This sleep dictionary explains the terms you are most likely to meet in a sleep article, clinic visit, or sleep-study report. Some entries describe symptoms, some describe measurements, and others name disorders or treatments. Those categories are not interchangeable.
A word or number rarely establishes a diagnosis by itself. Sleep-study rules differ by age, test type, sensors, and scoring standard. The International Classification of Sleep Disorders, Third Edition, Text Revision groups diagnoses into insomnia disorders, sleep-related breathing disorders, central disorders of hypersomnolence, circadian rhythm sleep-wake disorders, parasomnias, and sleep-related movement disorders 1. The AASM Scoring Manual separately governs how sleep stages, arousals, respiratory events, movements, and other signals are scored 2.
Use the alphabetical sections to find a term quickly. Links lead to focused guides when a definition is only the starting point.
A to E
Actigraphy: A clinical method that estimates sleep and wake patterns from movement recorded by a small device, usually worn on the wrist for several days. Actigraphy can help evaluate certain insomnia and circadian questions, but it does not directly measure brain-defined sleep stages. A consumer watch may use similar sensors without meeting the validation and clinical-use standards of medical actigraphy 34.
Apnea: A scored breathing event with a near-complete reduction in airflow. In obstructive apnea, breathing effort continues against a narrowed or closed airway. In central apnea, airflow and breathing effort fall together. Event duration and scoring rules depend on age and the applicable standard, so the adult rule should not be applied to an infant or child 5.
Apnea-hypopnea index (AHI): The number of scored apneas and hypopneas per hour of sleep on a polysomnogram. AHI is central to sleep-apnea diagnosis and severity categories, but it does not capture every symptom, oxygen effect, arousal, or health risk. Pediatric and adult interpretation differs. A machine-generated AHI, a PAP-device estimate, and a laboratory AHI may also be calculated from different signals 6.
Arousal: A brief shift toward lighter sleep or wakefulness identified from the electroencephalogram, or EEG, using formal scoring rules. An arousal may be linked to breathing, a limb movement, noise, or no clear cause. It is not the same as a full remembered awakening, and a heart-rate or movement change alone does not establish a scored cortical arousal 2.
Arousal index: The number of scored arousals per hour of sleep. The total can include respiratory, movement-related, and spontaneous arousals. It needs interpretation alongside age, sleep stages, test quality, symptoms, and the rest of the report rather than as a stand-alone diagnosis.
Atonia: Markedly reduced skeletal-muscle activity. Normal REM sleep includes low chin-muscle tone, often called REM atonia. It is not absolute paralysis of every muscle: breathing, eye movements, and small twitches can continue 72.
Awakening: A transition from sleep to wakefulness. A person may remember some awakenings and not others. On a sleep study, wakefulness is determined from recorded brain, eye, and muscle signals, not from memory alone.
Cataplexy: A brief loss of muscle tone while awake, usually triggered by emotion, with awareness preserved. It is strongly associated with narcolepsy type 1, but weakness, fainting, and seizures can look different and require proper evaluation 8.
Circadian rhythm: A biological pattern that repeats about every 24 hours and helps time sleep, alertness, hormones, temperature, and other functions. Humans have a central clock and clocks in tissues throughout the body. Circadian timing interacts with sleep pressure rather than acting as a simple on-off switch 9.
Chronotype: A person's tendency toward earlier or later sleep and activity timing. "Morning person" and "evening person" are informal descriptions. Chronotype reflects biology, age, light exposure, behavior, and obligations; it is not an immutable diagnosis.
Cognitive behavioral therapy for insomnia (CBT-I): A structured, multicomponent treatment for chronic insomnia. It commonly combines stimulus control, sleep restriction or sleep-compression methods, cognitive work, and other behavioral strategies. CBT-I is more than general sleep-hygiene advice, and its schedule should be adapted when another health or safety issue affects sleep 10.
Continuous, auto-adjusting, and bilevel positive airway pressure (CPAP, APAP, and BPAP): PAP modes that deliver pressurized air through a mask. CPAP provides one prescribed pressure, APAP varies pressure within a prescribed range, and BPAP provides different inspiratory and expiratory pressures. The modes are not interchangeable for every patient or every breathing disorder 11.
Daytime impairment: A meaningful effect of poor or mistimed sleep on daytime life, such as reduced concentration, mood disturbance, lower performance, or difficulty functioning socially, at school, or at work. It is broader than sleepiness.
Deep sleep: The everyday name for N3 sleep, also called slow-wave sleep. It is identified from EEG slow waves, not from how still a person looks or how hard they are to wake. "Deep sleep" on a consumer device is an estimate rather than a clinical stage measurement 74.
