Sleep Science Glossary: A-Z

A reference guide to the key terms used in sleep medicine and sleep science. Terms are alphabetically organized. Click any letter to jump to that section.

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

A

Adenosine: A neuromodulator that accumulates in the brain during waking hours and promotes sleepiness. The longer you're awake, the more adenosine builds — this is the "homeostatic sleep pressure" or sleep drive. Caffeine works by blocking adenosine receptors. During sleep, adenosine is cleared from the brain, reducing sleep pressure.
Advanced Sleep Phase (Syndrome): A circadian rhythm disorder in which the sleep-wake cycle is shifted significantly earlier than conventional timing — sleeping at 6-8 PM and waking at 2-4 AM. More common in older adults. The opposite of Delayed Sleep Phase Syndrome.
AHI (Apnea-Hypopnea Index): The number of apneas (complete breathing cessations) and hypopneas (partial breathing reductions) per hour of sleep. Used to classify sleep apnea severity: mild (5-14 events/hour), moderate (15-29), severe (30+).
Alpha Waves: Brain waves in the 8-12 Hz range, associated with relaxed wakefulness with eyes closed. Alpha activity decreases as sleep onset approaches and is replaced by theta waves in N1 sleep.

B

Ballistocardiography (BCG): A technique used by under-mattress sleep sensors (like Withings Sleep Analyzer) to measure micro-movements caused by heartbeat and respiration without any wearable device. Less accurate than wearable PPG for heart rate variability measurements.
BMAL1: A core clock gene that forms part of the transcription-translation feedback loop driving the circadian rhythm. Works in opposition to CLOCK protein. Mutations in BMAL1 are associated with disrupted circadian timing.

C

Cataplexy: Sudden, brief loss of muscle tone triggered by strong emotions (laughter, surprise, anger) while awake. A hallmark symptom of Type 1 narcolepsy. Cataplexy occurs when REM sleep's motor inhibition (atonia) intrudes into wakefulness.
CBT-I (Cognitive Behavioral Therapy for Insomnia): The first-line evidence-based treatment for chronic insomnia, superior to sleep medications in long-term outcomes. Consists of multiple components: stimulus control, sleep restriction therapy, sleep hygiene, cognitive restructuring, and relaxation techniques. Can be delivered by a therapist or digitally (dCBT-I).
Chronobiology: The scientific study of biological rhythms, including circadian (24-hour), ultradian (shorter than 24 hours), and infradian (longer than 24 hours) cycles.
Chronotype: An individual's biological predisposition toward morning (early chronotype/"morning lark") or evening (late chronotype/"night owl") activity patterns. Determined primarily by genetics, particularly variants in circadian clock genes (PER3, CLOCK). Shifts across the lifespan — children tend early, teens shift late, adults gradually return toward morning.
Circadian Rhythm: An approximately 24-hour biological cycle regulating sleep-wake timing, body temperature, hormone release, metabolism, immune function, and many other processes. The master clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. "Circadian" from Latin circa dies: "about a day."
CPAP (Continuous Positive Airway Pressure): The gold-standard treatment for obstructive sleep apnea. A CPAP machine delivers a steady stream of pressurized air through a mask, acting as a pneumatic splint to keep the upper airway open during sleep, preventing apnea events.

D

Dawn Simulator: An alarm clock that gradually increases light intensity in the 20-30 minutes before the set wake time, mimicking natural sunrise. Used to ease waking, particularly in winter or by evening chronotypes who must wake early.
Delayed Sleep Phase (Syndrome/Disorder): A circadian rhythm disorder in which the sleep-wake cycle is significantly shifted later than socially conventional timing. Individuals with DSPS cannot fall asleep until 1-4 AM even when trying. Prevalence is highest in adolescents (7-16%). Treated with morning light therapy, melatonin timing, and chronotherapy.
Delta Waves: High-amplitude, low-frequency (0.5-4 Hz) brain waves that define slow-wave (deep) sleep (N3 stage). Delta wave activity decreases with age and is significantly reduced by alcohol and many sedatives.

E

EEG (Electroencephalography): A method of recording electrical activity in the brain via electrodes on the scalp. EEG is the gold standard for measuring sleep stages — different stages produce distinct brainwave patterns. Used in clinical polysomnography (sleep studies).
Entrainment: The process by which an internal biological rhythm synchronizes to an external time cue (zeitgeber). The human circadian rhythm (with a natural period of ~24.2 hours) is entrained to the 24-hour day primarily by light exposure.

