Sleep Glossary

Sleep Glossary | Sleep Terms A – Z


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.

Altitude Insomnia

Insomnia that occurs when a person goes to higher altitudes. Usually accompanied by headaches, loss of appetite, and fatigue. Also called Acute mountain sickness, Acosta`s disease, Alpine sickness, or hypobaropathy.


Literally means “no breath”, the cessation of airflow at the nostrils and mouth for at least 10 seconds.

Apnea Index

A measure of the severity of sleep apnea; the number of apnea events per hour.

Apnea/Hypopnea Index

The number of apneas and hypopneas per hour. 5-20=mild, 21-50=moderate, above 51 severe.


Sudden change from sleep to wakefulness, or from a “deeper” stage of non-REM sleep to a “lighter” stage.

Arousal Disorder

Parasomnia disorder supposed to be due to an abnormal arousal function. Classical arousal disorders include sleepwalking, sleep terrors and confusional arousals.

Arousal Threshold

The ease that a sleeping person is awakened.

Auto Adjusting Continuous Positive Airway Pressure Device (SmartPAP) (Auto-PAP)

A type of CPAP machine monitoring changes in breathing and compensates automatically by making appropriate adjustments in pressure.

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.

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.


Bi-level pressure device used to treat sleep apnea. The “bi” refers to two pressures: a lower pressure for exhalation and a higher pressure for inhalation. Bi-Level machines are more expensive than a standard CPAP, but some patients tolerate it better because they can exhale comfortably against the constant inhalation pressure. (Sometimes called Bi-PAP, but that is a trademark name of one system).

Biological Clock

Term for the brain process causing us to have 24-hour fluctuations in body temperature, hormone secretion, and other bodily activities. The most important function fosters the daily alternation of sleep and wakefulness.


Teeth grinding or jaw clenching during sleep. The term clenching means you tightly clamp your top and bottom teeth together, especially the back teeth.


Sudden muscle weakness associated with narcolepsy. It is often triggered by emotions such as anger, surprise, laughter, and exhilaration. No loss of consciousness is involved – i.e. it is not a black out or a faint, and, despite the phonetic similarity of ‘narcolepsy’ and ‘cataplexy’ with ‘epilepsy’, cataplexy is not epileptic in nature. You are fully conscious, you just can’t move.

Central Apnea

Absence of airflow and inspiratory effort; apnea caused by irregularity in the brain’s control of breathing.

Circadian Rhythm

Innate, daily, fluctuation of behavioral and physiological functions, including sleep waking, generally tied to the 24-hour day-night cycle but sometimes to a different (e.g., 23 or 25 hour) periodicity when light/dark and other time cues are removed.


Continuous Positive Airway Pressure; the device used to treat sleep apnea by sending positive airway pressure at a constant, continuous pressure to help keep an open airway, allowing the patient to breathe normally through his/her nose and airway.

CPAP Pressure

Pressure needed to maintain an open airway in a sleep apnea patient treated with CPAP, expressed in centimeters of water. The positive pressure can range from 5 – 20 cm H20. Different patients require different pressures.

Deep Sleep

Refers to combined non-REM sleep stages 3 and 4 in sleep studies.

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 lab or in a patient’s home with portable recording equipment.


A disorder of sleep or wakefulness; not a parasomnia.

Electrocardiography (EKG)

A method of measuring the electrical activity of the heart.

Electroencephalogram (EEG)

Recording through the scalp of electrical potentials from the brain and the changes in these potentials. The EEG is one of the three basic variables (along with the EOG & EMG) used to score sleep stages and waking. Surface electrodes are used to record sleep in humans, recording potential differences between brain regions and a neutral reference point, or between brain regions.

Electromyogram (EMG)

Recording of electrical activity from the muscular system; in sleep recording, synonymous with resting muscle activity or potential. The chin EMG, along with EEG and EOG, is one of the three basic variables used to score sleep stages and waking. Surface electrodes are used to record sleep in humans, measuring activity from the submental or masseter muscles. These reflect the changes in resting muscle activity. 

Electro-oculogram (EOG)

Recording of voltage changes resulting from shifts in position of the eyeball-possible because each globe is a positive (anterior) and negative (posterior) dipole; along with the EEG and the EMG, one of the three basic variables used to score sleep stages and waking. Human sleep recordings utilize surface electrodes placed near the eyes to record the movement of the eyeballs. Rapid eye movements in sleep indicate a certain stage of sleep.