Electroencephalogram (EEG): A recording of electrical activity from electrodes placed on the scalp. In a sleep study, EEG helps distinguish wake, N1, N2, N3, REM sleep, and arousals. It is one part of polysomnography, not a complete sleep study by itself 2.
Entrainment: Alignment of an internal biological rhythm with an external cycle. For the circadian system, the light-dark cycle is the strongest cue, although timing of activity, meals, and social schedules may also contribute.
Excessive daytime sleepiness (EDS): Difficulty staying awake or alert when wakefulness is expected, with unintended dozing or a strong drive to sleep. EDS is a symptom with many possible causes, including insufficient sleep, medicines, sleep apnea, circadian disorders, and central disorders of hypersomnolence. It is not the same as low energy.
F to M
Fatigue: A sense of exhaustion, low energy, or reduced capacity for effort. A fatigued person may feel depleted without being able to fall asleep. Sleepiness and fatigue can occur together, but separating them helps a clinician investigate the right causes.
Home sleep apnea test (HSAT): A medically ordered test that records a limited set of breathing and related signals outside a sleep center. It can diagnose obstructive sleep apnea in selected, uncomplicated adults after a clinical evaluation. It is not a general sleep study, a consumer oxygen monitor, or the right test for every suspected breathing, movement, neurologic, pediatric, or complex medical disorder 6.
Homeostatic sleep drive: The biological pressure for sleep that generally increases with time awake and decreases during sleep. It works with the circadian alerting system, which helps explain why someone can feel a temporary second wind despite being awake for a long time 9.
Hypersomnolence and hypersomnia: Hypersomnolence describes excessive sleepiness or an increased tendency to fall asleep. Hypersomnia is used for some disorders and is also used less precisely to mean unusually long sleep, so the surrounding context matters. Neither term should be assigned from one long night or one questionnaire score 1.
Hypnagogic and hypnopompic: Hypnagogic experiences occur while falling asleep; hypnopompic experiences occur while waking. Vivid imagery, sounds, or a sense of presence can occur at either boundary. They may appear in healthy people as well as in narcolepsy and other conditions, so the words describe timing rather than a diagnosis.
Hypnogram: A graph showing sleep stages and wakefulness across a sleep period. It makes stage transitions, awakenings, and overall sleep architecture easier to see.
Hypopnea: A scored partial reduction in breathing accompanied by the required oxygen change, EEG arousal, or both under the scoring rule being used. It is not simply "shallow breathing." Different accepted definitions can change the reported event count, so the report's scoring method matters 5.
Insomnia: Repeated difficulty falling asleep, staying asleep, or returning to sleep after an early awakening despite adequate opportunity, together with daytime consequences. A bad night or voluntary short sleep is not automatically insomnia. Duration and frequency determine the formal diagnosis, and another sleep, medical, substance, or circadian problem may need consideration 1. See the focused insomnia guide for evaluation and treatment.
Jet lag: A temporary mismatch between internal circadian timing and local time after rapid travel across time zones. Direction, number of time zones, light exposure, and individual timing affect the pattern. It differs from ordinary travel fatigue 9.
Light therapy: Deliberately timed exposure to bright light to shift circadian timing or support alertness in selected settings. Morning and evening light can shift the clock in different directions, so brightness alone does not determine the effect. Clinical timing matters because poorly timed light can shift sleep in an unwanted direction 9.
Melatonin: A hormone whose release normally rises in dim evening conditions and helps signal biological night. The word may also refer to a supplement or prescription product, but taking melatonin is not the same as reproducing the body's natural signal. Product, dose, timing, age, and treatment goal matter 9.
Microsleep: A very brief, involuntary sleep episode or sleep-like lapse, often occurring when a person is severely sleepy. It may pass without awareness. A microsleep while driving or operating equipment is dangerous; the term is not a reassuring label for ordinary mind-wandering.
Multiple sleep latency test (MSLT): A daytime laboratory test that measures how quickly a person falls asleep during a series of scheduled nap opportunities and whether REM sleep appears soon after sleep onset. Preparation, the preceding night's PSG, medicines, substances, sleep schedule, and treatment of other sleep disorders can affect the result. It is used in a clinical evaluation of narcolepsy and selected hypersomnolence disorders, not as a general test for feeling tired 12.
N to R
N1, N2, and N3 sleep: The three non-REM stages. N1 is the transition from wake to sleep. N2 is established sleep marked by features such as sleep spindles and K complexes on EEG. N3 is slow-wave sleep. These stages are defined from recorded signals, and their proportions change with age, prior sleep, medicines, illness, and time of night 72.
