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Actigraphy

A method of estimating sleep by recording movement for days with a small device worn on the wrist.

For expertsdefinition and thresholds

A movement sensor worn on the wrist records for days or weeks; algorithms classify still periods as sleep and active periods as wake. The AASM 2018 guideline conditionally recommends actigraphy for assessing insomnia and circadian rhythm sleep wake disorders, for monitoring total sleep time before an MSLT, and for estimating total sleep time when added to a home sleep apnea test. It strongly recommends that it not be used in place of electromyography for diagnosing periodic limb movement disorder.

Smith MT, McCrae CS, Cheung J et al. Use of actigraphy for the evaluation of sleep disorders and circadian rhythm sleep-wake disorders: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2018;14(7):1231-1237. doi:10.5664/jcsm.7230

Related: Circadian rhythm · Insomnia · Home sleep apnea test (HSAT) · MSLT (multiple sleep latency test)

AHI (apnea hypopnea index)

A number that shows how many times an hour your breathing stops or becomes very weak during sleep. You can think of it as counting the stumbles in an hour of sleep.

For expertsdefinition and thresholds

The number of apneas and hypopneas per hour of total sleep time. In adults, the generally accepted severity thresholds are 5 to 15 mild, 15 to 30 moderate, and 30 or more severe; the diagnosis is made with polysomnography or a home sleep apnea test.

Kapur VK, Auckley DH, Chowdhuri S et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479-504. doi:10.5664/jcsm.6506

Related: Apnea · Hypopnea · REI (respiratory event index) · Obstructive sleep apnea (OSA) · Polysomnography (PSG)

Apnea

When breathing stops completely, or almost completely, for at least a few seconds during sleep. Like a hose that is blocked for a while.

For expertsdefinition and thresholds

Under the AASM 2012 rule, a drop of at least 90% from baseline in the oronasal thermal sensor signal, lasting at least 10 seconds. If respiratory effort continues it is classed as obstructive, if there is no effort at all as central, and if both occur as mixed apnea.

Berry RB, Budhiraja R, Gottlieb DJ et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. J Clin Sleep Med. 2012;8(5):597-619. doi:10.5664/jcsm.2172

Related: Hypopnea · AHI (apnea hypopnea index) · Obstructive sleep apnea (OSA) · Central sleep apnea

Arousal

When the brain slips out of sleep for a few seconds and goes back. You do not remember it in the morning, but if it happens often, sleep is not restful.

For expertsdefinition and thresholds

Under the AASM rule, an abrupt shift in EEG frequency lasting at least 3 seconds after at least 10 seconds of stable sleep; in REM an increase in chin EMG is also required. It plays a part in ending respiratory events and in scoring hypopneas.

Bonnet MH, Doghramji K, Roehrs T et al. The scoring of arousal in sleep: reliability, validity, and alternatives. J Clin Sleep Med. 2007;3(2):133-145. doi:10.5664/jcsm.26815

Related: RERA (respiratory effort related arousal) · Hypopnea · Sleep stages

Bruxism (teeth grinding)

Clenching or grinding the teeth. It can happen during sleep or while awake.

For expertsdefinition and thresholds

Under the 2018 international consensus, sleep bruxism is rhythmic or non-rhythmic masticatory muscle activity during sleep. Awake bruxism is defined by repetitive or sustained tooth contact and by clenching or thrusting of the lower jaw. The consensus suggests that in healthy people bruxism should be seen not as a disorder but as a behaviour that can be a risk or protective factor for certain clinical outcomes. Self-report and electromyography can be used in assessment; no standard threshold is used to say it is present or absent in healthy people.

Lobbezoo F, Ahlberg J, Raphael KG et al. International consensus on the assessment of bruxism: report of a work in progress. J Oral Rehabil. 2018;45(11):837-844. doi:10.1111/joor.12663

Related: Polysomnography (PSG) · Oral appliance · Snoring

CBT-I (cognitive behavioural therapy for insomnia)

A therapy without medicine that changes sleep habits and thoughts step by step over a few weeks.

For expertsdefinition and thresholds

A multicomponent treatment combining sleep restriction, stimulus control, cognitive restructuring, relaxation and sleep education. The AASM 2021 guideline gives a strong recommendation for CBT-I in chronic insomnia; the meta-analyses behind the guideline showed clinically meaningful improvements in remission and response rates, sleep quality, sleep latency and wake after sleep onset (WASO).

