Sleep Glossary

Common sleep medicine terms, abbreviations, and concepts explained in plain language for patients and caregivers.

A B C D E F H I J K L M N O P R S T W Z

A

AASM(American Academy of Sleep Medicine): The leading professional medical society for sleep medicine in the United States. The AASM publishes clinical practice guidelines, accredits sleep centers, and sets scoring standards for sleep studies.
Actigraphy: A non-invasive method of monitoring rest and activity cycles using a small wrist-worn device (actigraph) that detects movement. Often used over one to two weeks to estimate sleep patterns outside a lab.
Adenosine: A chemical that accumulates in the brain during wakefulness and promotes sleep pressure. Caffeine works by blocking adenosine receptors, temporarily reducing the feeling of sleepiness.
AHI(Apnea-Hypopnea Index): The average number of apneas (breathing pauses) and hypopneas (shallow breaths) per hour of sleep. Used to diagnose and classify sleep apnea severity: 5-14 mild, 15-29 moderate, 30+ severe.
Alpha waves: Brain waves at 8-13 Hz that appear during relaxed wakefulness, especially with eyes closed. They diminish as a person transitions into the lighter stages of sleep.
Apnea: A complete pause in breathing lasting at least 10 seconds during sleep. Apneas may be obstructive (airway blocked), central (no effort to breathe), or mixed.
Arousal: A brief shift to a lighter stage of sleep or to wakefulness, often lasting only a few seconds. Frequent arousals fragment sleep and can cause daytime sleepiness even when total sleep time seems adequate.
ASV(Adaptive Servo-Ventilation): A specialized form of positive airway pressure therapy that automatically adjusts pressure breath by breath. Used primarily for complex or central sleep apnea, not routine obstructive sleep apnea.

B

BiPAP(Bilevel Positive Airway Pressure): A PAP device that delivers a higher pressure when you inhale and a lower pressure when you exhale. Often prescribed when CPAP is not tolerated or for certain lung and neuromuscular conditions.
Bruxism: Grinding or clenching of the teeth, which may occur during sleep (sleep bruxism). Can cause tooth wear, jaw pain, and headaches, and is sometimes associated with arousals or sleep apnea.

C

Cataplexy: A sudden loss of muscle tone triggered by strong emotions such as laughter. Cataplexy is a hallmark feature of narcolepsy type 1 and ranges from slight facial drooping to full body collapse.
CBT-I(Cognitive Behavioral Therapy for Insomnia): A structured, evidence-based first-line treatment for chronic insomnia. Combines sleep restriction, stimulus control, cognitive restructuring, sleep hygiene, and relaxation techniques over several weeks.
Central Sleep Apnea(CSA): A form of sleep apnea in which the brain temporarily fails to signal the muscles to breathe. Unlike obstructive apnea, there is no airway blockage and no breathing effort during the event.
Chronotype: A person's natural tendency toward being an early bird, a night owl, or somewhere in between. Chronotype is influenced by genetics and shifts gradually across the lifespan.
Circadian Rhythm: The roughly 24-hour internal biological cycle that regulates sleep, alertness, body temperature, and hormone release. It is set by light exposure and other cues called zeitgebers.
Cortisol: A hormone released by the adrenal glands that follows a daily rhythm: lowest around midnight and peaking shortly after waking. Elevated cortisol at night is associated with insomnia and stress.
CPAP(Continuous Positive Airway Pressure): The first-line treatment for obstructive sleep apnea. A CPAP machine delivers a constant stream of pressurized air through a mask to keep the upper airway open during sleep.

D

Delayed Sleep-Wake Phase Disorder(DSWPD, DSPS): A circadian rhythm disorder where a person's natural sleep-wake schedule is shifted significantly later than conventional timing, often causing difficulty falling asleep before 2-3 AM and waking in the morning.
Delta Waves: Slow brain waves under 4 Hz that dominate deep sleep (N3, also called slow-wave sleep). Delta activity is associated with physical restoration and memory consolidation.

