Sleep Glossary: Sleep Medicine and CPAP Terms Explained

A

APAP (auto-adjusting PAP): An APAP machine automatically raises and lowers pressure within a prescribed range in response to detected breathing events, flow limitation and snoring. It is often used to start therapy without a lab titration in suitable adults.See also: CPAP (continuous positive airway pressure), CPAP titration, CPAP pressure
ASV (adaptive servo-ventilation): Adaptive servo-ventilation is a PAP mode that monitors breathing and adjusts pressure support breath by breath to smooth out central apneas and periodic breathing. It is not recommended for central sleep apnea with heart failure and an ejection fraction of 45 percent or less.See also: Central sleep apnea (CSA), Cheyne-Stokes breathing, Pressure support
Actigraphy: Actigraphy uses a wrist worn motion sensor, worn for days to weeks, to estimate sleep and wake patterns. Clinicians use it to evaluate circadian rhythm disorders and insomnia, often alongside a sleep diary.See also: Sleep diary, Circadian rhythm sleep-wake disorder
Adenosine: Adenosine is a chemical that builds up in the brain during waking hours and promotes sleepiness. Sleep clears it. Caffeine works by blocking adenosine receptors.See also: Sleep pressure (homeostatic sleep drive), Caffeine
Adenotonsillectomy: Adenotonsillectomy is surgery to remove the tonsils and adenoids. It is the usual first treatment for obstructive sleep apnea in children with enlarged tonsils and adenoids.See also: Obstructive sleep apnea (OSA), Upper airway
Advanced sleep-wake phase disorder: In advanced sleep-wake phase disorder the body clock runs early, so people feel sleepy in the early evening and wake very early. It is more common in older adults. Evening light exposure is sometimes used.See also: Circadian rhythm sleep-wake disorder, Chronotype, Bright light therapy
Aerophagia: Aerophagia means swallowing air. Some CPAP users swallow air during the night, causing bloating, burping or stomach discomfort. Adjustments to pressure, comfort settings or mask type may help, so report it to your clinician.See also: CPAP pressure, EPR (expiratory pressure relief)
Apnea: An apnea is a pause in breathing during sleep. In adult sleep scoring it is a drop in airflow of at least 90 percent lasting at least 10 seconds. Apneas are classed as obstructive, central or mixed depending on whether breathing effort continues.See also: Hypopnea, Obstructive sleep apnea (OSA), Central apnea, Apnea hypopnea index (AHI)
Apnea hypopnea index (AHI): The AHI is the number of apneas plus hypopneas per hour of sleep. It is the main measure of sleep apnea severity: in adults, under 5 is normal, 5 to under 15 mild, 15 to under 30 moderate and 30 or more severe. CPAP machines also report an estimated AHI during treatment.See also: Apnea, Hypopnea, Respiratory disturbance index (RDI), Respiratory event index (REI), Residual AHI
Arousal: In sleep scoring, an arousal is an abrupt shift in brain wave frequency lasting at least 3 seconds, often without full awakening. Frequent arousals from breathing events or leg movements fragment sleep and cause daytime sleepiness.See also: Arousal index, Respiratory effort related arousal (RERA), Hypopnea
Arousal index: The arousal index is the number of arousals per hour of sleep in a sleep study. A high arousal index indicates fragmented sleep, which may come from breathing events, limb movements or other causes.See also: Arousal, Polysomnography (PSG)

B

Backup rate: A backup rate is a minimum number of breaths per minute that a bilevel device will deliver if your own breathing slows or stops. Devices with this feature are called BPAP ST and are used for some central apnea and hypoventilation conditions.See also: BiPAP (bilevel positive airway pressure), Central sleep apnea (CSA), Obesity hypoventilation syndrome (OHS)
BiPAP (bilevel positive airway pressure): Bilevel PAP, often called BiPAP after a brand name, uses a higher pressure when you breathe in (IPAP) and a lower pressure when you breathe out (EPAP). It helps people who cannot tolerate high CPAP pressures and those with hypoventilation.See also: IPAP (inspiratory positive airway pressure), EPAP (expiratory positive airway pressure), CPAP (continuous positive airway pressure), Backup rate
Bright light therapy: Light therapy uses timed exposure to bright light, usually from a light box, to shift the body clock. Morning light moves it earlier and evening light moves it later. It is used for circadian rhythm disorders and some mood disorders.See also: Light exposure and sleep, Delayed sleep-wake phase disorder (DSWPD), Zeitgeber