Edinger JD, Arnedt JT, Bertisch SM et al. Behavioral and psychological treatments for chronic insomnia disorder in adults: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2021;17(2):255-262. doi:10.5664/jcsm.8986

Related: Insomnia · Sleep hygiene · Sleep efficiency

Central sleep apnea

In this type the throat is open; for a while the brain does not send the command to breathe.

For expertsdefinition and thresholds

Defined by apneas in which respiratory effort is also lost because the respiratory centre temporarily stops sending signals. Heart failure (Cheyne Stokes breathing), opioid use and high altitude are common causes.

Eckert DJ, Jordan AS, Merchia P, Malhotra A. Central sleep apnea: pathophysiology and treatment. Chest. 2007;131(2):595-607. doi:10.1378/chest.06.2287

Related: Apnea · Obstructive sleep apnea (OSA) · AHI (apnea hypopnea index)

Chronotype

Describes whether your body is more alert in the morning or in the evening. Some people get up early, some are night owls.

For expertsdefinition and thresholds

The time of day at which a person tends to place sleep and wakefulness within the 24-hour day, determined largely by the circadian clock. In a population, sleep and wake times are almost normally distributed; extreme morning types are getting up when extreme evening types have just fallen asleep. It is measured with the Horne Östberg morningness eveningness questionnaire (MEQ) and the Munich Chronotype Questionnaire (MCTQ); the MCTQ uses sleep timing on both work and free days. Chronotype changes with age and sex.

Roenneberg T, Kuehnle T, Juda M et al. Epidemiology of the human circadian clock. Sleep Med Rev. 2007;11(6):429-438. doi:10.1016/j.smrv.2007.07.005

Related: Circadian rhythm · Social jet lag · Melatonin · Sleep debt

Circadian rhythm

Your body's internal clock of about 24 hours. It largely sets when sleep comes and when you are awake.

For expertsdefinition and thresholds

An endogenous rhythm of about 24 hours, run by the suprachiasmatic nucleus in the hypothalamus and based on feedback loops of clock genes. Light is the strongest synchroniser; a shifted rhythm can lead to circadian rhythm sleep wake disorders.

Patke A, Young MW, Axelrod S. Molecular mechanisms and physiological importance of circadian rhythms. Nat Rev Mol Cell Biol. 2020;21(2):67-84. doi:10.1038/s41580-019-0179-2

Related: Sleep hygiene · Insomnia

Desaturation

A short drop in the level of oxygen in the blood. When breathing weakens, the blood's oxygen store empties a little.

For expertsdefinition and thresholds

A drop in oxygen saturation of at least 3% (4% under some rules) from baseline. It is used in scoring hypopneas and in calculating the ODI.

Berry RB, Budhiraja R, Gottlieb DJ et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. J Clin Sleep Med. 2012;8(5):597-619. doi:10.5664/jcsm.2172

Related: SpO2 (oxygen saturation) · ODI (oxygen desaturation index) · Hypopnea

Epworth Sleepiness Scale

A short questionnaire that asks how easily you doze off in ordinary situations during the day.

For expertsdefinition and thresholds

A self-report scale developed by Johns in 1991 that scores the chance of dozing in eight situations from 0 to 3; the total ranges from 0 to 24. In the study by Johns and Hocking, the scores of people without a sleep disorder stayed between 0 and 10, and above 10 was taken as excessive daytime sleepiness. It is licensed; it does not diagnose, it only rates subjective sleepiness.

Johns MW. A new method for measuring daytime sleepiness: the Epworth sleepiness scale. Sleep. 1991;14(6):540-545. doi:10.1093/sleep/14.6.540; Johns M, Hocking B. Daytime sleepiness and sleep habits of Australian workers. Sleep. 1997;20(10):844-849. doi:10.1093/sleep/20.10.844

Related: Excessive daytime sleepiness · STOP-Bang questionnaire

Excessive daytime sleepiness

Dozing off unintentionally during the day, or finding it hard to stay awake, even though you spend enough time in bed at night.

For expertsdefinition and thresholds

An increased tendency to sleep during periods when one is expected to be awake; it is assessed subjectively with the Epworth scale and objectively with the MSLT and the maintenance of wakefulness test (MWT). Obstructive sleep apnea, narcolepsy and insufficient sleep are the main causes.

Littner MR, Kushida C, Wise M et al. Practice parameters for clinical use of the multiple sleep latency test and the maintenance of wakefulness test. Sleep. 2005;28(1):113-121. doi:10.1093/sleep/28.1.113

Related: Epworth Sleepiness Scale · Sleep latency · Obstructive sleep apnea (OSA)

Home sleep apnea test (HSAT)

A simple sleep test done at home, in your own bed, with a few sensors. It does not measure brain waves; it follows breathing and oxygen.