F

Free-Running Rhythm: A circadian rhythm that runs on its own internal period without synchronization to external time cues. In total darkness (no zeitgebers), the human circadian period is approximately 24.2 hours, causing gradual drifting of the sleep-wake cycle.

G

Glymphatic System: A waste-clearance system in the brain that uses cerebrospinal fluid to flush out metabolic byproducts — including amyloid-beta and tau proteins associated with Alzheimer's disease. Glymphatic activity increases approximately 60% during slow-wave sleep. First described by Maiken Nedergaard in 2013.
Growth Hormone: A hormone secreted primarily during slow-wave (N3) sleep, particularly in the first sleep cycle. Critical for physical growth in children, muscle protein synthesis, tissue repair, immune function, and fat metabolism. Sleep disruption reduces growth hormone secretion.

H

Half-Life (Caffeine): The time for half of a substance to be metabolized. Caffeine's half-life is approximately 5-7 hours in average adults, meaning 100mg consumed at 2 PM leaves ~50mg active at 8 PM. Varies by genetics (CYP1A2), age, medications, and pregnancy.
Homeostatic Sleep Drive (Process S): One of the two main processes regulating sleep (alongside the circadian process C). Process S is driven by adenosine accumulation during waking — the longer you're awake, the stronger the drive to sleep. It interacts with the circadian rhythm to determine timing and quality of sleep.
HRV (Heart Rate Variability): The variation in time interval between heartbeats. Higher HRV generally indicates better autonomic nervous system function and recovery. HRV is highest during deep and REM sleep and serves as a proxy for sleep quality in wearable devices. Reduced by alcohol, overtraining, illness, and chronic stress.
Hypnagogic / Hypnopompic: Hypnagogic refers to the transitional state between wakefulness and sleep (falling asleep). Hypnopompic refers to the transition from sleep to waking. Both states can involve hallucinations, vivid imagery, and the sensation of sleep paralysis, which are normal.
Hypnic Jerk: An involuntary muscle twitch or jerk that occurs during the transition to sleep (N1). Also called a "sleep start." Extremely common and benign — experienced by most people at some point. Often accompanied by a sensation of falling.

I

Image Rehearsal Therapy (IRT): A cognitive-behavioral technique for nightmare disorder in which the patient writes down a recurring nightmare, modifies any element of it, and rehearses the modified version during waking hours. Shown to significantly reduce nightmare frequency and intensity.
Insomnia: A sleep disorder characterized by difficulty falling asleep, staying asleep, or waking too early, occurring at least 3 nights per week for at least 3 months, with resulting daytime impairment. Affects ~10% of adults chronically; up to 30% transiently. First-line treatment is CBT-I.

K

K-Complex: A characteristic EEG waveform occurring during N2 sleep — a sharp negative deflection followed by a slower positive component. K-complexes appear in response to external stimuli (sounds, etc.) and are thought to reflect the sleeping brain's mechanism for suppressing arousal responses while maintaining sleep.

L

Locus Coeruleus: A brain region that produces norepinephrine and plays a key role in arousal and wakefulness. Activity in the locus coeruleus is near-zero during REM sleep, which is why memory encoding (which requires norepinephrine) is impaired during dreaming — a fact that contributes to why dreams are quickly forgotten.
Lucid Dreaming: The state of being aware that you are dreaming while the dream is occurring, often with some degree of voluntary control. Occurs during REM sleep. Scientifically validated by Keith Hearne and Stephen LaBerge using pre-agreed eye movement signals in EEG-monitored subjects. Prevalence: ~55% of people experience at least one in their lifetime; ~23% monthly.

M

Melatonin: A hormone produced by the pineal gland in response to darkness. Acts as a circadian timing signal, communicating "it is night" to the brain and body. Does not cause sleep directly but promotes circadian alignment. Suppressed by blue-spectrum light. Naturally lower in older adults. Optimal supplemental dose is 0.3-0.5 mg — far lower than most commercial products.
MILD (Mnemonic Induction of Lucid Dreams): A lucid dreaming induction technique developed by Stephen LaBerge. Involves waking after 5-6 hours of sleep, recalling a recent dream, setting the intention to recognize the dream state, and returning to sleep. Based on prospective memory — the ability to remember to do something in the future.
MSLT (Multiple Sleep Latency Test): A diagnostic test for narcolepsy and idiopathic hypersomnia. Measures how quickly a patient falls asleep during 4-5 scheduled nap opportunities throughout the day. Average sleep onset latency under 8 minutes, combined with two or more sleep-onset REM periods (SOREMPs), is diagnostic for narcolepsy.