Narcolepsy: A chronic disorder of sleep-wake regulation that causes excessive daytime sleepiness and disrupted nighttime sleep. Some people also have cataplexy, sleep paralysis, or vivid sleep-boundary experiences. Diagnosis requires a clinical assessment and properly conducted testing; falling asleep easily or having one sleep attack does not establish it 8.
Nightmare: A disturbing, well-remembered dream that usually leads to awakening. Nightmares can occur occasionally without a disorder. Nightmare disorder involves a repeated pattern with distress or impaired functioning and needs more than vivid dream content alone 1.
Non-rapid eye movement (NREM) sleep: The phase that includes N1, N2, and N3. Brain activity, muscle tone, autonomic activity, and ease of awakening differ across those stages, so NREM should not be treated as one uniform "inactive" state 7.
Obstructive and central sleep apnea (OSA and CSA): In OSA, the upper airway repeatedly narrows or closes while breathing effort continues. In CSA, airflow and breathing effort reduce or stop together. A person can have both patterns, and the distinction changes evaluation and treatment. See the sleep apnea guide for the full diagnostic context.
Oxygen desaturation: A measured fall in blood oxygen saturation from a preceding level. A desaturation can accompany a respiratory event, but it is not synonymous with apnea or hypopnea and does not reveal the cause by itself.
Oxygen desaturation index (ODI): The number of qualifying oxygen drops per hour of the measurement period. The required size of the drop, reference period, sensors, and denominator can vary. ODI is not interchangeable with AHI, and a consumer oximeter's event count is not a sleep-apnea diagnosis 2.
Parasomnia: An unwanted event or experience that occurs during sleep, at sleep onset, or while waking, such as sleepwalking, confusional arousals, nightmares, or REM sleep behavior disorder. Different parasomnias arise from different sleep states and need different evaluation 1.
Periodic limb movements of sleep (PLMS): Repetitive limb movements scored during sleep using timing and muscle-signal rules. PLMS may occur with restless legs syndrome and other conditions or without symptoms. A PLMS index is a measurement; periodic limb movement disorder requires clinical consequences and exclusion of better explanations 132.
Polysomnography (PSG): A sleep study that records several body signals, usually including EEG, eye movements, chin muscle activity, airflow, breathing effort, oxygen saturation, and heart rhythm. Extra sensors may answer specific questions. PSG can identify stages and many events, but the report still needs clinical interpretation 2.
Positive airway pressure (PAP): Treatment that sends pressurized air through a mask. PAP is the umbrella term; CPAP, APAP, BPAP, and more specialized modes deliver pressure differently. The prescription depends on the breathing disorder and clinical context 11.
Rapid eye movement (REM) sleep: A sleep stage defined by a characteristic combination of EEG activity, rapid eye movements, and low chin-muscle tone. Vivid dreams are common in REM, but dreams can occur in non-REM sleep too. REM is neither "deep sleep" nor ordinary wakefulness 72.
Respiratory disturbance index (RDI): A report measure that may include apneas, hypopneas, and respiratory-effort-related arousals per hour of sleep. Usage can vary across laboratories and payers, so check the report's definition instead of assuming RDI always equals AHI 6.
Respiratory event index (REI): The number of scored breathing events divided by monitoring or recording time, commonly reported by an HSAT that cannot determine sleep time in the same way as PSG. Because awake time may remain in the denominator, REI can differ from a PSG AHI 6.
Restless legs syndrome (RLS): A clinical pattern of an urge to move the legs, usually with unpleasant sensations, that begins or worsens at rest, improves at least temporarily with movement, and is more prominent in the evening or at night. RLS is experienced while awake and is not the same as PLMS recorded during sleep 13.
S to Z
Sedative and hypnotic: Sedative describes a medicine or substance that reduces alertness or activity; hypnotic describes one intended to promote sleep. Some medicines have both effects. These terms do not mean a product is safe for every person, improves normal sleep architecture, or treats the cause of insomnia.
Sleep architecture: The arrangement of wake, N1, N2, N3, and REM sleep across the night. It includes stage order, timing, transitions, and proportions rather than one universal ideal pattern.
Sleep cycle: A progression through non-REM and REM sleep that repeats across a sleep period. Cycles vary in length and composition within one night and across age, health, and prior sleep. Multiplying a fixed "90-minute cycle" is not a reliable way to choose an exact bedtime 7.
Sleep debt: A planning metaphor for cumulative sleep shortfall, not a precise biological balance. A calculator may compare estimated need with estimated sleep, but it cannot determine an exact hour-for-hour repayment. See what sleep debt means for the limits of the concept.