For expertsdefinition and thresholds

An unattended recording that includes at least airflow, respiratory effort and oximetry. The AASM 2017 guideline recommends polysomnography or a technically adequate HSAT in uncomplicated adults with a high likelihood of moderate to severe sleep apnea; if the HSAT is negative or inadequate, polysomnography should be done.

Kapur VK, Auckley DH, Chowdhuri S et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479-504. doi:10.5664/jcsm.6506

Related: Polysomnography (PSG) · REI (respiratory event index) · Obstructive sleep apnea (OSA)

Hypnogram

A stepped chart that shows which stage of sleep you were in through the night.

For expertsdefinition and thresholds

In polysomnography, sleep is scored in 30-second epochs as wake (W), N1, N2, N3 and REM (R); the hypnogram is these scores plotted against time. Sleep onset, REM periods, awakenings and the distribution of stages through the night can be seen at a glance. Measures such as sleep efficiency and sleep latency are calculated from the same scoring.

Silber MH, Ancoli-Israel S, Bonnet MH et al. The visual scoring of sleep in adults. J Clin Sleep Med. 2007;3(2):121-131. doi:10.5664/jcsm.26814

Related: Sleep stages · REM and NREM sleep · Polysomnography (PSG) · Sleep efficiency

Hypopnea

When breathing becomes clearly weaker during sleep without stopping. Like a tap turned down rather than off.

For expertsdefinition and thresholds

Under the AASM 2012 recommended rule, a reduction in flow of at least 30% in the nasal pressure signal, lasting at least 10 seconds; it must be accompanied by an oxygen desaturation of at least 3% or an arousal. Some payers, such as Medicare, use a 4% desaturation requirement.

Berry RB, Budhiraja R, Gottlieb DJ et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. J Clin Sleep Med. 2012;8(5):597-619. doi:10.5664/jcsm.2172

Related: Apnea · AHI (apnea hypopnea index) · Desaturation · Arousal

Insomnia

Having trouble falling asleep, staying asleep or waking too early even when you have the chance to sleep, and this affecting your day.

For expertsdefinition and thresholds

Under ICSD-3 and DSM-5, chronic insomnia is difficulty falling or staying asleep despite adequate opportunity for sleep, with daytime impairment, on at least 3 nights a week for at least 3 months. The European guideline recommends CBT-I as first-line treatment.

Riemann D, Espie CA, Altena E et al. The European Insomnia Guideline: an update on the diagnosis and treatment of insomnia 2023. J Sleep Res. 2023;32(6):e14035. doi:10.1111/jsr.14035

Related: CBT-I (cognitive behavioural therapy for insomnia) · Sleep hygiene · Sleep latency

Melatonin

A natural hormone that helps the body set its sleep time.

For expertsdefinition and thresholds

A physiological hormone that plays a part in sleep timing, also used from outside as a medicine or supplement. In a meta-analysis pooling 12 randomised controlled trials, the most convincing evidence was for a shorter time to fall asleep in primary insomnia and delayed sleep phase syndrome, and for regulating the sleep wake pattern in blind people. The AASM 2017 guideline, on the other hand, gives a weak recommendation against using melatonin to fall or stay asleep in chronic insomnia. You should talk to a doctor before using it.

Auld F, Maschauer EL, Morrison I et al. Evidence for the efficacy of melatonin in the treatment of primary adult sleep disorders. Sleep Med Rev. 2017;34:10-22. doi:10.1016/j.smrv.2016.06.005; Sateia MJ, Buysse DJ, Krystal AD et al. Clinical practice guideline for the pharmacologic treatment of chronic insomnia in adults: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(2):307-349. doi:10.5664/jcsm.6470

Related: Circadian rhythm · Chronotype · Insomnia · Sleep latency

MSLT (multiple sleep latency test)

A test in which you are given several short chances to sleep during the day and how quickly you fall asleep is measured.

For expertsdefinition and thresholds

After overnight polysomnography, five nap opportunities are given during the day, two hours apart; the first starts 1.5 to 3 hours after the end of the night recording. In each nap, the time to fall asleep and whether there is an early REM period are recorded. According to the AASM it is indicated in the evaluation of suspected narcolepsy and may be useful in idiopathic hypersomnia; it is not routinely used in the initial diagnosis of OSA or to assess change after CPAP. The maintenance of wakefulness test (MWT), in contrast, measures a person's ability to stay awake.