C

CPAP (continuous positive airway pressure): CPAP is a treatment that delivers a steady stream of pressurized air through a mask to hold the upper airway open during sleep. It is the standard first treatment for moderate to severe obstructive sleep apnea. Benefits depend on using it every night for most of the night.See also: APAP (auto-adjusting PAP), BiPAP (bilevel positive airway pressure), CPAP mask, PAP adherence (compliance), Obstructive sleep apnea (OSA)
CPAP desensitization: Desensitization is a gradual practice program for people who feel anxious or claustrophobic with a CPAP mask. It typically starts with holding the mask to the face while awake, then wearing it with airflow while relaxing, before using it for sleep.See also: PAP adherence (compliance), CPAP mask
CPAP filter: Filters clean the air drawn into a CPAP machine. Disposable filters are replaced often (Medicare allows up to 2 a month) and reusable filters are rinsed and replaced about every 6 months.See also: CPAP (continuous positive airway pressure), CPAP tubing
CPAP mask: A CPAP mask (interface) delivers pressurized air to your airway. The main types are nasal pillows, nasal masks and full face masks. Fit and comfort strongly affect how consistently people use CPAP.See also: Nasal pillows, Nasal mask, Full face mask, Mask leak
CPAP pressure: CPAP pressure is the air pressure the machine delivers, measured in centimeters of water (cm H2O). Most CPAP machines can deliver about 4 to 20 cm H2O, and the prescribed setting comes from titration or auto PAP data.See also: cm H2O (centimeters of water), CPAP titration, APAP (auto-adjusting PAP)
CPAP titration: Titration is the process of finding the PAP pressure that prevents apneas, hypopneas, snoring and flow limitation. It is done in a sleep lab with a technologist adjusting pressure overnight, or by reviewing APAP data at home.See also: Split-night study, Polysomnography (PSG), APAP (auto-adjusting PAP), CPAP pressure
CPAP tubing: CPAP tubing (hose) connects the machine to the mask. Standard tubing is usually about 6 feet long. Medicare allows replacement up to once every 3 months for standard or heated tubing.See also: Heated tubing, CPAP mask
Caffeine: Caffeine is a stimulant found in coffee, tea, soft drinks and energy drinks that blocks adenosine and reduces sleepiness. Its half-life averages around 5 hours in adults, so afternoon caffeine can still disturb night sleep.See also: Adenosine, Sleep hygiene
Cataplexy: Cataplexy is a sudden, brief loss of muscle tone triggered by strong emotions such as laughter, while the person stays conscious. It can range from a slack jaw to collapse and is characteristic of narcolepsy type 1.See also: Narcolepsy, REM atonia, Orexin (hypocretin)
Central apnea: A central apnea is a breathing pause in which the brain does not send the signal to breathe, so there is no breathing effort. Occasional central apneas at sleep onset can be normal, but frequent ones suggest central sleep apnea.See also: Apnea, Central sleep apnea (CSA), Mixed apnea
Central sleep apnea (CSA): Central sleep apnea is a disorder in which breathing repeatedly stops because the brain's drive to breathe pauses, not because the airway is blocked. It is associated with heart failure, stroke, opioid use and high altitude, and can appear when starting CPAP. Treatment depends on the cause and may include CPAP, bilevel PAP with a backup rate or ASV.See also: Central apnea, Cheyne-Stokes breathing, ASV (adaptive servo-ventilation), Treatment emergent central sleep apnea
Cheyne-Stokes breathing: Cheyne-Stokes breathing is a waxing and waning pattern in which breathing gradually gets deeper, then shallower, then pauses as a central apnea. It is seen most often in heart failure and after stroke.See also: Central sleep apnea (CSA), ASV (adaptive servo-ventilation)
Chinstrap: A chinstrap is a fabric strap that supports the jaw to help keep the mouth closed during CPAP use with a nasal mask or pillows. Medicare allows one every 6 months.See also: Mouth leak, Nasal pillows
Chronic insomnia disorder: Chronic insomnia disorder is diagnosed when sleep difficulty occurs at least three nights a week for at least three months and causes daytime problems. CBT-I is the recommended first line treatment for adults.See also: Insomnia, Cognitive behavioral therapy for insomnia (CBT-I), Hyperarousal
Chronotype: Chronotype is a person's natural preference for earlier or later sleep and activity, from morning types (larks) to evening types (owls). It is partly genetic and changes with age, becoming later in adolescence and earlier in older age.See also: Circadian rhythm, Social jet lag, Delayed sleep-wake phase disorder (DSWPD)
Circadian rhythm: A circadian rhythm is a roughly 24 hour cycle in the body's physiology and behavior, including sleepiness, body temperature and hormone release. It is driven by a master clock in the brain and kept in time mainly by light.See also: Suprachiasmatic nucleus (SCN), Melatonin, Zeitgeber, Chronotype
Circadian rhythm sleep-wake disorder: Circadian rhythm sleep-wake disorders occur when the body clock is out of step with the desired or required sleep schedule, causing insomnia or sleepiness. Examples include delayed and advanced sleep phase, shift work disorder, jet lag and non-24 hour rhythm disorder.See also: Delayed sleep-wake phase disorder (DSWPD), Advanced sleep-wake phase disorder, Shift work disorder, Jet lag disorder, Non-24-hour sleep-wake rhythm disorder
Cognitive behavioral therapy for insomnia (CBT-I): CBT-I is a structured program, usually 4 to 8 sessions, that changes the behaviors and thoughts that keep insomnia going. It combines stimulus control, sleep restriction, cognitive therapy, relaxation and sleep education. It is the recommended first treatment for chronic insomnia in adults.See also: Stimulus control, Sleep restriction therapy, Cognitive therapy for insomnia, Chronic insomnia disorder
Cognitive therapy for insomnia: The cognitive part of CBT-I identifies and challenges unhelpful beliefs about sleep, such as catastrophic worry about the effects of a poor night. Reducing these worries lowers the arousal that keeps people awake.See also: Cognitive behavioral therapy for insomnia (CBT-I), Hyperarousal
Core body temperature rhythm: Core body temperature follows a circadian rhythm, falling in the evening and reaching its lowest point in the early morning hours. The evening decline is linked with sleep onset, which is one reason a cool bedroom helps sleep.See also: Circadian rhythm, Sleep hygiene
cm H2O (centimeters of water): Centimeters of water is the unit used for CPAP pressure. One cm H2O is the pressure of a 1 cm column of water, equal to 0.98 hectopascals (hPa).See also: CPAP pressure

D

DME supplier: A durable medical equipment (DME) supplier provides CPAP machines, masks and supplies, fits masks, and often downloads usage data for insurers. Medicare requires suppliers to give instruction on using and caring for the equipment.See also: CPAP (continuous positive airway pressure), PAP adherence (compliance)
Delayed sleep-wake phase disorder (DSWPD): In DSWPD the body clock runs late, so people cannot fall asleep until very late and have great difficulty waking early, but sleep normally when allowed to keep their own schedule. It is most common in teens and young adults. Timed light and melatonin may help.See also: Circadian rhythm sleep-wake disorder, Chronotype, Bright light therapy
Dim light melatonin onset (DLMO): DLMO is the time in the evening when melatonin levels start to rise under dim light. It is the most reliable marker of circadian timing and is used in research and in diagnosing circadian rhythm disorders.See also: Melatonin, Circadian rhythm, Delayed sleep-wake phase disorder (DSWPD)
Distilled water: CPAP manufacturers generally recommend distilled water in the humidifier because minerals in tap water can leave deposits in the chamber. Follow the instructions for your machine.See also: Heated humidifier, Water chamber

