Sleep Glossary

A |B |C |D |E |F |G |H |I |J |K |L |M |N |O |P |Q |R |S |T |U |V |W |X |Y |Z |Abbreviations


Abdominal Movement - In diagnostic sleep studies, abdominal movement is recorded. This is one of the measures of respiratory effort, reflecting movement of the diaphragm. 

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. 

Ambulatory Monitor - Portable system used to record (continuously) multiple physiological variables during sleep. 

Apnea- Literally means "no breath"; the cessation of airflow at the nostrils and mouth for at least 10 seconds.

Apnea index (AI) - A measure of the severity of sleep apnea;  the number of apnea events per hour. - Apnea literally means "no breath"; the cessation of airflow at the nostrils and mouth for at least 10 seconds.

Apnea/Hypopnea index (AHI) - the number of apneas and hypopneas per hour.

Arousal - abrupt change from sleep to wakefulness, or from a "deeper" stage of non-REM sleep to a "lighter" stage 

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 is 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. 

Benzodiazepines - developed in the 1950's, this class of compounds tranquilize and sedates. 

Bi-Level - 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. The biological clock is found in a pair of tiny bilateral brain areas called the suprachiasmatic nuclei. 

Body Position - four positions are identified 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. 

Bruxism – teeth grinding during sleep 


Cataplexy - sudden, dramatic decrement in muscle tone and loss of deep reflexes that leads to muscle weakness, paralysis, or postural collapse.  Usually caused by outburst of emotion:  laughter, startle, or sudden physical exercise; one of the tetrad of symptoms of narcolepsy. 

Central sleep apnea - This is a sleep disorder in which the airway is not blocked but the brain fails to signal the muscles to breathe.

Cheyne-Stokes respiration - breathing pattern typified by regular "crescendo-decrescendo" or waxing and waning fluctuations in respiratory rate and tidal volume. 

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

Circadian rhythms - These biological rhythms 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 might contribute to insomnia. 

Compliance - adhering to or conforming with a regimen of treatment such as CPAP 

CPAP - 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 (cm H20). The positive pressure can range from 5 - 20 cm H20. Different patients require different pressures. The value is determined in a CPAP titration study. 


Deep Sleep - refers to combined non-REM sleep stages 3 and 4 in sleep studies 

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. 

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

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. 

DME - Durable Medical Equipment. Equipment such as wheelchairs and walkers which are prescribed for use by or on the order of a physician, also includes CPAP and BI-Level machines. 

Drowsiness, Drowsy - quiet wakefulness occurring prior to sleep onset. 


Electrodes - small devices transmitting biological electrical activity from subject to polygraph Sleep Glossary

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 and reflect the changes in resting muscle activity.  

ENT - Ear, Nose and Throat.  A doctor specializing in diseases of the Ear, Nose and Throat.  These specialists often do surgery as well, and may be referred to as an ENT surgeon. 

Epworth Sleepiness Scale - index of sleep propensity during the day as perceived by patients, and derived from the answers to 8 questions. 

Excessive daytime sleepiness or somnolence (EDS) - subjective report of difficulty in staying awake, accompanied by a ready entrance into sleep when the individual is sedentary 


Fatigue - feeling of tiredness or weariness usually associated with performance decrements 


Habitual Snorers - those who snore nearly every night.

Humidification - moisture is added to the airflow as an adjunct to CPAP (Continuous Positive Airway Pressure) therapy in treating obstructive sleep apnea (OSA). 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. 

Hypersomnia – excessive, prolonged sleep.

Hypertension -High blood pressure. 

Hypnotics - Sleep-inducing drugs. 

Hypocretin-  A natural hormone that helps regulate periods of sleep or wakefulness.

Hypopnea - shallow breathing in which the air flow in and out of the airway is less than half of normal--usually associated with oxygen desaturation. 


Insomnia – complaint describing difficulty in sleeping 


Jet Lag - disturbance induced by a major rapid shift in environmental time during travel to a new time zone.


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

Light Sleep - term used to describe non-REM sleep stage 1, and sometimes, stage 2. 

Light Therapy - used in the treatment of SAD (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. 


Maintenance of Wakefulness Test (MWT) - measures how alert you are during the day and shows whether or not you are able to stay awake for a defined period of time. .

Melatonin - hormone secreted by the brain’s pineal gland.

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

Mixed sleep apnea - This is a combination of central sleep apnea and obstructive sleep apnea.

Multiple Sleep Latency Test (MSLT) - a series “nap tests” utilized in the assessment of excessive daytime sleepiness. 