Littner MR, Kushida C, Wise M et al. Practice parameters for clinical use of the multiple sleep latency test and the maintenance of wakefulness test. Sleep. 2005;28(1):113-121. doi:10.1093/sleep/28.1.113; Krahn LE, Arand DL, Avidan AY et al. Recommended protocols for the Multiple Sleep Latency Test and Maintenance of Wakefulness Test in adults: guidance from the American Academy of Sleep Medicine. J Clin Sleep Med. 2021;17(12):2489-2498. doi:10.5664/jcsm.9620

Related: Narcolepsy · Excessive daytime sleepiness · Sleep latency · Polysomnography (PSG)

Narcolepsy

A rare brain disease that causes irresistible sleep attacks during the day.

For expertsdefinition and thresholds

Reflects the selective loss or dysfunction of orexin (hypocretin) neurons in the lateral hypothalamus. In type 1 narcolepsy, excessive daytime sleepiness is accompanied by cataplexy; hallucinations, sleep paralysis and broken night sleep can also occur. The diagnosis rests on clinical findings and is supported by markers such as REM periods starting soon after falling asleep, orexin deficiency in the cerebrospinal fluid and HLA-DQB1*06:02 positivity. On the MSLT, a mean sleep latency of 8 minutes or less and at least 2 early REM periods are considered consistent with narcolepsy.

Bassetti CLA, Adamantidis A, Burdakov D et al. Narcolepsy: clinical spectrum, aetiopathophysiology, diagnosis and treatment. Nat Rev Neurol. 2019;15(9):519-539. doi:10.1038/s41582-019-0226-9; Krahn LE, Arand DL, Avidan AY et al. Recommended protocols for the Multiple Sleep Latency Test and Maintenance of Wakefulness Test in adults: guidance from the American Academy of Sleep Medicine. J Clin Sleep Med. 2021;17(12):2489-2498. doi:10.5664/jcsm.9620

Related: MSLT (multiple sleep latency test) · Excessive daytime sleepiness · Sleep paralysis · REM and NREM sleep

Obstructive sleep apnea (OSA)

When the throat repeatedly narrows and closes during sleep, so breathing is cut off. The chest tries to breathe but air cannot get through.

For expertsdefinition and thresholds

Under ICSD-3 criteria, the diagnosis is made with 5 or more predominantly obstructive respiratory events per hour on polysomnography or a home test together with symptoms (sleepiness, snoring, witnessed apnea, waking up gasping or choking) or an associated condition, or with 15 or more events regardless of symptoms. ICSD-3-TR (2023) keeps these core criteria.

Sateia MJ. International Classification of Sleep Disorders, third edition: highlights and modifications. Chest. 2014;146(5):1387-1394. doi:10.1378/chest.14-0970

Related: Apnea · AHI (apnea hypopnea index) · Upper airway · PAP therapy (CPAP) · Central sleep apnea

ODI (oxygen desaturation index)

A number that shows how many times an hour the oxygen in your blood drops noticeably.

For expertsdefinition and thresholds

The number of drops in oxygen saturation of at least 3% (or 4%) from baseline per hour. It is calculated with pulse oximetry; on its own it is not considered enough to diagnose sleep apnea.

Kapur VK, Auckley DH, Chowdhuri S et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479-504. doi:10.5664/jcsm.6506

Related: Desaturation · SpO2 (oxygen saturation) · AHI (apnea hypopnea index)

Oral appliance

A custom-made mouthpiece worn on the teeth during sleep that moves the lower jaw slightly forward to keep the airway in the throat open.

For expertsdefinition and thresholds

A custom-made, adjustable device fitted by a dentist that widens the pharyngeal airway by moving the mandible forward. The AASM and AADSM 2015 guideline recommends it in primary snoring when treatment is wanted, and in patients with obstructive sleep apnea who cannot tolerate PAP or prefer another treatment.

Ramar K, Dort LC, Katz SG et al. Clinical practice guideline for the treatment of obstructive sleep apnea and snoring with oral appliance therapy: an update for 2015. J Clin Sleep Med. 2015;11(7):773-827. doi:10.5664/jcsm.4858

Related: PAP therapy (CPAP) · Snoring · Obstructive sleep apnea (OSA)

PAP therapy (CPAP)

During sleep, air at a slight pressure is given through a mask worn on the nose or face. This air keeps the throat open like an air cushion.