Expiratory Positive Airway Pressure. Pressure prescribed for the expiratory (breathing out) phase of an individual on Bi-level CPAP therapy for OSA (obstructive sleep apnea).

Epworth Sleepiness Scale

Index of sleep propensity during the day as perceived by patients, and derived from the answers to 8 questions.

Esophageal Pressure

Measurement used to determine respiratory effort and by inference, airway resistance. Considered an invasive measure, generally used only in polysomnographic testing, conducted in sleep disorders centers.

Excessive Daytime Sleepiness

Subjective report of difficulty in staying awake, accompanied by a ready entrance into sleep when the individual is sedentary. EDS suggests the presence of a sleep disorder and is different from fatigue. Depression, anxiety, stress, and boredom are commonly thought to cause excessive sleepiness, but in fact these conditions cause fatigue and apathy.


Feeling of tiredness or weariness usually associated with performance decrements.

Flattening Index

Number indicating the amount of airflow limitation caused by partial closure of the upper airway. 0.3 indicates an open airway, 0.15 is mildly obstructed, 0.1 is severely limited airflow, and 0.0 reflects a totally closed airway.  Flattening Index is used to identify the condition known as Upper Airway Resistance Syndrome (UARS), and is continuously recorded in both diagnostic sleep studies and CPAP titrations.

Flow Limitation

The partial closure of the upper airway impeding the flow of air into the lungs.

GABA (Gamma-Amniobutyric Acid)

Major neurotransmitter in the brain, which is considered to be involved in muscle relaxation, sleep, diminished emotional reaction and sedation.

Genioglossus Tongue Advancement

A possible surgical treatment used for sleep apnea and/or snoring, improving the airway behind the base of the tongue. The genioglossus, the main tongue muscle, relaxes during sleep, often allowing the tongue to fall into the airway. The muscle attaches to the middle of the lower jaw. A segment of bone containing this muscle is pulled forward and stabilized, opening the airway space behind the tongue. 

Habitual Snorers

Those who snore nearly every night.

Histogram (sleep)

Graph indicating sleep stages throughout the night.


Moisture is added to the airflow as an adjunct to CPAP therapy in treating obstructive sleep apnea. Humidification can be added to the CPAP by diverting the airflow over or through a cool or heated water reservoir (humidifier) to prevent the upper airway from drying out.

Hyoid Suspension

A possible surgical procedure sometimes used in the treatment of sleep apnea and/or snoring, designed to improve the airway behind the base of the tongue. The hyoid bone is located in the neck where some tongue muscles attach. The hyoid bone is pulled forward in front of the voice box and can open the airway space behind the tongue.


Extreme irritability; seen in sleep deprived subjects.


Excessive, prolonged sleep.

Hypnagogic Imagery (Hallucinations)

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.


Morbid fear of falling asleep.


Sleep-inducing drugs.

Inappropriate Sleep Episodes

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


Complaint describing difficulty in sleeping.


Inspiratory Positive Airway Pressure. Physician prescribed pressure for the inspiratory phase on a Bi-level CPAP device.

Jet Lag

Disturbance induced by a major rapid shift in environmental time during travel to a new time zone.

Light-Dark Cycle

Periodic pattern of light (artificial or natural) alternating with darkness.

Light Sleep

Term used to describe non-REM sleep stage 1, and sometimes, stage 2.

Light Therapy

Used in the treatment of Seasonal Affective Disorder and other conditions. Exposes the eyes to light of appropriate intensity and duration and at the appropriate time of day to effect the timing, duration and quality of sleep.

Limit-Setting Sleep Disorder

Disorder due to child’s difficulty in falling asleep by delaying and refusing to go to bed.

Mandibular Maxillary Osteotomy and Advancement (MMOA)

Procedure developed for patients with retrolingual obstruction, patients with retropalatal and retrolingual obstruction who have not responded to CPAP and uvulopalatopharyngoplasty (UPPP).


Hormone secreted by the brain’s pineal gland.


Partial awakening from sleep.


Period lasting up to a few seconds during which the polysomnogram suddenly shifts from waking characteristics to sleep.