E

EPAP (expiratory positive airway pressure): EPAP is the lower pressure a bilevel machine delivers while you breathe out. It keeps the airway open at the end of each breath, which prevents obstruction.See also: IPAP (inspiratory positive airway pressure), BiPAP (bilevel positive airway pressure), Pressure support
EPR (expiratory pressure relief): EPR is a comfort setting, named by ResMed, that lowers pressure slightly when you breathe out so exhaling feels easier. Other manufacturers use similar features under different names, such as Philips C-Flex. It is different from true bilevel therapy.See also: Pressure relief, CPAP (continuous positive airway pressure), BiPAP (bilevel positive airway pressure)
Electroencephalogram (EEG): An EEG records the brain's electrical activity through small sensors on the scalp. In sleep studies it is used to identify wake, the stages of NREM sleep and REM sleep.See also: Polysomnography (PSG), Electrooculogram (EOG), Electromyogram (EMG)
Electromyogram (EMG): An EMG records muscle activity. In sleep studies, chin EMG helps identify REM sleep, when muscle tone drops, and leg EMG detects periodic limb movements.See also: Electroencephalogram (EEG), REM atonia, Periodic limb movement disorder (PLMD)
Electrooculogram (EOG): An EOG records eye movements using sensors near the eyes. It helps identify REM sleep, with its rapid eye movements, and the slow rolling eye movements of drowsiness.See also: Electroencephalogram (EEG), REM sleep
Epworth Sleepiness Scale (ESS): The Epworth Sleepiness Scale is a questionnaire about how likely you are to doze in eight everyday situations, giving a score from 0 to 24. Scores above 10 suggest excessive daytime sleepiness. It was developed by Dr. Murray Johns in 1991.See also: Excessive daytime sleepiness (EDS), STOP-Bang questionnaire, Multiple Sleep Latency Test (MSLT)
Excessive daytime sleepiness (EDS): Excessive daytime sleepiness is difficulty staying awake and alert during the day, with unintended dozing. Common causes include insufficient sleep, sleep apnea, narcolepsy, medicines and depression.See also: Epworth Sleepiness Scale (ESS), Narcolepsy, Obstructive sleep apnea (OSA)

F

Full face mask: A full face (oronasal) mask covers both the nose and mouth. It is often used by mouth breathers, people with frequent nasal congestion and people on higher pressures.See also: CPAP mask, Hybrid mask, Mouth leak

H

Headgear: Headgear is the set of straps that holds a CPAP mask in place. It should be snug but not tight; over-tightening often worsens leaks. Medicare allows replacement once every 6 months.See also: CPAP mask, Mask leak
Heated humidifier: A heated humidifier warms and adds moisture to the air from a CPAP machine. It helps with dry mouth, nosebleeds and nasal congestion. Most modern machines have one built in, filled with distilled water.See also: Heated tubing, Water chamber, Rainout, Distilled water
Heated tubing: Heated tubing has a built in wire that keeps the air warm on its way to the mask. It reduces rainout (condensation) and allows higher humidity settings in a cool bedroom.See also: Heated humidifier, Rainout, CPAP tubing
Home sleep apnea test (HSAT): A home sleep apnea test is a simplified sleep study you do in your own bed. It records breathing, oxygen and heart rate but usually not brain waves. It is suitable for adults with a high likelihood of uncomplicated moderate to severe obstructive sleep apnea.See also: Polysomnography (PSG), Respiratory event index (REI), Obstructive sleep apnea (OSA)
Hybrid mask: A hybrid mask covers the mouth and seals at or under the nose with pillows or a small cushion, without resting on the bridge of the nose. It is an option for mouth breathers who find full face masks confining.See also: Full face mask, Nasal pillows
Hyperarousal: Hyperarousal is a state of heightened mental and physical activation that is thought to underlie chronic insomnia. It can show up as racing thoughts at bedtime, elevated heart rate and a sense of being tired but wired.See also: Chronic insomnia disorder, Relaxation training
Hypnagogic hallucinations: Hypnagogic hallucinations are vivid sensory experiences while falling asleep; hypnopompic ones happen on waking. They are common in narcolepsy but can occur in healthy people, especially with sleep deprivation.See also: Sleep paralysis, Narcolepsy
Hypnic jerk: A hypnic jerk (sleep start) is a sudden, brief muscle twitch while falling asleep, sometimes with a feeling of falling. It is common and harmless.See also: N1 sleep
Hypnogram: A hypnogram is a graph that shows the sleep stages a person passes through across the night, from wake to N1, N2, N3 and REM. It makes the cycling pattern of sleep easy to see.See also: Polysomnography (PSG), Sleep cycle, N3 sleep (deep sleep)
Hypoglossal nerve stimulation: Hypoglossal nerve stimulation is an implanted device that stimulates the nerve controlling the tongue during sleep, moving the tongue forward to keep the airway open. It is used for selected adults with moderate to severe obstructive sleep apnea who cannot use CPAP.See also: Obstructive sleep apnea (OSA), Upper airway, CPAP (continuous positive airway pressure)
Hypopnea: A hypopnea is a partial reduction in breathing during sleep. Under the AASM recommended rule it is a drop in airflow of at least 30 percent for at least 10 seconds with an oxygen drop of at least 3 percent or an arousal. Medicare requires a 4 percent oxygen drop when scoring for coverage.See also: Apnea, Apnea hypopnea index (AHI), Oxygen desaturation, Arousal
Hypoxemia: Hypoxemia means a lower than normal level of oxygen in the blood. During sleep it can occur in bursts with apneas and hypopneas, or for longer periods with hypoventilation or lung disease.See also: SpO2 (oxygen saturation), Oxygen desaturation, Obesity hypoventilation syndrome (OHS)