Sleep deficiency and sleep deprivation: Sleep deprivation means not obtaining enough sleep. Sleep deficiency is broader and can include too little sleep, sleep at the wrong biological time, poor or fragmented sleep, or a disorder that prevents adequate sleep 14.
Sleep efficiency: Total sleep time divided by time in bed, expressed as a percentage. It describes how much of the sleep opportunity was spent asleep. It does not grade how restorative the sleep was, and one cutoff should not be applied across a diary, actigraphy, PSG, all ages, and all disorders 23.
Sleep fragmentation: Repeated interruption of sleep by awakenings or arousals. Fragmentation can occur from breathing events, movement, pain, environment, caregiving, substances, medicines, or other causes. A person may not remember each interruption.
Sleep hygiene: Habits and environmental conditions that support sleep, such as appropriate timing, a workable sleep opportunity, and managing light, noise, caffeine, and alcohol. Sleep hygiene can remove barriers, but the AASM guideline advises against using sleep hygiene alone as the treatment for chronic insomnia 10.
Sleep inertia: Temporary grogginess, slowed thinking, or reduced performance after waking. It can be stronger after abrupt awakening, deeper sleep, a nap, circadian low points, or insufficient sleep. Persistent or severe impairment needs context rather than a generic "sleep inertia" label.
Sleep latency: The time from the start of an intended sleep opportunity to sleep onset. In a diary it is estimated; in PSG it is determined from recorded sleep stages. An MSLT uses a separate nap-based latency definition.
Sleep maintenance and wake after sleep onset (WASO): Sleep maintenance is the ability to remain asleep after initially falling asleep. WASO totals the time scored or estimated awake between sleep onset and the final awakening. Frequent brief awakenings and one long awake period can produce a similar total but feel different.
Sleep onset: The transition from wakefulness into sleep. Its exact scoring definition depends on the test or research protocol. Feeling that you were awake all night can differ from EEG-scored sleep without either experience being "fake."
Sleep opportunity and time in bed (TIB): Sleep opportunity is the period available and intended for sleep. TIB is the clock time spent in bed and may include reading, trying to sleep, wakefulness, and sleep. Neither term equals total sleep time.
Sleep paralysis: Temporary inability to move while awareness returns at sleep onset or awakening, when REM-related muscle atonia persists briefly into wakefulness. It can occur by itself or with narcolepsy. Recurrent episodes, injuries, breathing concerns, or significant distress warrant clinical discussion 8.
Sleep quality: A person's overall experience of sleep, including continuity, satisfaction, restoration, and daytime function. It is useful but subjective. A high consumer score, a long duration, or a favorable stage graph does not prove that sleep was healthy 4.
Sleep stage: One of the EEG-defined states of sleep: N1, N2, N3, or REM. Clinical staging uses brain, eye, and chin-muscle signals in short time segments. A phone or general movement tracker cannot reproduce that measurement directly 24.
Snoring: Sound produced by vibration of upper-airway tissues during sleep. Snoring may occur without obstructive sleep apnea, and OSA can occur without loud reported snoring. Sound cannot show airflow, breathing effort, arousal, or oxygen well enough to make the diagnosis.
Suprachiasmatic nucleus (SCN): A small region in the hypothalamus that serves as the central circadian pacemaker. It receives information about environmental light and helps coordinate clocks and rhythms across the body 9.
Total sleep time (TST): The total time scored or estimated as sleep within a study or sleep period. In PSG it is the sum of N1, N2, N3, and REM sleep. It is not the same as time in bed.
Wake after sleep onset (WASO): See sleep maintenance and wake after sleep onset above.
Zeitgeber: An external time cue that helps synchronize a biological rhythm with the environment. Light is the strongest circadian zeitgeber for humans. The term means "time giver"; it does not refer to a sleep stage or hormone 9.
How to read a sleep report without self-diagnosing
Start by identifying what kind of test produced the report. A laboratory PSG, an HSAT, PAP-device data, actigraphy, overnight oximetry, and a consumer wearable measure different signals. Numbers with similar labels may therefore have different denominators, algorithms, or purposes.
Then ask:
- Which values were directly measured, and which were estimated?
- Which scoring rule, age range, and reference values apply?
- Do the results explain the symptoms and reason for testing?
- Were there technical limitations, little sleep, unusual sleep position, or missing signals?
- What diagnosis, if any, did the interpreting clinician make from the complete record?
Consumer technology can help document patterns and start a conversation, but the AASM states that it is not a substitute for medical evaluation or validated diagnostic testing 4. If a term on your report remains unclear, ask the ordering or interpreting clinician to explain what it means in that test and how much it matters for your symptoms.
