Glossary of Sleep Terminology

Below is a list of terminology used by our sleep clinicians.

Use the letters below to navigate.

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Advanced Sleep Phase Syndrome (ASPS)

Phases of the daily sleep/wake cycle are advanced with respect to clock time. This is classified as a circadian rhythm disorder. The sleep phase occurs well ahead of the conventional bedtime and the tendency is to wake up too early.

Alpha Intrusion

Brief occurrence of alpha activity during a stage of sleep. Alpha rhythm – EEG oscillations, prominent over the occipital cortex, with a frequency of 8-13 Hz in adults; indicative of the awake state; present in most, but not all, normal individuals; most consistent and predominant during relaxed wakefulness.


An apnea is a period of time during which breathing stops or is markedly reduced. There are two types of apneas, the more common obstructive sleep apnea and the less common central sleep apnea.

Apnea-Hypopnea Index

Number of pauses in breathing or insufficient breathing usually with decrease in oxygenation events per hour of sleep.


Swift change of EEG frequency for at least 3 seconds taking place in any sleep stage.

Arousal Disorder

Type of parasomnia involving atypical arousal from sleep.

Arousal Threshold

Ease with which a sleeping person is awakened.


Ease with which a sleeping person is awakened.


Basic Sleep Cycle

Progression through orderly succession of sleep states and stages. For the healthy adult, the first cycle begins by going from wakefulness to non-REM sleep. The first REM period follows the first period of non-REM sleep, and the two sleep states continue to alternate throughout the night with an average period of about 90 minutes. A night of normal human sleep usually consists of 4-6 non-REM/REM sleep cycles.


Clock time when one attempts to fall asleep, as differentiated from clock time when one gets into bed.

Biological clock

A collection of cells that regulates an overt biological rhythm, such as the sleep/wake cycle, or some other aspect of biological timing, including reproductive cycles or hibernation.

Body Position

Four positions are identified in which a patient may be sleeping; back, left side, right side or abdomen. The time spent sleeping in each position and the number of respiratory events in a particular position are tabulated.


Teeth grinding during sleep.



Symptom of narcolepsy; consists of a sudden loss of muscle tone that leads to feelings of weakness and a loss of voluntary muscle control.

Central Sleep Apnea

Sleep disorder in which the airway is not blocked but the brain fails to signal the muscles to breathe.

Cheyne Stokes Respiration

Breathing pattern characterized by regular waxing and waning of respiratory rate and tidal volume.


A behavioral technique in which bedtime is systematically adjusted; used in cases when the patient’s sleep-wake pattern is out-of-synch with the external environment.

Circadian Rhythms

Biological rhythms that include the internal clock which influences when, how much, and how well people sleep.

Cognitive Therapy

In some cases of insomnia, this therapy includes interventions that are meant to help people identify and correct inappropriate thoughts and beliefs that may contribute to insomnia.

Conditioned Insomnia

Type of psychophysiological insomnia produced by the development, during an earlier experience of sleeplessness.

CPAP (continuous positive airway pressure)

An effective treatment for sleep apnea patients; delivers air into airways through a specially designed nasal mask or pillows.


Delayed Sleep Phase

A condition occurring when the clock hour at which sleep normally occurs is moved back in time in a given, 24 hour sleep-wake cycle. The result is a temporarily displaced (delayed) occurrence of sleep within the 24 hour cycle.

Delayed Sleep Phase Syndrome

A clock problem, when the sleep-wake cycle is delaying regular bedtime. A Circadian Rhythm Disorder.

Delta Sleep

Also called slow-wave sleep, this deep sleep occurs in stages 3 and 4 of NREM sleep.

Delta Waves

Brain waves with a frequency of 1 to 3 hertz that emanate from the forward portion of the brain during deep sleep in normal adults.

Diagnostic Sleep Study

Monitoring of several physiological activities in a sleeping individual. Usually performed to determine the absence or presence of a specific sleep disorder. The sleep study can occur in a sleep disorders center or in a patient’s home with portable recording equipment.


A primary disturbance of sleep initiation, maintenance or excessive sleepiness.



To reset or align with the biological clock.




A standard 30 second duration of the sleep recording that is assigned a sleep stage designation; for special purposes, occasionally longer or shorter epochs are scored.


Summer hibernation when animals sleep to get away from heat, no water, or their food source disappears.

Excessive Daytime Sleepiness

Subjective report of daytime over- propensity to fall asleep.

Extrinsic Sleep Disorders

Sleep disorders that arise from an out of the body cause.



Feeling of tiredness or weariness usually associated with performance decrements.

Final Awakening

Time of wakefulness that occurs after the final wake-up time until lights on.

First Night Effect

Altered sleep physiological variables caused by unfamiliar sleep environment and/or recording equipment.