I

IPAP (inspiratory positive airway pressure): IPAP is the higher pressure a bilevel machine delivers while you breathe in. The difference between IPAP and EPAP is the pressure support that helps each breath.See also: EPAP (expiratory positive airway pressure), BiPAP (bilevel positive airway pressure), Pressure support
Idiopathic hypersomnia: Idiopathic hypersomnia is a disorder of excessive daytime sleepiness without a known cause, often with long sleep times and extreme difficulty waking up. Unlike narcolepsy, it does not involve cataplexy.See also: Excessive daytime sleepiness (EDS), Narcolepsy, Multiple Sleep Latency Test (MSLT)
Insomnia: Insomnia is difficulty falling asleep, staying asleep or waking too early despite adequate opportunity to sleep, with daytime effects such as fatigue or poor concentration. Short term insomnia is common after stress; chronic insomnia lasts at least three months.See also: Chronic insomnia disorder, Cognitive behavioral therapy for insomnia (CBT-I), Insomnia Severity Index (ISI)
Insomnia Severity Index (ISI): The ISI is a seven item questionnaire, scored 0 to 28, that rates the severity and impact of insomnia over the past two weeks. Scores of 15 to 21 indicate moderate and 22 to 28 severe clinical insomnia. It is widely used to track response to treatment.See also: Insomnia, Cognitive behavioral therapy for insomnia (CBT-I), Sleep diary

J

Jet lag disorder: Jet lag is a temporary mismatch between the body clock and local time after flying across time zones, causing poor sleep, daytime fatigue and digestive upset. It is usually worse after eastward travel and when crossing more than 3 time zones.See also: Circadian rhythm sleep-wake disorder, Bright light therapy, Melatonin

K

K-complex: A K-complex is a large, sharp negative wave followed by a positive wave on the EEG, lasting at least half a second. It is a defining feature of N2 sleep and can occur in response to sounds.See also: N2 sleep, Sleep spindle, Electroencephalogram (EEG)

L

Light exposure and sleep: Light is the main signal that sets the body clock. Morning light tends to shift the clock earlier, while bright light in the evening and at night shifts it later and suppresses melatonin.See also: Zeitgeber, Melatonin, Bright light therapy

M

Maintenance of Wakefulness Test (MWT): The MWT measures how long you can stay awake while sitting quietly in a dim room over several trials. It is used to assess the ability to stay alert, for example in people with safety critical jobs or to check response to treatment.See also: Multiple Sleep Latency Test (MSLT), Excessive daytime sleepiness (EDS)
Mask leak: Mask leak is air escaping between the mask and face, beyond the vent leak built into the mask design. Large leaks reduce therapy effectiveness and can be noisy or dry the eyes. Better fitting, a different size or style, or replacing worn cushions often helps.See also: CPAP mask, Mouth leak, Mask liner
Mask liner: A mask liner is a soft fabric layer placed between the mask cushion and the skin. It can reduce skin irritation, marks and small leaks.See also: Mask leak, CPAP mask
Maxillomandibular advancement (MMA): MMA is a jaw surgery that moves the upper and lower jaws forward, enlarging the airway behind the tongue and palate. It is one of the more effective surgical options for obstructive sleep apnea in selected patients.See also: Uvulopalatopharyngoplasty (UPPP), Obstructive sleep apnea (OSA)
Melatonin: Melatonin is a hormone made by the pineal gland, mainly in darkness, that signals biological night to the body. Bright light, especially in the evening, suppresses it. In the US melatonin is also sold as a dietary supplement, which is not regulated as a drug and can vary in actual content.See also: Circadian rhythm, Dim light melatonin onset (DLMO), Suprachiasmatic nucleus (SCN)
Microsleep: A microsleep is a brief, unintended episode of sleep lasting a fraction of a second up to several seconds, often without the person realizing it. Microsleeps are a major danger in drowsy driving.See also: Sleep deprivation, Excessive daytime sleepiness (EDS)
Mixed apnea: A mixed apnea starts as a central apnea, with no breathing effort, and ends with obstructive effort against a closed airway. Mixed apneas are usually counted with obstructive events when grading severity.See also: Central apnea, Apnea, Obstructive sleep apnea (OSA)
Mouth leak: Mouth leak happens when air escapes through an open mouth while using a nasal mask or nasal pillows. It causes dry mouth and reduced effectiveness. Options include a chinstrap, a full face mask or treating nasal congestion.See also: Mask leak, Chinstrap, Full face mask
Multiple Sleep Latency Test (MSLT): The MSLT is a daytime test of sleepiness made up of 4 or 5 scheduled nap opportunities two hours apart. It measures how quickly you fall asleep and whether you enter REM sleep, and is used to diagnose narcolepsy and idiopathic hypersomnia.See also: Maintenance of Wakefulness Test (MWT), Narcolepsy, Sleep onset REM period (SOREMP), Sleep latency