Narcolepsy - This is a neurological condition in which people experience excessive daytime sleepiness, cataplexy, sleep paralysis, hallucinations, and intermittent, uncontrollable sleep attacks during the daytime.

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

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. 

Nocturia - excessive, often frequent, urination during the night.

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.  It consists of stages 1, 2 (light sleep) and 3,4 (deep sleep). 


Obstructive sleep apnea (OSA) - This is the most common kind of sleep apnea. It is caused by a blockage of the upper airway that leads to a cessation of airflow (at least 10 seconds);  cessation of breathing during sleep, due to an obstruction, such as a semi-collapsed trachea, tongue relaxed to back of the throat, or a large amount of tissue in the throat.

Optimum Sleep - average amount of sleep needed every night by an individual. 


Parasomnias - These are abnormal behaviors that occur during sleep that interrupt sleep and can result in injury,Sleep Glossary insomnia, and/or excessive daytime sleepiness.

Periodic Limb Movement Disorder - also known as periodic leg movements and nocturnal myoclonus. Characterized by periodic episodes of repetitive and highly stereotyped limb movements occuring 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. There can be marked night-to-night variability in the number of movements or in the existence of movements. 

Phase advance - movement to a position earlier in the 24 hour sleep - wake cycle of a period of sleep or wake; for example, a shift of the sleep phase from 11 p.m. - 7 a.m. to 8 p.m. - 4 a.m. 

Polysomnography - This is a test that records sleep architecture (such as the amount of NREM and REM sleep, number of arousals) and a variety of body functions during sleep, including breathing patterns, heart rhythms, and limb movements.

Polysomnographic Technologist - health care professional trained in performing diagnostic sleep studies.

Positional Sleep Apnea - a total apnea-hypopnea index (AHI) > or = 5 with a > 50% reduction in the AHI between the supine and nonsupine postures, and an AHI that normalizes (AHI < 5) in the nonsupine posture.


RDI - Respiratory Disturbance Index, includes all respiratory events per hour. 

Rapid Eye Movement (REM) Sleep- one of the two basic states of sleep.  REM sleep is 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 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. 

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. 


Shiftwork - working hours outside of the conventional daytime hours of 9:00 a.m. to 5:00 p.m.

Sleep apnea - This is a serious sleep disorder that occurs when a person’s breathing is interrupted during sleep. Means the cessation of breathing for 10 or more seconds during sleep.

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

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 latency -  amount of time it takes to fall asleep after the lights have been turned off 

Sleep paralysis - This is a symptom of narcolepsy. It involves the temporary inability to move or speak while falling asleep or waking up. 

Sleep stage NREM - major sleep state apart from REMS; comprises sleep stages 1-4.

Sleep stage 1 - a stage of NREM sleep occurring after wake. Stage 1 normally assumes 4-5% of total sleep. 

Sleep stage 2 - a stage of NREM sleep characterized by sleep spindles and K complexes and usually accounts for 45-55% of total sleep time. 

Sleep stage 3 - 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 - 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.

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.

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). 

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. 

Slow wave sleep (SWS) - sleep stages 3 and 4.

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. 


Titration - progressive, stepwise increase in CPAP pressure applied during a polysomnogram to establish the optimal treatment pressure.

Transient Insomnia - difficulty sleeping for only a few nights'


Uvulopalatopharyngoplasty (UPPP) - also abbreviated as UPP or UP3 this operation is performed on the throat to treat snoring and sleep apnea.  UPPP is an accepted means of surgical treatment has a curative rate of less than 50%.  


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


AHI – Apnea/Hypopnea Index 

AI – Apnea Index 

APAP (Auto-CPAP) - auto-titrating, self-adjusting device

Bi-PAP (BPAP) - Trademark name of a Bi-Level cpap machine.

CPAP - Continuous Positive Airway Pressure (a nasal device to relieve obstructed breathing in a sleeping patient) 

EDS - Excessive Daytime Sleepiness 

MSLT - Multiple Sleep Latency Test 

MWT - Maintenance of Wakefulness Test 

NREM – Non-Rapid Eye Movement (sleep)

OSA - Obstructive sleep apnea

PLMD - Periodic Leg Movement Disorder

PLMS - Periodic Leg Movements of Sleep

PSG - Polysomnogram 

REM - Rapid Eye Movement (sleep)

RLS - Restless Leg Syndrome

UPPP - Uvulopalatopharyngoplasty 

VPAP - Variable positive airway pressure

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