Forbidden Zone

The period of strongest clock-dependent alerting, usually in the evening. Prevents falling asleep.

Fragmentation (pertaining to Sleep Architecture)

Interruption of a sleep stage as a result of the appearance of a lighter stage, or to the occurrence of wakefulness, which leads to disrupted non-REM-REM sleep cycles.

Fragmented Sleep

Repeated arousals and/or awakenings causing interrupted and insufficient sleep. One of the most common findings in child and adolescent sleep disorders.

Free Running Rhythm

Irregular circadian rhythm with no 24 hrs wake-sleep cycle. Possible causes are genetic and mostly unknown origin.


Habitual Snorers

Those who snore nearly every night.


Winter hibernation where the animal’s temperature, heart rate, and body slow down so much that it might appear to be dead.

Homeostatic Regulation of Sleep

Refers to the neurobiological signals mediating the pressure or urge to sleep.


Typical behavior in a child with a sleep disorder which is causing lack of quality sleep.


Synonym to excessive sleepiness.

Hypnagogic Hallucination

A “greater-than-life-like” dream experience that occurs during sleep. Hypnagogic hallucinations are sometimes associated with narcolepsy.

Hypnagogic Imagery

Vivid sensory images occurring at sleep onset but particularly vivid with sleep-onset REM periods; feature of narcoleptic REM naps.

Hypnagogic Startle

“Sleep start” or sudden body jerk, observed normally just at sleep onset, resulting in at least momentary awakening


A graphical summary of the electrical activities occurring during a night’s sleep.


Morbid fear of falling asleep.


Sleep-inducing drugs.


Referred to 50% reduction of oro-nasal airflow for 10 seconds or more during sleep, usually associated with an arousal and/or oxygen desaturation.


Inappropriate Sleep Episodes

Unplanned sleep periods often occurring in an unsafe situation (i.e., while driving). These episodes are always due to sleep deprivation.


Sleeplessness; chronic difficulty with sleep onset or maintenance of sleep, or a perception of nonrefreshing sleep.


Jet Lag

A temporary disorder that causes fatigue, insomnia, and other symptoms as a result of rapid air travel across time zones.


K Complex

An EEG wave. One of the markers for stage 1 sleep, light sleep.


Leg Movement

Leg movements are recorded in both diagnostic sleep studies and titration studies.

Light Sleep

Usually referred to stage-1 sleep.
Limit-Setting Sleep Disorder
Disorder due to child’s difficulty in falling asleep by delaying and refusing to go to bed.



The hormone that considered to regulate the sleep- wake cycle to the outside world’s time giver signs, mainly light.


A sleep occurrence lasting up to 10 seconds during which no external stimulus is perceived.


Partial awakening from sleep

Mixed Sleep Apnea

A combination of central sleep apnea and obstructive sleep apnea.


A single major sleep period and a single major wake period in a 24-hour day.

Motor Activity in Sleep

Any muscular movement during sleep.

Motor Atonia

Absence of muscle activity during sleep.

Movement Arousal

A body movement associated with arousal or awakening; a sleep scoring variable.



A short sleep episode during routine wakefulness.


A neurological condition in which people experience excessive daytime sleepiness, cataplexy, sleep paralysis, hallucinations and intermittent, uncontrollable sleep attacks during the day.

Night Terrors

Also known as sleep terrors, or pavor nocturnus. Night terrors are characterized by an incomplete arousal from slow wave sleep. If, the individual is awakened during a night terror, he/she is usually confused and does not remember details of the event. Night terrors are different from nightmares; if an individual is awakened during a nightmare, he/she functions well and may have some recall of the nightmare.


Unpleasant and/or frightening dream occurring in REM sleep (different from a night terror).


Excessive, often frequent, urination during the night.


Relating to or taking place at night.

Nocturnal Confusion

Episodes of delirium and/or disorientation near or during nighttime sleep; often seen in victims of Alzheimer’s Disease and more common in the elderly.

Nocturnal Enuresis (Bedwetting)

Urinating while asleep.

Nocturnal Sleep-Related Eating Disorder (NS-RED)

Getting up during the night and eating while sleepwalking. No recall in the morning.

Non 24-Hour Sleep Wake Disorder

A circadian rhythm disorder in which the sleep-wake pattern does not conform to the usual 24-hour cycle.

Non-Rapid Eye Movement (NREM) Sleep

One of the two basic states of sleep; consists of Stages 1, 2 (light sleep) and 3, 4 (deep sleep).

NREM Sleep Intrusion

Brief period of NREM sleep patterns appearing in REM sleep; a portion of NREM sleep not appearing in its usual sleep cycle position.


