hallucinations: Often terrifying dream-like sounds or images occurring just prior to sleep; a symptom of narcolepsy that can be mistaken for psychosis.

hypnogram: A diagram that summarizes the stages of sleep recorded in the sleep laboratory.

insomnia: Trouble falling asleep or staying asleep, or sleep that is nonrestorative.

melatonin: A hormone produced in a predictable daily rhythm by the pineal gland.

narcolepsy: A sleep disorder marked by excessive sleepiness or sudden sleep attacks.

obstructive sleep apnea: Sleep apnea resulting from blockage of the airway.

parasomnias: Episodic disruptive behaviors occurring during sleep, indicating abnormal or partial arousal.

periodic limb movement disorder (PLMD): Syndrome characterized by periodic jerking of the legs during sleep.

polysomnography: Simultaneous recording of brain waves and other measures of physiological functioning to assess sleep.

quiet sleep: All sleep except REM sleep. In the quiet phase of sleep, thinking and most physiological activities slow, but movement still occurs. Also called non-REM sleep.

rapid eye movement (REM) sleep: A period of intense brain activity often associated with dreams; named for the rapid eye movements that occur during this time. Also called dreaming sleep.

restless legs syndrome (RLS): 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.

sleep apnea: Cessation of breathing during sleep, lasting at least 10 seconds and associated with a fall in blood oxygen or arousal from sleep.

sleep architecture: The pattern made when sleep stages are charted on a hypnogram.

slow-wave sleep: Sleep Stages 3 and 4; during slow-wave sleep the brain becomes less responsive to external stimuli.

somnambulism: Sleepwalking.

somniloquy: Talking in one's sleep.

Last updated:	January 23, 2007