For expertsdefinition and thresholds

Positive airway pressure (fixed-pressure CPAP, auto-adjusting APAP, bilevel BPAP) prevents the upper airway from closing during sleep pneumatically. The AASM 2019 guideline gives a strong recommendation for PAP therapy in adults with obstructive sleep apnea and excessive sleepiness.

Patil SP, Ayappa IA, Caples SM et al. Treatment of adult obstructive sleep apnea with positive airway pressure: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2019;15(2):335-343. doi:10.5664/jcsm.7640

Related: Obstructive sleep apnea (OSA) · Upper airway · Oral appliance

Pcrit (critical closing pressure)

A measure of the pressure at which the throat closes during sleep. The higher the number, the more easily the throat closes.

For expertsdefinition and thresholds

Passive critical closing pressure (Pcrit) is the nasal pressure at which the upper airway closes when muscle activity is taken out of play, expressed in cmH2O; it is measured in the laboratory by briefly lowering the pressure given through the nose. In the 75-person study by Eckert and colleagues, Pcrit was 0.3 cmH2O in patients with OSA and −6.2 cmH2O in controls. Even so, in 19% of patients the airway was relatively hard to collapse, similar to controls (−2 to −5 cmH2O). In 56% of patients, non-anatomical traits such as muscle response, arousal threshold and ventilatory control also played an important role.

Eckert DJ, White DP, Jordan AS et al. Defining phenotypic causes of obstructive sleep apnea. Identification of novel therapeutic targets. Am J Respir Crit Care Med. 2013;188(8):996-1004. doi:10.1164/rccm.201303-0448OC; Gold AR, Schwartz AR. The pharyngeal critical pressure. The whys and hows of using nasal continuous positive airway pressure diagnostically. Chest. 1996;110(4):1077-1088. doi:10.1378/chest.110.4.1077

Related: Upper airway · Obstructive sleep apnea (OSA) · PAP therapy (CPAP)

Polysomnography (PSG)

A detailed sleep test in which you stay a night at a sleep centre while your brain waves, breathing, oxygen and heartbeat are recorded together.

For expertsdefinition and thresholds

The simultaneous recording of channels such as EEG, EOG, chin EMG, airflow, chest and abdominal respiratory effort, SpO2, ECG and leg EMG; it is usually done under the supervision of a sleep technologist. It is the gold standard test that measures sleep stages and respiratory events.

Kushida CA, Littner MR, Morgenthaler T et al. Practice parameters for the indications for polysomnography and related procedures: an update for 2005. Sleep. 2005;28(4):499-523. doi:10.1093/sleep/28.4.499

Related: Home sleep apnea test (HSAT) · Sleep stages · AHI (apnea hypopnea index) · Sleep centre (sleep laboratory)

Positional apnea

Sleep apnea in which most pauses in breathing happen while lying on the back and become clearly fewer when lying on the side.

For expertsdefinition and thresholds

Under a commonly used definition, the supine AHI is at least twice the non-supine AHI. In more than half of patients with obstructive sleep apnea, apnea worsens when lying on the back; positional therapy is one of the treatment options.

Omobomi O, Quan SF. Positional therapy in the management of positional obstructive sleep apnea: a review of the current literature. Sleep Breath. 2018;22(2):297-304. doi:10.1007/s11325-017-1561-y

Related: Obstructive sleep apnea (OSA) · AHI (apnea hypopnea index) · Snoring

REI (respiratory event index)

The number of breathing events per hour counted in a sleep test done at home. Because it is not known whether you were asleep, it is divided by the total recording time.

For expertsdefinition and thresholds

Found by dividing the number of apneas and hypopneas in a home sleep apnea test by the monitoring time instead of the sleep time. Because time spent awake enters the denominator, the REI can understate the AHI of the same night.

Kapur VK, Auckley DH, Chowdhuri S et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479-504. doi:10.5664/jcsm.6506

Related: AHI (apnea hypopnea index) · Home sleep apnea test (HSAT) · ODI (oxygen desaturation index)

REM and NREM sleep

Sleep has two main types: one in which the eyes move quickly and dreams are vivid, and one that is calm and gets deeper. Through the night the two come in turn.

For expertsdefinition and thresholds

NREM sleep consists of stages N1, N2 and N3 (slow wave sleep); REM sleep shows rapid eye movements, low-amplitude mixed-frequency EEG and muscle atonia. In adults the cycle repeats about every 90 minutes; N3 is concentrated in the first half of the night and REM in the second half.