N

N1 sleep: N1 is the lightest stage of sleep, the transition from wakefulness. The EEG slows from alpha to theta activity and slow rolling eye movements appear. It usually makes up about 5 percent of adult sleep.See also: N2 sleep, NREM sleep, Hypnic jerk
N2 sleep: N2 is a stage of light sleep marked on the EEG by sleep spindles and K-complexes. It takes up roughly half of adult sleep time, more than any other stage.See also: Sleep spindle, K-complex, N3 sleep (deep sleep)
N3 sleep (deep sleep): N3, also called slow wave or deep sleep, is defined by high amplitude slow delta waves. It is hardest to wake from, concentrated in the first half of the night and declines with age.See also: Slow wave sleep, N2 sleep, NREM sleep
NREM sleep: Non-rapid eye movement sleep includes stages N1, N2 and N3 and makes up about three quarters of adult sleep. Brain activity slows progressively from N1 to N3.See also: N1 sleep, N2 sleep, N3 sleep (deep sleep), REM sleep
Nap: A nap is a short sleep during the day. Short naps of about 10 to 20 minutes can boost alertness with little grogginess, while long or late naps can make it harder to sleep at night.See also: Sleep inertia, Sleep pressure (homeostatic sleep drive)
Narcolepsy: Narcolepsy is a chronic neurological disorder causing excessive daytime sleepiness and abnormal intrusions of REM sleep. Type 1 includes cataplexy or low orexin levels; type 2 does not. It is diagnosed with an overnight sleep study followed by an MSLT.See also: Cataplexy, Orexin (hypocretin), Multiple Sleep Latency Test (MSLT), Sleep paralysis
Nasal mask: A nasal mask has a cushion that covers the nose, either over the bridge or under the nose. It delivers air through the nose only and is used by many CPAP users who keep their mouths closed during sleep.See also: CPAP mask, Nasal pillows, Full face mask
Nasal pillows: Nasal pillows are small soft inserts that seal at the openings of the nostrils. They are the least bulky mask style and suit people who wear glasses or feel claustrophobic. They work best for people who breathe through the nose.See also: CPAP mask, Nasal mask, Chinstrap
Nightmare disorder: Nightmare disorder involves repeated, vivid, frightening dreams that wake the person and cause distress or daytime problems. Nightmares arise from REM sleep and are common after trauma. Imagery rehearsal therapy is one treatment.See also: Parasomnia, REM sleep
Non-24-hour sleep-wake rhythm disorder: In non-24 disorder, the body clock is not synchronized to the 24 hour day, so sleep times drift later day by day. It is most common in totally blind people who cannot perceive light.See also: Circadian rhythm sleep-wake disorder, Melatonin

O

Obesity hypoventilation syndrome (OHS): OHS is a condition in which people with obesity breathe too shallowly, so carbon dioxide builds up in the blood during the day as well as at night. Most people with OHS also have obstructive sleep apnea. It is treated with PAP therapy, often bilevel, and weight management.See also: Hypoxemia, BiPAP (bilevel positive airway pressure), Obstructive sleep apnea (OSA)
Obstructive sleep apnea (OSA): Obstructive sleep apnea is a disorder in which the upper airway repeatedly narrows or closes during sleep while the chest keeps trying to breathe. It causes loud snoring, breathing pauses, oxygen drops and broken sleep. Untreated, it is linked with high blood pressure, heart disease, stroke and drowsy driving. It is the most common form of sleep apnea.See also: Apnea, CPAP (continuous positive airway pressure), Snoring, Apnea hypopnea index (AHI), Upper airway
Oral appliance therapy: An oral appliance, most often a mandibular advancement device, is a custom mouthpiece that holds the lower jaw forward during sleep to keep the airway open. It is an option for obstructive sleep apnea in people who cannot tolerate or prefer not to use CPAP, and is fitted by a qualified dentist.See also: Obstructive sleep apnea (OSA), CPAP (continuous positive airway pressure), Upper airway
Orexin (hypocretin): Orexin, also called hypocretin, is a brain chemical made in the hypothalamus that promotes wakefulness and stabilizes sleep and wake states. Loss of orexin producing neurons causes narcolepsy type 1, and orexin receptor antagonists are used to treat insomnia.See also: Narcolepsy, Orexin receptor antagonist, Cataplexy
Orexin receptor antagonist: Orexin receptor antagonists are prescription insomnia medicines that block the wake promoting signal orexin. US examples are suvorexant, lemborexant and daridorexant. They are not used in people with narcolepsy.See also: Orexin (hypocretin), Insomnia, Z-drugs
Oxybates: Oxybates, such as sodium oxybate and lower sodium mixed salt oxybate, are prescription medicines taken at night to treat cataplexy and excessive daytime sleepiness in narcolepsy. They are tightly controlled through a restricted distribution program.See also: Narcolepsy, Cataplexy, Wake promoting medicines
Oxygen desaturation: An oxygen desaturation is a drop in blood oxygen saturation from the baseline level, commonly scored when it is 3 or 4 percent or more. Repeated desaturations during sleep are a feature of sleep apnea and are counted in the ODI.See also: SpO2 (oxygen saturation), Oxygen desaturation index (ODI), Hypopnea, Hypoxemia
Oxygen desaturation index (ODI): The ODI is the number of times per hour that blood oxygen falls by a set amount, usually 3 or 4 percent, from baseline. It often tracks closely with the AHI and is used in overnight oximetry and home testing.See also: Oxygen desaturation, SpO2 (oxygen saturation), Pulse oximetry, Apnea hypopnea index (AHI)