E

EEG(Electroencephalogram): A recording of the brain's electrical activity using scalp electrodes. EEG is the core measurement used to identify sleep stages during a polysomnogram.
EMG(Electromyogram): A recording of muscle electrical activity. During a sleep study, chin and leg EMG channels help identify REM sleep, bruxism, and periodic limb movements.
EOG(Electrooculogram): A recording of eye movements used in sleep studies to identify REM sleep, when rapid eye movements occur beneath closed eyelids.
Excessive Daytime Sleepiness(EDS): Difficulty staying awake or alert during the day despite opportunity for sleep. EDS is a core symptom of narcolepsy, sleep apnea, and insufficient sleep, and is distinct from general fatigue.

F

Ferritin: A blood marker of iron stores. Low ferritin (generally under 75 ng/mL) is strongly linked to restless legs syndrome, and iron repletion is a first-line treatment under the 2025 AASM guidelines.

H

Hypersomnia: A condition of excessive sleepiness or excessive sleep duration. Idiopathic hypersomnia and narcolepsy are the primary central disorders of hypersomnolence.
Hypnagogic: Referring to the transition from wakefulness to sleep. Hypnagogic hallucinations are vivid sensory experiences that can occur during sleep onset and are common in narcolepsy.
Hypnogram: A graph showing the progression of sleep stages across the night. It is a standard visual summary produced from polysomnography.
Hypnopompic: Referring to the transition from sleep to wakefulness. Hypnopompic hallucinations occur upon waking and, like hypnagogic ones, can be a feature of narcolepsy.
Hypopnea: A partial reduction in airflow during sleep (typically at least 30%) accompanied by an oxygen drop or arousal. Hypopneas are counted alongside apneas in the AHI.

I

Insomnia: Difficulty falling asleep, staying asleep, or waking too early, despite adequate opportunity, with daytime consequences. Chronic insomnia occurs at least three nights per week for three months or longer.

J

Jet Lag: A temporary circadian misalignment caused by rapid travel across time zones. Symptoms include insomnia, daytime sleepiness, and gastrointestinal disturbance, typically resolving within days.

K

K-complex: A distinctive sharp, high-amplitude waveform that appears in stage N2 sleep. K-complexes are thought to help suppress arousal from external stimuli and may play a role in memory processing.

L

Light Therapy: Timed exposure to bright light (typically 10,000 lux) used to shift circadian rhythms. Used for delayed sleep-wake phase disorder, seasonal affective disorder, and shift work adaptation.

M

Melatonin: A hormone released by the pineal gland in the evening that signals darkness to the body. Low-dose timed melatonin can help shift circadian rhythms; higher doses are less effective and not generally recommended.
Microarousal: A brief (3-15 second) shift toward lighter sleep or wakefulness that the sleeper usually does not remember. Frequent microarousals degrade sleep quality.
MSLT(Multiple Sleep Latency Test): A daytime test in which a person is given five nap opportunities at two-hour intervals. It measures how quickly sleep onset occurs and whether REM sleep appears, helping diagnose narcolepsy and idiopathic hypersomnia.

N

Narcolepsy: A chronic neurological disorder characterized by excessive daytime sleepiness and abnormal REM sleep regulation. Type 1 includes cataplexy and is caused by loss of orexin-producing neurons.
Nocturia: Waking one or more times during the night to urinate. It is a common complaint that can be caused by age, fluid timing, diuretics, or sleep apnea.
NREM(Non-Rapid Eye Movement Sleep): The three stages of sleep other than REM: N1 (light transition), N2 (bulk of sleep), and N3 (deep slow-wave sleep). NREM makes up about 75-80% of total sleep time in adults.

O

Obstructive Sleep Apnea(OSA): The most common form of sleep apnea, caused by repeated collapse of the upper airway during sleep. Leads to oxygen drops, arousals, and increased cardiovascular risk if untreated.
Orexin(Hypocretin): Neuropeptides produced in the hypothalamus that promote wakefulness and regulate REM sleep. Loss of orexin neurons causes narcolepsy type 1.

P

Parasomnia: A category of sleep disorders involving abnormal behaviors, movements, emotions, or dreams during sleep. Examples include sleepwalking, night terrors, and REM sleep behavior disorder.
Periodic Limb Movements of Sleep(PLMS): Repetitive, involuntary limb movements (typically leg twitches) during sleep. When frequent and associated with symptoms, they are called periodic limb movement disorder (PLMD).
Polysomnography(PSG, Sleep Study): The standard overnight test for diagnosing sleep disorders. It records EEG, EOG, EMG, airflow, respiratory effort, oxygen saturation, and ECG simultaneously in a lab or at home.