Obstructive Hypopnea

Periodic and partial closure of the throat during sleep resulting in reduced air exchange at the level of the mouth and/or nostril.

Obstructive Sleep Apnea (OSA)

The most common kind of sleep apnea. It is caused by a blockage of the upper airway.

Optimum Sleep

Average amount of sleep needed every night by an individual.


Paradoxical Sleep

REM sleep.


Abnormal behaviors during sleep that interrupt sleep and can result in injury, insomnia, and/or excessive daytime sleepiness.

Paroxysmal Nocturnal Dyspnea (PND)

Respiratory distress and shortness of breath due to pulmonary edema, appearing suddenly and often awakening the sleeping individual.

Pathological Sleep

Abnormal sleep patterns.

Perceptual Disengagement

Change in consciousness at the onset of sleep when environmental stimuli are no longer perceived, and there is no longer any conscious, meaningful interaction with the environment.

Periodic Limb Movement Disorder (PLMD)

A disorder in which rhythmic jerking of the legs interrupts sleep, causing insomnia and/or excessive daytime sleepiness.

Persistent Insomnia

Continuing insomnia responding poorly to treatment.

Phasic (Event/Activity)

Brain, muscle, or autonomic related event of a brief and episodic nature occurring in sleep. Usually occur during REM sleep, such as eye movements and/or muscle twitches.


Multiple sleep periods and wake periods in a 24-hour day.


The brainstem region critical for initiating REM sleep.

Post-Prandial Drowsiness

Sleepiness that occurs after a meal, usually lunch.

Premature Morning Awakening

Early termination of the sleep period in a sleep maintenance dims due to inability to return to sleep after the last of several awakenings.


Quiet Sleep

The term used to describe NREM sleep during infancy.


Rapid Eye Movement (REM) Sleep

One of the two basic states of sleep. REM sleep, also known as dream sleep, is characterized by rapid eye movements, and more irregular breathing and heart rate compared to NREM sleep.

REM Density

A function that expresses the frequency of eye movements per unit of time during REM sleep.

REM Motor Atonia

The active suppression of activity in the antigravity and voluntary muscles during REM sleep. The muscles are completely flaccid and limp.

REM Onset

Designation for commencement of a REM period; used also as a shorthand term for a sleep-onset REM period.

REM Period

REM portion of a NREM-REM cycle; early in the night it may be as short as a half-minute, whereas in later cycles longer than an hour.

REM Rebound

Increased frequency, duration and percentage of dream sleep commonly seen after REM sleep deprivation and during CPAP titration study.

REM Sleep Behavior Disorder (RBD)

Disorder in which REM motor atonia is partially or completely absent and the individual acts out the ongoing dream. The behavior in REM behavior disorder is often correlates with the ongoing, hallucinatory REM dream episode.

REM Sleep Episode

REM sleep portion of a NREM-REM sleep cycle. Early in the first sleep period, episodes may be only several minutes in duration. Later REM episodes almost are always longer, 20 to 30 minutes up to an hour.

REM Sleep Intrusion

Brief interval of REM sleep appearing out of its usual positioning in the NREM-REM sleep cycle.

REM Sleep Latency

Designation for the first epoch of a REM sleep episode.

REM Sleep Onset

Designation for the first epoch of a REM sleep episode.

REM-Associated Disorders

Sleep disturbances that occur in REM sleep. REM sleep Onset – Beginning of REM sleep.

Restless Legs Syndrome (RLS)

A neurologic movement disorder that is often associated with a sleep complaint.

Restlessness (Referring to Quality of Sleep)

Persistent or recurrent body movements, arousals, and/or brief awakenings in the course of sleep.



Parasomnia of a sexual, sometimes, violent nature towards the bed partner.


A state marked by lessened consciousness, lessened movement of the skeletal muscles, and slowed-down metabolism.

Sleep Apnea

Sleep disorder that occurs when a person’s breathing is interrupted during sleep.

Sleep Architecture

Sleep stages composition: NREM-REM cycles. Usually given as percentage per age.

Sleep Cycle

Synonymous with NREM-REM cycle.

Sleep Debt

Result of recurrent sleep deprivation which occurs over time when an individual does not experience a sufficient amount of the restorative daily sleep that is required to maintain a sense of feeling rested and refreshed.

Sleep Deprivation

Acute or chronic lack of sufficient sleep.

Sleep Disorders

Broad range of illnesses arising from many causes, including, dysfunctional sleep mechanisms, abnormalities in physiological functions during sleep, abnormalities of the biological clock, and sleep disturbances that are induced by factors extrinsic to the sleep process.

Sleep Episode

Interval of sleep that may be voluntary or involuntary.

Sleep Extension

Extending sleep time by increasing the time in bed.