Silber MH, Ancoli-Israel S, Bonnet MH et al. The visual scoring of sleep in adults. J Clin Sleep Med. 2007;3(2):121-131. doi:10.5664/jcsm.26814

Related: Sleep stages · Polysomnography (PSG)

RERA (respiratory effort related arousal)

Breathing becomes harder without stopping completely, and the brain notices and wakes up for a moment. It is not remembered the next morning, but sleep has been broken up.

For expertsdefinition and thresholds

A respiratory event lasting at least 10 seconds, with increasing respiratory effort or flattening of the nasal pressure curve and ending in an arousal, that does not meet the criteria for an apnea or hypopnea. The AHI plus RERAs gives the respiratory disturbance index (RDI).

Berry RB, Budhiraja R, Gottlieb DJ et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. J Clin Sleep Med. 2012;8(5):597-619. doi:10.5664/jcsm.2172

Related: Arousal · Hypopnea · AHI (apnea hypopnea index)

Restless legs syndrome

When the urge to move your legs while resting becomes irresistible. It gets worse in the evening and goes away when you move.

For expertsdefinition and thresholds

Also known as Willis Ekbom disease. It is defined by an urge to move one or both legs (sometimes the arms) while at rest; the urge often comes with unpleasant sensations, is relieved by movement and becomes marked in the evening or at night. The IRLSSG 2014 criteria added, as a fifth essential criterion, that the symptoms are not confused with another condition such as cramps or positional discomfort, and merged the criteria for children with those for adults. Brain iron deficiency and heredity play a part; the AASM 2025 guideline emphasises checking iron levels before treatment.

Allen RP, Picchietti DL, Garcia-Borreguero D et al. Restless legs syndrome/Willis-Ekbom disease diagnostic criteria: updated International Restless Legs Syndrome Study Group (IRLSSG) consensus criteria. Sleep Med. 2014;15(8):860-873. doi:10.1016/j.sleep.2014.03.025; Winkelman JW, Berkowski JA, DelRosso LM et al. Treatment of restless legs syndrome and periodic limb movement disorder: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2025;21(1):137-152. doi:10.5664/jcsm.11390

Related: Insomnia · Actigraphy · Polysomnography (PSG)

Sleep centre (sleep laboratory)

A unit where sleep tests are done, where you stay overnight, and that works with sleep physicians and technologists.

For expertsdefinition and thresholds

A unit with sleep medicine physicians and technologists where tests such as attended polysomnography, titration studies and the MSLT are done. It is where the diagnosis is made when a home test is not suitable or is inconclusive.

Kapur VK, Auckley DH, Chowdhuri S et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479-504. doi:10.5664/jcsm.6506

Related: Polysomnography (PSG) · Home sleep apnea test (HSAT)

Sleep debt

The hours you sleep less than you need, piling up on top of each other. Like small debts that grow over time.

For expertsdefinition and thresholds

The recommended sleep duration for adults is 7 hours or more per night on a regular basis (joint AASM and SRS statement). In the experiment by Van Dongen and colleagues, in healthy adults whose time in bed was restricted to 4 or 6 hours for 14 days, cognitive performance got progressively worse in a dose-dependent way; 6 hours of sleep or less a night created a deficit equivalent to 2 nights without any sleep. The participants' sleepiness ratings, however, rose only slightly; that is, people largely did not notice the deficit.

Watson NF, Badr MS, Belenky G et al. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. J Clin Sleep Med. 2015;11(6):591-592. doi:10.5664/jcsm.4758; Van Dongen HPA, Maislin G, Mullington JM, Dinges DF. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep. 2003;26(2):117-126. doi:10.1093/sleep/26.2.117

Related: Social jet lag · Excessive daytime sleepiness · Chronotype

Sleep efficiency

How much of your time in bed you actually spend asleep. If you lie in bed for eight hours and sleep for six, the ratio is three quarters.

For expertsdefinition and thresholds

Total sleep time as a percentage of time in bed (or recording time). In sleep restriction therapy, time in bed is usually adjusted against an 85% threshold; the definition of the denominator differs between studies.

Reed DL, Sacco WP. Measuring sleep efficiency: what should the denominator be? J Clin Sleep Med. 2016;12(2):263-266. doi:10.5664/jcsm.5498

Related: Sleep latency · Insomnia · CBT-I (cognitive behavioural therapy for insomnia)

Sleep hygiene

Everyday habits that make good sleep easier: a regular bedtime, avoiding caffeine in the evening, a dark and cool room.

For expertsdefinition and thresholds

Behavioural advice on caffeine, alcohol, nicotine, exercise, sleep schedule and the bedroom environment. It is valuable for public health, but on its own it is not recommended as a treatment for chronic insomnia.