R

REM Rebound: An increase in the amount and intensity of REM sleep following a period of REM deprivation, such as after stopping alcohol, certain medications, or recovery from sleep loss.
REM Sleep(Rapid Eye Movement Sleep): The sleep stage characterized by rapid eye movements, vivid dreaming, and muscle atonia (paralysis). REM is critical for memory consolidation and emotional regulation and comprises about 20-25% of adult sleep.
REM Sleep Behavior Disorder(RBD): A parasomnia in which the normal muscle paralysis of REM sleep is absent, allowing the sleeper to act out dreams. RBD can be an early warning sign of Parkinson's disease and related disorders.
Restless Legs Syndrome(RLS, Willis-Ekbom Disease): A sensorimotor disorder characterized by an urge to move the legs, usually accompanied by uncomfortable sensations, worse at rest and in the evening, and relieved by movement.

S

Sleep Architecture: The structural organization of sleep, including the distribution, timing, and cycling of NREM and REM stages across the night.
Sleep Debt: The cumulative difference between the sleep a person needs and the sleep they actually get. Chronic sleep debt contributes to cognitive impairment, metabolic disease, and mood disturbance.
Sleep Drive: The homeostatic pressure to sleep that builds during wakefulness and dissipates during sleep. Along with the circadian rhythm, it is one of the two main regulators of sleep-wake behavior.
Sleep Efficiency: The percentage of time spent in bed that is actually spent asleep, calculated as total sleep time divided by time in bed. Healthy sleep efficiency is typically 85% or higher.
Sleep Hygiene: A collection of behaviors and environmental practices that support healthy sleep, such as a consistent schedule, a dark cool bedroom, and avoiding caffeine late in the day. Alone, it is rarely sufficient for chronic insomnia.
Sleep Inertia: The temporary grogginess and impaired performance experienced immediately after waking, especially from deep sleep. It typically lasts 15-30 minutes.
Sleep Latency: The time it takes to fall asleep after getting into bed with the intention to sleep. Normal sleep latency ranges from about 10 to 20 minutes.
Sleep Restriction Therapy: A core component of CBT-I that temporarily limits time in bed to build sleep drive and consolidate fragmented sleep. Time in bed is gradually expanded as sleep efficiency improves.
Sleep Spindles: Brief bursts of 11-16 Hz activity that are a defining feature of stage N2 sleep. They are thought to support memory consolidation and protect sleep from external disturbance.
Slow-Wave Sleep(SWS, N3, Deep Sleep): The deepest stage of NREM sleep, dominated by delta waves. SWS is associated with physical restoration, growth hormone release, and glymphatic clearance of brain metabolites.
Somnambulism(Sleepwalking): A parasomnia involving complex behaviors during partial arousal from deep sleep, including walking and other activities. Most common in children and usually outgrown by adolescence.
Stimulus Control: A CBT-I technique that strengthens the association between bed and sleep by using the bed only for sleep and intimacy, and leaving the bed when unable to sleep within about 15-20 minutes.
Suprachiasmatic Nucleus(SCN): A small region of the hypothalamus that serves as the body's master circadian clock. It receives light input from the retina and coordinates peripheral clocks throughout the body.

T

Theta Waves: Brain waves at 4-7 Hz, prominent during light sleep stages N1 and N2 and during REM sleep. They represent a transition between wakefulness and deeper sleep.
Titration Study: A type of in-lab sleep study used to determine the optimal CPAP or BiPAP pressure setting for treating sleep apnea.

W

WASO(Wake After Sleep Onset): The total amount of time spent awake after initially falling asleep and before final awakening. High WASO is a hallmark of sleep-maintenance insomnia.

Z

Zeitgeber: An external cue that synchronizes the circadian rhythm to the 24-hour day. Light is the most powerful zeitgeber; others include meals, exercise, and social activity.

Definitions are provided for general educational purposes and are not a substitute for professional medical advice.