P

PAP adherence (compliance): Adherence describes how consistently a person uses PAP therapy. Medicare defines it as at least 4 hours a night on at least 70 percent of nights in a consecutive 30 day period during the first 3 months. Longer nightly use is linked with better results.See also: CPAP (continuous positive airway pressure), CPAP desensitization
Parasomnia: A parasomnia is an unwanted event or experience during sleep or at the transitions into and out of sleep. Examples include sleepwalking, sleep terrors, confusional arousals, nightmares and REM sleep behavior disorder.See also: Sleepwalking, Sleep terrors, REM sleep behavior disorder (RBD), Nightmare disorder
Periodic limb movement disorder (PLMD): PLMD involves repetitive jerking or kicking movements of the limbs during sleep that disturb sleep and cause daytime symptoms, without another sleep disorder explaining them. The movements are measured on a sleep study as the periodic limb movement index.See also: Restless legs syndrome (RLS), Electromyogram (EMG), Polysomnography (PSG)
Polysomnography (PSG): Polysomnography is a comprehensive overnight sleep study, usually done in a sleep lab. It records brain waves, eye movements, muscle activity, heart rhythm, breathing, oxygen levels and leg movements. It is the reference test for diagnosing sleep apnea and many other sleep disorders.See also: Home sleep apnea test (HSAT), Electroencephalogram (EEG), Hypnogram, Split-night study
Positional sleep apnea: Positional sleep apnea is obstructive sleep apnea that is much worse when lying on the back (supine) than on the side. It may be treated with positional therapy devices that discourage back sleeping.See also: Obstructive sleep apnea (OSA), Positional therapy
Positional therapy: Positional therapy uses devices or techniques that keep a person off their back during sleep. It is used for positional sleep apnea, where breathing events happen mainly when lying supine.See also: Positional sleep apnea, Obstructive sleep apnea (OSA)
Pressure relief: Pressure relief is a general term for comfort features that reduce pressure at the start of exhalation on a CPAP or APAP device. It can make therapy feel less forceful for people who find exhaling against pressure uncomfortable.See also: EPR (expiratory pressure relief), Ramp
Pressure support: Pressure support is the difference between inspiratory and expiratory pressure on a bilevel or ASV device. A larger difference gives each breath more help, which is used for hypoventilation and to stabilize central apnea.See also: IPAP (inspiratory positive airway pressure), EPAP (expiratory positive airway pressure), ASV (adaptive servo-ventilation)
Pulse oximetry: Pulse oximetry measures blood oxygen saturation (SpO2) and pulse rate using light shone through a fingertip or earlobe. Overnight oximetry can show repeated oxygen drops but cannot by itself diagnose the type of sleep apnea.See also: SpO2 (oxygen saturation), Oxygen desaturation index (ODI)

R

REM atonia: REM atonia is the near total loss of skeletal muscle tone during REM sleep, which keeps people from acting out dreams. Loss of atonia causes REM sleep behavior disorder, and atonia intruding into wakefulness causes sleep paralysis and cataplexy.See also: REM sleep, REM sleep behavior disorder (RBD), Sleep paralysis, Cataplexy
REM latency: REM latency is the time from falling asleep to the first period of REM sleep, typically around 90 minutes in adults. Unusually short REM latency can occur in narcolepsy, depression and after REM deprivation.See also: REM sleep, Sleep onset REM period (SOREMP), Narcolepsy
REM sleep: Rapid eye movement sleep is the stage when most vivid dreaming happens. The brain is active, the eyes move rapidly and most skeletal muscles are temporarily paralyzed (atonia). It makes up about a fifth to a quarter of adult sleep, mostly in the second half of the night.See also: REM atonia, REM latency, REM sleep behavior disorder (RBD), NREM sleep
REM sleep behavior disorder (RBD): In REM sleep behavior disorder, the normal muscle paralysis of REM sleep is lost, so people act out dreams, sometimes violently. It is diagnosed with polysomnography and can be an early sign of neurodegenerative diseases such as Parkinson disease.See also: REM sleep, REM atonia, Parasomnia
Rainout: Rainout is water condensation that forms in CPAP tubing or the mask when warm, humid air cools on the way. It can cause gurgling or water on the face. Heated tubing, a tubing cover, a warmer room or a lower humidity setting reduce it.See also: Heated tubing, Heated humidifier
Ramp: Ramp is a CPAP comfort feature that starts at a low pressure and increases gradually to the prescribed pressure over a set time, or until the machine detects you are asleep (often called auto ramp). It helps people fall asleep before full pressure is reached.See also: CPAP (continuous positive airway pressure), Pressure relief, CPAP pressure
Relaxation training: Relaxation training uses methods such as progressive muscle relaxation, slow breathing and guided imagery to reduce physical and mental tension at bedtime. It is one component of CBT-I.See also: Cognitive behavioral therapy for insomnia (CBT-I), Hyperarousal
Residual AHI: Residual AHI is the number of apneas and hypopneas per hour that a PAP machine detects while you use it. It is the device's estimate, not a sleep study measurement, and values under 5 are commonly targeted.See also: Apnea hypopnea index (AHI), CPAP (continuous positive airway pressure), APAP (auto-adjusting PAP)
Respiratory disturbance index (RDI): The RDI counts apneas, hypopneas and respiratory effort related arousals (RERAs) per hour of sleep. Because it includes RERAs, it is equal to or higher than the AHI. Some insurers accept either AHI or RDI for coverage decisions.See also: Apnea hypopnea index (AHI), Respiratory effort related arousal (RERA), Upper airway resistance syndrome (UARS)
Respiratory effort related arousal (RERA): A RERA is a sequence of breaths with increasing effort or flattening of the airflow signal that ends in a brief arousal from sleep, without meeting the criteria for an apnea or hypopnea. RERAs fragment sleep and are counted in the RDI.See also: Respiratory disturbance index (RDI), Arousal, Upper airway resistance syndrome (UARS)
Respiratory event index (REI): The REI is the number of respiratory events per hour of recording time on a home sleep apnea test. Because home tests often cannot tell when you are asleep, the REI divides by monitoring time rather than sleep time, which can underestimate severity compared with an in-lab AHI.See also: Apnea hypopnea index (AHI), Home sleep apnea test (HSAT), Total sleep time (TST)
Restless legs syndrome (RLS): Restless legs syndrome, also called Willis-Ekbom disease, is an urge to move the legs, usually with uncomfortable sensations, that starts or worsens at rest and in the evening and is relieved by movement. Low iron stores can contribute, so iron levels are often checked.See also: Periodic limb movement disorder (PLMD), Insomnia