Mixed (sleep) Apnea

Interruption in breathing during sleep beginning as a central apnea then becoming an obstructive apnea.

Mixed (sleep) Apnea

Interruption in breathing during sleep beginning as a central apnea then becoming an obstructive 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

The absence of muscle activity during sleep.

Multiple sleep latency test (MSLT)

A series “nap tests” utilized in the assessment of excessive daytime sleepiness.


Muscle contractions in the form of “jerks” or twitches.


Sleep disorder characterized by excessive sleepiness, cataplexy, sleep paralysis, hypnogogic hallucinations, and an abnormal tendency to pass directly from wakefulness into REM sleep.

Natural Short Sleeper

Person who normally sleeps less than five hours a night with no adverse effects.


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

Night Terrors

Also known as sleep terrors. 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.


Excessive, often frequent, urination during the 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 Sleep-Related Eating Disorder (NS-RED)

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

Nocturnal Enuresis (Bedwetting)

Urinating while asleep.

NREM or Non-REM Sleep

Characterized by slower and larger brain waves and little or no dream behavior; quiet sleep, slow-wave sleep; approximately 80% of 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.

Obesity-Hypoventilation Syndrome

Term applied to obese individuals hypoventilating during wakefulness.

Obstructive Apnea

Cessation of airflow (at least 10 seconds) in the presence of continued inspiratory effort, cessation of breathing during sleep, due to a mechanical obstruction, such as a semi-collapsed trachea, tongue relaxed to back of the throat, or a large among of tissue in the uvula area.

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.

Ondine’s Curse

The respiratory center in the brain is unable to stimulate breathing in response to an increased amount of carbon dioxide in the blood. Ondine’s Curse or central alveolar hypoventilation typically worsens during sleep.

Optimum Sleep

Average amount of sleep needed every night by an individual.


An event happening during sleep, or induced or exacerbated by sleep, such as sleepwalking or asthma; not a dyssomnia.

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

Also known as periodic leg movements and nocturnal myoclonus. Characterized by periodic episodes of repetitive and highly stereotyped limb movements occurring during sleep. The movements are often associated with a partial arousal or awakening; however, the patient is usually unaware of the limb movements or frequent sleep disruption. Between the episodes, the legs are still.

Persistent Insomnia

Continuing insomnia responding poorly to treatment.

Pickwickian Syndrome

Obesity accompanied by somnolence, lethargy, chronic hypoventilation, hypoxia, and secondary polycythemia (a condition marked by an abnormal increase in the number of circulating red blood cells); usually has severe obstructive sleep apnea.

Pineal Gland

Gland in the brain secreting the hormone melatonin.

Polysomnogram (PSG)

Continuous and simultaneous recording of physiological variables during sleep, i.e., EEG, EOG, EMG (the three basic stage scoring parameters), EKG, respiratory air flow, respiratory excursion, lower limb movement, and other electrophysiological variables.


Biomedical instrument for the measurement of multiple physiological variables of sleep.

Post-Traumatic Stress Disorder

Re-experiencing of a traumatic event in the form of repetitive dreams, recurrent and intrusive daytime recollections, and/or dissociative flashback episodes.

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.

Prescribed CPAP Pressure

Pressure(s) or settings determined by a CPAP titration sleep study, which a physician prescribes for a patient’s CPAP therapy machine.


Respiratory Disturbance Index, includes all respiratory events per hour.

REM Sleep, Rapid Eye Movement Sleep

Sleep characterized by the active brain waves, flitting motions of the eyes, and weakness of the muscles; most dreaming occurs in this stage, which accounts for about 20% of sleep in adults.

REM-Associated Disorders

Sleep disturbances that occur in REM sleep.


REMS Latency

The period of time in the sleep period from sleep onset to the first appearance of stage REMS.

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 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 Rebound

Compensatory increase in REM sleep following experimental reduction. Extension of time in, and an increase in frequency and density of REM sleep episodes; usually an increase in REM sleep percent of total sleep time above baseline values.

Restless Legs Syndrome (RLS)

Sleep disorder characterized by a deep creeping, or crawling sensation in the legs that tends to occur when an individual is not moving. There is an almost irresistible urge to move the legs; the sensations are relieved by movement.