N

Narcolepsy: A chronic neurological disorder characterized by excessive daytime sleepiness, sleep attacks, and often cataplexy (Type 1). Caused primarily by loss of hypocretin (orexin)-producing neurons in the lateral hypothalamus, often through an autoimmune mechanism. Affects approximately 1 in 2,000 people.
NREM Sleep (Non-REM): The portion of the sleep cycle outside of REM sleep, consisting of three stages: N1 (light sleep transition), N2 (light sleep with sleep spindles and K-complexes), and N3 (slow-wave/deep sleep). NREM makes up approximately 75-80% of total sleep time in adults.
NSERC (Night Sleep Efficiency Rating): See Sleep Efficiency.

O

Obstructive Sleep Apnea (OSA): The most common form of sleep apnea (vs central sleep apnea). The upper airway repeatedly collapses during sleep, causing breathing cessation events followed by brief arousals. Associated with snoring, daytime sleepiness, hypertension, cardiovascular disease, and metabolic disorders. Diagnosed by sleep study (AHI ≥5 events/hour).
Orexin / Hypocretin: Neuropeptides produced in the lateral hypothalamus that promote and stabilize wakefulness. Loss of orexin neurons causes narcolepsy with cataplexy (Type 1 narcolepsy). Orexin receptor antagonists (suvorexant/Belsomra, lemborexant/Dayvigo) are approved sleep medications that block this wake-promoting system.

P

Parasomnia: Abnormal behaviors or experiences during sleep. Categories include NREM parasomnias (sleepwalking, sleep terrors, confusional arousals — occurring in N3) and REM parasomnias (REM Sleep Behavior Disorder, nightmare disorder). Most NREM parasomnias are more common in children and tend to resolve with age.
Polysomnography (PSG): The gold-standard clinical sleep study, conducted in a sleep laboratory. Simultaneously records EEG (brain waves), EOG (eye movements), EMG (muscle activity), respiration, oxygen saturation, ECG (heart rhythm), and body position. Used to diagnose sleep disorders including sleep apnea, narcolepsy, parasomnias, and REM Behavior Disorder.
PPG (Photoplethysmography): A technique used by wearable sleep trackers (Fitbit, Apple Watch, Oura Ring) to measure blood volume changes in the skin using LED light sensors. Used to estimate heart rate, HRV, and sleep stages. More accurate in rings (finger arteries) than wrists.

R

REM Atonia: The temporary paralysis of voluntary muscles (except eyes and respiratory muscles) that normally occurs during REM sleep. Prevents physical acting out of dream content. Failure of REM atonia causes REM Sleep Behavior Disorder. Brief intrusion of REM atonia into waking causes sleep paralysis.
REM Sleep (Rapid Eye Movement): A distinct sleep stage characterized by rapid eye movements, vivid dreaming, muscle atonia, and a brain state resembling wakefulness in some measures. Comprises 20-25% of total sleep in adults. REM periods are shortest in early sleep cycles and longest in the final cycles. Critical for emotional processing, memory consolidation, and creativity.
REM Sleep Behavior Disorder (RBD): A parasomnia in which REM atonia is absent, allowing physical acting out of dream content — shouting, punching, kicking, or running while asleep. More common in men over 50. RBD is a significant risk factor for synucleinopathies (Parkinson's disease, Lewy body dementia) — up to 80% of RBD patients develop one within 10-15 years. Requires medical evaluation.
Restless Legs Syndrome (RLS): A neurological movement disorder causing uncomfortable sensations in the legs (typically described as crawling, itching, or tingling) that are relieved by movement. Symptoms are worst in the evening and at rest, making sleep onset difficult. Associated with iron deficiency, pregnancy, and kidney disease. Treated with dopamine agonists, iron supplementation, or alpha-2-delta ligands.