Irish LA, Kline CE, Gunn HE et al. The role of sleep hygiene in promoting public health: a review of empirical evidence. Sleep Med Rev. 2015;22:23-36. doi:10.1016/j.smrv.2014.10.001

Related: Insomnia · CBT-I (cognitive behavioural therapy for insomnia) · Circadian rhythm

Sleep latency

The time it takes you to fall asleep after turning off the light.

For expertsdefinition and thresholds

The time from lights out to the first epoch of sleep. A National Sleep Foundation expert panel counted 30 minutes or less as one of the indicators of good sleep quality. Daytime sleepiness is assessed with the Multiple Sleep Latency Test (MSLT), in which the same measure is repeated during the day; a short mean latency supports pathological sleepiness.

Ohayon M, Wickwire EM, Hirshkowitz M et al. National Sleep Foundation's sleep quality recommendations: first report. Sleep Health. 2017;3(1):6-19. doi:10.1016/j.sleh.2016.11.006

Related: Sleep efficiency · Excessive daytime sleepiness · Insomnia

Sleep paralysis

Being unable to move for a short time while falling asleep or waking up. The mind is awake, but the body does not obey for a while.

For expertsdefinition and thresholds

A state of being unable to move voluntarily while conscious, at the transition into sleep or on waking. In a systematic review pooling 35 studies (36,533 people in total), the lifetime rate of experiencing it at least once was 7.6% in the general population, 28.3% in students and 31.9% in psychiatric patients; in people with panic disorder the rate was 34.6%. When it occurs together with excessive daytime sleepiness and cataplexy, sleep paralysis can be one of the symptoms of type 1 narcolepsy.

Sharpless BA, Barber JP. Lifetime prevalence rates of sleep paralysis: a systematic review. Sleep Med Rev. 2011;15(5):311-315. doi:10.1016/j.smrv.2011.01.007; Bassetti CLA, Adamantidis A, Burdakov D et al. Narcolepsy: clinical spectrum, aetiopathophysiology, diagnosis and treatment. Nat Rev Neurol. 2019;15(9):519-539. doi:10.1038/s41582-019-0226-9

Related: Narcolepsy · REM and NREM sleep · Excessive daytime sleepiness

Sleep stages

Sleep comes in steps like a staircase: you go down from light sleep into deep sleep, then move into dreaming sleep. This round repeats several times a night.

For expertsdefinition and thresholds

In AASM scoring, 30-second epochs are marked as W (wake), N1, N2, N3 and R. Stages 3 and 4 of the pre-2007 Rechtschaffen and Kales system have been merged into N3.

Silber MH, Ancoli-Israel S, Bonnet MH et al. The visual scoring of sleep in adults. J Clin Sleep Med. 2007;3(2):121-131. doi:10.5664/jcsm.26814

Related: REM and NREM sleep · Polysomnography (PSG) · Sleep efficiency

Snoring

The sound made when the soft tissues in the throat vibrate with breathing during sleep. Like a flag flapping in the wind.

For expertsdefinition and thresholds

A breathing sound produced during sleep by partial narrowing of the upper airway and tissue vibration. Snoring without apnea (primary snoring) and snoring that accompanies obstructive sleep apnea cannot be reliably told apart on the basis of sound; objective measurement methods are not standardised either.

Main C, Liu Z, Welch K et al. Surgical procedures and non-surgical devices for the management of non-apnoeic snoring: a systematic review of clinical effects and associated treatment costs. Health Technol Assess. 2009;13(3). doi:10.3310/hta13030

Related: Obstructive sleep apnea (OSA) · Upper airway · Oral appliance

Snoring index

A number that shows how many times you snore in an hour of sleep.

For expertsdefinition and thresholds

The number of snoring events per hour of sleep (events per hour); it is usually measured during polysomnography with a microphone or a vibration sensor. Because the sound level from which snoring is counted differs from study to study, the numbers cannot be compared directly. In the 121-person study by Levartovsky and colleagues, the index was 23% higher in men and fell by about half over the night; snoring intensity, however, was not related to the AHI. In a meta-regression pooling 13 studies, snoring measures were generally strongly related to the AHI; however, the snoring index remained a weaker indicator than intensity and frequency measures.