Sleep Fragmentation

Brief arousals occurring throughout the night, reducing the total amount of time spent in the deeper levels of sleep.

Sleep Hygiene

Practices, habits, and environmental factors that are important for getting sound sleep.

Sleep Hyperhydrosis

Excessive sweating during sleep.

Sleep Inertia

Feelings of grogginess and/or sleepiness that persist longer than 10 to 20 minutes after waking up.

Sleep Interruption

Breaks in the sleep architecture resulting in arousal and wakefulness.

Sleep Latency

Time from lights off to sleep onset.

Sleep Mentation

Thoughts, feelings, images, perceptions, hallucinations, and active dreams taking place during sleep.

Sleep Onset

Conversion from wakefulness to sleep period after lights off.

Sleep Onset Imagery

Images and experiences during the moments following the transition from wake to sleep.

Sleep Paralysis

Symptom of narcolepsy; involves the temporary inability to move or speak while falling asleep or waking up.

Sleep Pattern (24 hour sleep-wake pattern)

Individual’s clock hour schedule of bedtimes and rise times as well as nap behavior: may also include time and duration of sleep interruptions.

Sleep Related Accidents

Accidents caused by individuals who were sleep deprived and who, as a result, had impaired judgment.

Sleep Restriction

Limitation of the number of hours in bed.

Sleep Spindle

Spindle shaped EEG bursts. They are one of the signs for sleep initiation.

Sleep Structure

Similar to sleep architecture. Sleep structure, in addition to encompassing sleep stage and cycle relationships, assesses the within-stage qualities of the EEG and other physiological attributes.

Sleep Talking

Talking in sleep takes place during stage REMS, representing a motor breakthrough of dream speech, or in the course of transitory arousals from NREMS and other stages. Full consciousness is not achieved and no memory of the event remains.

Sleep-Maintenance DIMS or Insomnia

Disturbance in maintaining sleep once achieved; persistently interrupted sleep without difficulty falling asleep.

Sleep-Onset REM Period

Atypical beginning of sleep by entrance directly into stage REM.

Sleep-Wake Shift (-change, -reversal)

Sleep wholly or partially moved to a time of customary waking activity, and the latter is moved to the habitual sleep period; common in jet lag and shift work.

Sleep-Wake Transition Disorder

Disorder occurring during the transition from wakefulness to sleep or from one sleep stage to another; a form of parasomnia.

Sleep-wake, 24 Hour Cycle

The clock hour relationships of the major sleep and wake phases in the 24 hour cycle: similar to sleep pattern.

Sleepiness (somnolence, drowsiness)

Difficulty in maintaining the wakeful state so that the individual falls asleep if not actively kept aroused; not simply a feeling of physical tiredness or listlessness.

Sleeping Pills

Compounds that have a sedative effect, used to produce sleepiness.

Sleepwalker or Sleepwalking

Individual subject to somnambulism (one who walks while sleeping). Sleepwalking typically occurs in the first third of the night during deep NREM sleep (stages 3 and 4).


Noise produced primarily with inspiratory respiration during sleep owing to vibration of the soft palate and the pillars of the oropharyngeal inlet. Many snorers have incomplete obstruction of the upper airway, and may develop obstructive sleep apnea.


Sleep Walking.


Prolonged drowsiness or sleepiness.


Causing or tending to cause sleep.

Spindle REMS

Condition in which sleep spindles persist atypically in REMS; seen in chronic DIMS conditions.

Subjective Sleepiness

Feelings of sleepiness.

Subwakefulness Syndrome

Syndrome defined as a defect in the CNS support system for waking. The few individuals reported with subwakefulness syndrome have daytime drowsiness and daytime sleep episodes that are always composed of NREMS stages 1 or 2. The naps occur repetitively.


Scheduling sleep to synchronize with the biological clock.



A feeling of a lessened capacity for work and reduced efficiency of accomplishment, usually accompanied by a sense of weariness and fatigue.

Total Sleep Period

Period of time measured from sleep onset to final awakening. In addition to total sleep time, it is comprised of the time taken up by arousals and movement time until wake-up.

Total Sleep Time

Amount of NREM and REM sleep from lights off to lights on.

Transient Arousals

Brief awakenings from sleep.

Transient Insomnia

Difficulty sleeping for only a few nights.


Unihemispheric Sleep

A type of sleep in which one side of the brain is asleep while the other is awake. This phenomenon is observed most notably in birds (like those that make long, transoceanic flights) and aquatic mammals (like dolphins and porpoises).

Unintended Sleep Episode

Sleep episode that is not planned and may happen during an activity in which such an episode is hazardous, such as when driving a car or working with machinery.


Wake Time

Total time that is scored awake in a polysomnogram occurring between sleep onset and final wake-up.