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STOP-Bang questionnaire: STOP-Bang is an eight question screening tool for obstructive sleep apnea covering Snoring, Tiredness, Observed apneas, blood Pressure, BMI, Age, Neck size and Gender. A score of 5 to 8 suggests high risk. It is a screening tool, not a diagnosis.See also: Obstructive sleep apnea (OSA), Epworth Sleepiness Scale (ESS), Home sleep apnea test (HSAT)
Shift work disorder: Shift work disorder is insomnia or excessive sleepiness, with reduced total sleep, linked to work hours that overlap the normal sleep period, such as night or rotating shifts. Strategic napping, light management and a protected daytime sleep window help.See also: Circadian rhythm sleep-wake disorder, Excessive daytime sleepiness (EDS)
Sleep bruxism: Sleep bruxism is clenching or grinding of the teeth during sleep. It can cause jaw pain, headaches and tooth wear, and is sometimes associated with sleep apnea. Dentists often treat it with a night guard.See also: Parasomnia, Obstructive sleep apnea (OSA)
Sleep cycle: A sleep cycle is one pass through the stages of NREM sleep followed by REM sleep, lasting about 90 minutes on average. Adults usually have four to six cycles a night, with more deep sleep early and more REM sleep later.See also: NREM sleep, REM sleep, Hypnogram
Sleep debt: Sleep debt is the accumulated difference between the sleep a person needs and the sleep they get. Extra sleep can partly pay it back, but research suggests catch-up sleep does not fully reverse the effects of chronic short sleep.See also: Sleep deprivation, Social jet lag
Sleep deprivation: Sleep deprivation means not getting enough sleep, either through total sleep loss or ongoing short sleep. It impairs attention, mood and judgment, and over time is linked with heart disease, obesity, diabetes and accidents.See also: Sleep debt, Microsleep
Sleep diary: A sleep diary (sleep log) is a daily record of bedtime, time to fall asleep, awakenings, wake time and naps, usually kept for one to two weeks. It is a core tool in diagnosing insomnia and guiding CBT-I.See also: Actigraphy, Cognitive behavioral therapy for insomnia (CBT-I), Sleep efficiency
Sleep efficiency: Sleep efficiency is the percentage of time in bed actually spent asleep. It is calculated as total sleep time divided by time in bed. Values of about 85 percent or more are commonly considered healthy and are a target in sleep restriction therapy.See also: Total sleep time (TST), Time in bed (TIB), Sleep restriction therapy, Wake after sleep onset (WASO)
Sleep hygiene: Sleep hygiene refers to habits and surroundings that support sleep, such as a regular schedule, a dark, quiet, cool bedroom, and limiting caffeine, alcohol and late heavy meals. On its own it is not an effective treatment for chronic insomnia, but it supports other treatments.See also: Cognitive behavioral therapy for insomnia (CBT-I), Caffeine, Core body temperature rhythm
Sleep inertia: Sleep inertia is the grogginess and reduced performance right after waking, which usually fades over minutes to an hour. It is stronger after waking from deep sleep and after sleep loss.See also: N3 sleep (deep sleep), Nap
Sleep latency: Sleep latency (sleep onset latency) is the time it takes to fall asleep after lights out. Healthy adults usually fall asleep within about 10 to 20 minutes; very short latency can indicate sleep deprivation and long latency is common in insomnia.See also: Multiple Sleep Latency Test (MSLT), Wake after sleep onset (WASO), Sleep efficiency
Sleep onset REM period (SOREMP): A SOREMP is REM sleep that begins within 15 minutes of falling asleep. Two or more SOREMPs on an MSLT and the preceding night's study are part of the diagnostic criteria for narcolepsy.See also: Multiple Sleep Latency Test (MSLT), REM latency, Narcolepsy
Sleep paralysis: Sleep paralysis is a temporary inability to move or speak while falling asleep or waking, caused by REM atonia continuing into wakefulness. It can be frightening but is harmless; frequent episodes can occur with narcolepsy.See also: REM atonia, Narcolepsy, Hypnagogic hallucinations
Sleep pressure (homeostatic sleep drive): Sleep pressure is the growing need for sleep that builds the longer you are awake and falls during sleep. Together with the circadian rhythm, it determines when you feel sleepy. Naps and lying in bed awake both reduce it.See also: Adenosine, Two-process model of sleep, Circadian rhythm
Sleep related hypoventilation: Sleep related hypoventilation is shallow or slow breathing during sleep that lets carbon dioxide rise. It occurs with obesity hypoventilation, neuromuscular disease, chest wall problems and some medicines, and is often treated with bilevel PAP.See also: Obesity hypoventilation syndrome (OHS), BiPAP (bilevel positive airway pressure), Backup rate
Sleep restriction therapy: Sleep restriction therapy limits time in bed to roughly the time actually spent asleep, then gradually extends it as sleep efficiency improves. It builds sleep pressure and consolidates sleep. It can cause temporary sleepiness, so it is best done with guidance.See also: Cognitive behavioral therapy for insomnia (CBT-I), Sleep efficiency, Time in bed (TIB), Sleep pressure (homeostatic sleep drive)
Sleep spindle: A sleep spindle is a brief burst of brain wave activity at about 11 to 16 Hz, lasting at least half a second. Spindles are a hallmark of N2 sleep and are thought to play a role in memory and in shielding sleep from outside noise.See also: N2 sleep, K-complex, Electroencephalogram (EEG)
Sleep terrors: Sleep terrors (night terrors) are episodes of sudden screaming, intense fear and racing heart from deep sleep, usually with little memory afterward. They are most common in children and often outgrown.See also: Parasomnia, Slow wave sleep, Nightmare disorder
Sleepwalking: Sleepwalking (somnambulism) is a parasomnia in which a person gets up and walks or does other complex actions while still partly asleep, usually from deep N3 sleep in the first part of the night. It is more common in children.See also: Parasomnia, Slow wave sleep, Sleep terrors
Slow wave sleep: Slow wave sleep is another name for N3 sleep, named for its slow, high amplitude delta brain waves of 0.5 to 2 Hz. Sleepwalking and night terrors usually arise from it.See also: N3 sleep (deep sleep), Sleepwalking, Sleep terrors
Snoring: Snoring is the sound of vibrating tissues in the throat as air passes through a narrowed airway during sleep. It is common and not always a sign of disease, but loud, regular snoring with gasping or pauses is a key symptom of obstructive sleep apnea.See also: Obstructive sleep apnea (OSA), Upper airway, STOP-Bang questionnaire
Social jet lag: Social jet lag is the mismatch between sleep timing on workdays and free days, measured as the difference in the midpoint of sleep. It is common in evening types who sleep in on weekends and has been linked with poorer health outcomes in studies.See also: Chronotype, Jet lag disorder, Circadian rhythm
SpO2 (oxygen saturation): SpO2 is the percentage of hemoglobin carrying oxygen, as measured by a pulse oximeter on the finger or ear. Sleep studies record it continuously to detect drops linked with breathing events.See also: Pulse oximetry, Oxygen desaturation, Hypoxemia
Split-night study: A split-night study combines diagnosis and treatment in one night in a sleep lab. If enough sleep apnea is recorded in the first part of the night, the technologist starts CPAP and adjusts the pressure for the rest of the night.See also: Polysomnography (PSG), CPAP titration
Stimulus control: Stimulus control is a CBT-I technique that rebuilds the link between bed and sleep. Rules include going to bed only when sleepy, getting out of bed if unable to sleep, using the bed only for sleep and sex, and getting up at the same time every day.See also: Cognitive behavioral therapy for insomnia (CBT-I), Sleep restriction therapy
Suprachiasmatic nucleus (SCN): The SCN is a small cluster of nerve cells in the hypothalamus that acts as the body's master circadian clock. It receives light signals from the eyes and coordinates daily rhythms throughout the body.See also: Circadian rhythm, Melatonin, Zeitgeber