S

SCN (Suprachiasmatic Nucleus): A tiny bilateral structure in the hypothalamus containing approximately 20,000 neurons. The master circadian clock, synchronizing virtually all biological rhythms to the 24-hour day. Primary zeitgeber input is light via the retinohypothalamic tract from intrinsically photosensitive retinal ganglion cells (ipRGCs) containing melanopsin.
Sleep Debt: The cumulative shortfall between the sleep obtained and the sleep needed. Short-term sleep debt (days to weeks) can be substantially recovered with subsequent adequate sleep. Chronic long-term sleep debt may have lasting physiological effects. Not to be confused with sleep inertia.
Sleep Efficiency: The percentage of time in bed actually spent asleep. Calculated as (total sleep time ÷ total time in bed) × 100. Normal is 85%+. Lower efficiency indicates frequent waking or extended time falling asleep. Sleep restriction therapy (part of CBT-I) aims to improve efficiency by limiting time in bed.
Sleep Inertia: The grogginess, disorientation, and impaired performance immediately after waking — particularly after waking from deep (N3) sleep. Can last 15-60 minutes. Minimized by waking at the end of a sleep cycle (during lighter REM phase), allowing adequate total sleep duration, and using light and caffeine strategically on waking.
Sleep Paralysis: A temporary inability to move or speak during the transition between sleep and wakefulness (most often upon waking). Caused by REM atonia persisting into waking. Often accompanied by hypnagogic/hypnopompic hallucinations. More frequent during sleep deprivation, irregular schedules, and stress. Usually benign and brief (seconds to a few minutes). Associated with narcolepsy when frequent.
Sleep Spindles: Bursts of oscillatory neural activity (11-15 Hz) visible in EEG during N2 sleep. Generated by the thalamus. Thought to play a role in memory consolidation and in protecting sleep by dampening the brain's response to sensory stimuli. People with more sleep spindles tend to be lighter sleepers.
Slow-Wave Sleep (SWS) / N3: The deepest stage of NREM sleep, defined by high-amplitude delta wave activity (≥20% of the epoch). Most restorative stage for physical processes: growth hormone release, tissue repair, immune function, glymphatic waste clearance. Decreases significantly with age and alcohol consumption. Most abundant in the first half of the night.
Social Jetlag: The chronic circadian misalignment experienced by evening chronotypes who must conform to early social/occupational schedules on weekdays, then revert to their natural schedule on weekends. Described by chronobiologist Till Roenneberg. Associated with metabolic dysfunction, mood disorders, and increased all-cause mortality risk.

T

Thermoregulation: The body's process of maintaining core temperature within a narrow range. Core temperature falls during sleep onset and the early part of sleep, reaches its minimum in the middle of the night, and rises toward waking. Bedroom temperature significantly influences this cycle. During REM sleep, thermoregulation is largely suspended.
Two-Process Model of Sleep: The dominant theoretical framework for sleep regulation, proposed by Alexander Borbély in 1982. Posits that sleep timing is determined by the interaction of two processes: Process S (homeostatic sleep drive, driven by adenosine buildup) and Process C (circadian alerting signal). When S exceeds C sufficiently, sleep occurs; when C overcomes S, waking is maintained.

U

Ultradian Rhythm: A biological rhythm shorter than 24 hours. Sleep cycles (approximately 90 minutes) are a prominent example of an ultradian rhythm. Other ultradian rhythms include basic rest-activity cycles during waking and 90-minute oscillations in alertness and cognitive performance.

W

Wake Windows: In infant sleep science, the period of time a baby can comfortably stay awake between sleep periods before accumulating too much sleep pressure. Wake windows vary by age — newborns: 45-60 minutes; 3-month-olds: 60-90 minutes; 6-month-olds: 2-3 hours. Exceeding the wake window causes overtiredness that paradoxically makes sleep more difficult.
WBTB (Wake Back to Bed): A lucid dreaming induction technique: wake after 5-6 hours of sleep, stay awake for 20-60 minutes (reading about lucid dreaming or doing reality testing), then return to sleep. The technique increases REM sleep pressure and places the returning sleep period more heavily into REM, making lucid dreaming more likely.

Z

Zeitgeber: German for "time giver." An environmental cue that synchronizes the circadian clock to the 24-hour day. The dominant zeitgeber for humans is light (specifically short-wavelength blue light detected by melanopsin-containing ipRGCs in the retina). Secondary zeitgebers include meal timing, exercise, social contact, and temperature. First described by Jürgen Aschoff.
Z-Drugs (Non-benzodiazepine Hypnotics): A class of sleep medications including zolpidem (Ambien), zaleplon (Sonata), and eszopiclone (Lunesta). Act on GABA-A receptors similarly to benzodiazepines but with some structural differences. Associated with dependency, tolerance, rebound insomnia on discontinuation, parasomnias (sleepwalking, sleep-eating), and — particularly in older adults — falls, cognitive impairment, and dementia risk. Listed on Beers Criteria as inappropriate for older adults.