Levartovsky A, Dafna E, Zigel Y, Tarasiuk A. Breathing and snoring sound characteristics during sleep in adults. J Clin Sleep Med. 2016;12(3):375-384. doi:10.5664/jcsm.5588; Chiang JK, Lin YC, Lu CM, Kao YH. Correlation between snoring sounds and obstructive sleep apnea in adults: a meta-regression analysis. Sleep Sci. 2022;15(4):463-470. doi:10.5935/1984-0063.20220068

Related: Snoring · AHI (apnea hypopnea index) · Polysomnography (PSG)

Social jet lag

The shift between your sleep times on weekdays and at the weekend. Like travelling to another time zone and back every weekend.

For expertsdefinition and thresholds

The mismatch between biological time and social time such as work and school; it is expressed as the difference between sleep timing on workdays and on free days. The difference grows most in late chronotypes: sleep debt builds up on workdays and they try to make up for it on free days. In the 501-person study by Wittmann and colleagues, smoking was markedly more common in late chronotypes; the authors suggested that these associations were explained more by social jet lag than by chronotype itself.

Wittmann M, Dinich J, Merrow M, Roenneberg T. Social jetlag: misalignment of biological and social time. Chronobiol Int. 2006;23(1-2):497-509. doi:10.1080/07420520500545979

Related: Chronotype · Sleep debt · Circadian rhythm

SpO2 (oxygen saturation)

Shows what percentage of the oxygen carriers in your blood are full. It is measured with a small clip worn on the finger.

For expertsdefinition and thresholds

An estimate of arterial haemoglobin oxygen saturation measured by pulse oximetry from light absorption. In healthy people at sea level it is usually 95% or higher; device accuracy falls at low saturation, with movement and with poor circulation.

Jubran A. Pulse oximetry. Crit Care. 2015;19:272. doi:10.1186/s13054-015-0984-8

Related: Desaturation · ODI (oxygen desaturation index) · Polysomnography (PSG)

STOP-Bang questionnaire

A short screening questionnaire that helps decide who should be assessed in more detail for sleep apnea.

For expertsdefinition and thresholds

Classifies the risk of obstructive sleep apnea as low, intermediate or high with eight yes or no items: snoring, tiredness, observed apnea, high blood pressure, body mass index, age, neck circumference and sex. It is a screening tool; the AASM 2017 guideline recommends that questionnaires not be used to diagnose without polysomnography or HSAT.

Chung F, Abdullah HR, Liao P. STOP-Bang questionnaire: a practical approach to screen for obstructive sleep apnea. Chest. 2016;149(3):631-638. doi:10.1378/chest.15-0903

Related: Obstructive sleep apnea (OSA) · Epworth Sleepiness Scale · Polysomnography (PSG)

Titration (pressure setting)

A setting night to find the air pressure on a CPAP machine that is right for you.

For expertsdefinition and thresholds

A study in the sleep laboratory, with polysomnography, in which PAP pressure is raised step by step until obstructive events disappear. Under the AASM 2008 guideline, CPAP starts at least at 4 cmH2O and is raised by at least 1 cmH2O at intervals of at least 5 minutes; the recommended upper limit at age 12 and over is 20 cmH2O. The aim is to eliminate apneas, hypopneas, RERAs and snoring; at the chosen pressure, SpO2 is expected to stay above 90% and leak to be acceptable. The AASM 2019 guideline states that in adults without significant comorbidities, treatment can be started at home with an auto-adjusting device (APAP) or with laboratory titration.

Kushida CA, Chediak A, Berry RB et al. Clinical guidelines for the manual titration of positive airway pressure in patients with obstructive sleep apnea. J Clin Sleep Med. 2008;4(2):157-171. doi:10.5664/jcsm.27133; Patil SP, Ayappa IA, Caples SM et al. Treatment of adult obstructive sleep apnea with positive airway pressure: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2019;15(2):335-343. doi:10.5664/jcsm.7640

Related: PAP therapy (CPAP) · Polysomnography (PSG) · Sleep centre (sleep laboratory) · RERA (respiratory effort related arousal)

Upper airway

The airway that runs from the nose and mouth down to the larynx. You can think of it as a flexible tube whose muscles relax during sleep.

For expertsdefinition and thresholds

Consists of the nose, nasopharynx, oropharynx and hypopharynx; the pharynx is a segment without bony support that stays open through muscle tone. During sleep, reduced dilator muscle activity combined with anatomical narrowing and obesity increases the tendency to collapse.

Dempsey JA, Veasey SC, Morgan BJ, O'Donnell CP. Pathophysiology of sleep apnea. Physiol Rev. 2010;90(1):47-112. doi:10.1152/physrev.00043.2008

Related: Snoring · Obstructive sleep apnea (OSA) · PAP therapy (CPAP)