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Time in bed (TIB): Time in bed is the period from getting into bed to get out of bed for the day, including time awake. It is the denominator in sleep efficiency and the quantity adjusted in sleep restriction therapy.See also: Total sleep time (TST), Sleep efficiency, Sleep restriction therapy
Tirzepatide for sleep apnea: Tirzepatide (brand name Zepbound) is a weekly injectable weight loss medicine. In December 2024 the FDA approved it for moderate to severe obstructive sleep apnea in adults with obesity, the first drug approved for OSA. It is used alongside diet and exercise.See also: Obstructive sleep apnea (OSA), Obesity hypoventilation syndrome (OHS)
Total sleep time (TST): Total sleep time is the amount of actual sleep during a sleep period, excluding time awake in bed. It is measured in sleep studies and estimated in sleep diaries and actigraphy.See also: Time in bed (TIB), Sleep efficiency
Travel CPAP: Travel CPAP machines are smaller, lighter devices designed for trips. Many run on batteries and some omit the heated humidifier. Under US airline rules, CPAP is a medical device and generally does not count toward carry-on limits.See also: CPAP (continuous positive airway pressure), Heated humidifier
Treatment emergent central sleep apnea: Also called complex sleep apnea, this is central sleep apnea that appears or persists when obstructive sleep apnea is treated with PAP. It often settles with continued CPAP over weeks, but persistent cases may need a different device such as ASV.See also: Central sleep apnea (CSA), CPAP (continuous positive airway pressure), ASV (adaptive servo-ventilation)
Two-process model of sleep: The two-process model, proposed by Alexander Borbely, explains sleep timing as the interaction of a homeostatic process (sleep pressure, Process S) and a circadian process (Process C). It underpins many behavioral sleep treatments.See also: Sleep pressure (homeostatic sleep drive), Circadian rhythm

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Upper airway: The upper airway is the passage from the nose and mouth through the throat (pharynx) to the voice box. During sleep its muscles relax, and in people with sleep apnea the soft walls can collapse. Jaw size, tonsils, tongue size and body weight affect how narrow it is.See also: Obstructive sleep apnea (OSA), Snoring, Oral appliance therapy
Upper airway resistance syndrome (UARS): UARS describes increased effort to breathe through a narrowed airway that causes frequent arousals (RERAs) without many full apneas or hypopneas. Current sleep disorder classifications generally treat it as part of the obstructive sleep apnea spectrum.See also: Respiratory effort related arousal (RERA), Respiratory disturbance index (RDI), Obstructive sleep apnea (OSA)
Uvulopalatopharyngoplasty (UPPP): UPPP is a surgery that removes or reshapes tissue in the throat, such as the uvula, part of the soft palate and the tonsils, to widen the airway. Results for sleep apnea vary, and it is generally considered when other treatments fail.See also: Obstructive sleep apnea (OSA), Upper airway, Maxillomandibular advancement (MMA)

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Wake after sleep onset (WASO): WASO is the total time spent awake after first falling asleep and before the final awakening. It is a key measure of sleep maintenance problems in insomnia.See also: Sleep efficiency, Sleep latency, Insomnia
Wake promoting medicines: Wake promoting medicines such as modafinil, armodafinil, solriamfetol and pitolisant are prescribed for excessive daytime sleepiness in narcolepsy and some other conditions. Some are approved for residual sleepiness in treated obstructive sleep apnea, but they do not treat the apnea itself.See also: Narcolepsy, Excessive daytime sleepiness (EDS), Oxybates
Water chamber: The water chamber is the tub that holds water for a CPAP humidifier. It should be emptied and cleaned regularly. Medicare allows replacement up to once every 6 months.See also: Heated humidifier, Distilled water

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Z-drugs: Z-drugs are nonbenzodiazepine prescription sleep medicines, such as zolpidem, eszopiclone and zaleplon, that act on the GABA receptor. Since 2019 they carry an FDA boxed warning about complex sleep behaviors such as sleepwalking and sleep driving.See also: Insomnia, Orexin receptor antagonist, Sleepwalking
Zeitgeber: A zeitgeber (German for time giver) is an outside cue that sets the body clock. Light is the strongest; meal timing, exercise and social schedules also have effects.See also: Circadian rhythm, Bright light therapy

Sleep glossary

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