Seasonal Affective Disorder (SAD)

A form of depression caused by inadequate bright light affecting the biological clock during the late autumn and winter. Treatment often involves the use of light therapy.


Compounds tending to calm, and reduce nervousness or excitement and foster sleep.


Neurotransmitter in the brain that modulates mood, appetite, sexual activity, aggression, body temperature and sleep.


Working hours outside of the conventional daytime hours of 9:00 a.m. to 5:00 p.m.


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

Sleep Apnea

Cessation of breathing for 10 or more seconds during sleep.

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 Efficiency (SE)

Proportion of sleep in the period potentially filled by sleep–ratio of total sleep time to time in bed.

Sleep Hygiene

Conditions and practices that promote continuous and effective sleep, including regularity of bedtime and arise time; conforming time spent in bed to the time necessary for sustained and individually adequate sleep (i.e., the total sleep time sufficient to avoid sleepiness when awake); restriction of alcohol and caffeine beverages in the period prior to bedtime; employment of exercise, nutrition, and environmental factors so that they enhance, not disturb, restful 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 period measured from “lights out,” or bedtime, to the beginning of sleep.

Sleep Log (-diary)

Daily, written record of an individual’s sleep-wake pattern containing such information as time of retiring and arising, time in bed, estimated total sleep period, number and duration of sleep interruptions, quality of sleep, daytime naps, use of medications or caffeine beverages, nature of waking activities, and other data.

Sleep Mentation

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

Sleep Paralysis

Waking and not being able to move for a short period of time, usually occurs out of REM (dream) sleep.

Sleep Related Accidents

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

Sleep stage 1

A stage of NREM sleep occurring after wake. Its criteria consist of a low-voltage EEG with slowing to theta frequencies, alpha activity less than 50%, EEG vertex spikes, and slow rolling eye movements; no sleep spindles, K-complexes, or REMS.  Stage 1 normally assumes 4-5% of total sleep.

Sleep stage 2

A stage of NREM sleep characterized by sleep spindles and K complexes against a relatively low-voltage, mixed-frequency EEG background; high-voltage delta waves may comprise up to 20% of stage 2 epochs; usually accounts for 45-55% of total sleep time.

Sleep stage 3

A stage of NREM sleep defined by at least 20 and not more than 50% of the period (30 second epoch) consisting of EEG waves less than 2 Hz and more than 75 uV (high -amplitude delta waves); a “delta” sleep stage; with stage 4, it constitutes “deep “NREM sleep; appears usually only in the first third of the sleep period; usually comprises 4-6% of total sleep time.

Sleep stage 4

All statements concerning NREM stage 3 apply to stage 4 except that high-voltage, slow EEG waves, cover 50% or more of the record; NREM stage 4 usually takes up 12-15% of total sleep time. Somnambulism, sleep terror, and sleep-related enuresis episodes generally start in stage 4 or during arousals from this stage

Sleep stage REM

The stage of sleep found in all mammal studies, including man, in which brain activity is extensive, brain metabolism is increased, and vivid hallucinatory imagery, or dreaming occurs (in humans).  Also called “paradoxical sleep” because, in the face of this intense excitation of the CNS and presence of spontaneous rapid eye movements, resting muscle activity is suppressed.  The EEG is a low-voltage, fast-frequency, non-alpha record.  Stage REMS is usually 20-25% of total sleep time.

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.

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).

SmartPAP (Smart CPAP)

(Smart Continuous Positive Airway Pressure) Medical device used in the treatment of obstructive sleep apnea providing preset levels of continuous airflow, and automatically adjusting to keep the breathing passages open by sensing changes in airway integrity. The air flows from the device through a tube that connects to a nose or face mask.
Snoring – 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.


Walking while asleep.

Transient Insomnia

Difficulty sleeping for only a few nights.

Unattended CPAP Titration Study

Sleep study that is usually performed in the home, after determining that a patient has a sleep related breathing disorder such as OSA or Upper Airway Resistance Syndrome, and is likely to benefit from CPAP therapy. 

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.

White Noise

Mixture of sound waves extending over a wide frequency range that may be used to mask unwanted noise that may interfere with sleep.

Wilkinson Addition Test

Performance test; numbers added for one hour. Often included in a battery of tests to measure the impact of acute or chronic sleep loss.