Glossary
Sleep Science
Adenosine — A chemical that accumulates in the brain during wakefulness, creating sleep pressure. Caffeine works by blocking adenosine receptors.
Circadian rhythm — The body's internal clock, running on an approximately 24-hour cycle. Regulates sleep timing, hormone release, body temperature, and many other functions.
Chronotype — Your genetically influenced natural tendency toward morning or evening alertness. Morning types are "larks," evening types are "owls."
Cortisol — The body's primary stress hormone. In a healthy pattern, highest in the morning and lowest at night. Chronic stress disrupts this cycle.
Delta waves — Large, slow brain waves characteristic of deep sleep (N3). Associated with physical restoration and memory consolidation.
Glymphatic system — The brain's waste-clearance system, most active during deep sleep. Flushes metabolic byproducts including beta-amyloid proteins.
Homeostatic sleep drive — The progressive increase in the need to sleep the longer you stay awake. Driven primarily by adenosine accumulation.
Melatonin — A hormone produced by the pineal gland that signals nighttime to the body. Production is suppressed by light exposure, especially blue light.
Pineal gland — A small brain structure that produces melatonin in response to darkness signals from the suprachiasmatic nucleus.
REM sleep — Rapid Eye Movement sleep. A sleep stage characterized by vivid dreaming, brain activity similar to waking, and temporary muscle paralysis. Critical for emotional processing and memory.
Sleep architecture — The structure and pattern of sleep stages throughout the night, including the timing and proportion of each stage.
Sleep cycle — One complete progression through all sleep stages, lasting approximately 90 minutes. A typical night includes 4–6 cycles.
Sleep inertia — The grogginess and impaired performance experienced immediately after waking, especially from deep sleep. Usually resolves within 15–30 minutes.
Sleep latency — The time it takes to fall asleep after intending to. Also called sleep onset latency.
Sleep pressure — The physiological need to sleep that builds during wakefulness. Driven by adenosine and the homeostatic sleep drive.
Sleep spindles — Short bursts of brain activity during Stage 2 sleep. Associated with memory consolidation and motor learning.
Slow-wave sleep (SWS) — Another term for deep sleep (N3), named for the slow delta brain waves that characterize it.
Suprachiasmatic nucleus (SCN) — A tiny brain region in the hypothalamus that serves as the body's master clock. Receives light input directly from the eyes.
Sleep Stages
N1 (Stage 1) — Light sleep. The transition between wakefulness and sleep. Lasts a few minutes. Easily disrupted.
N2 (Stage 2) — Intermediate sleep. Body temperature drops, heart rate slows. Comprises roughly 50% of total sleep time.
N3 (Stage 3) — Deep sleep, also called slow-wave sleep. Most physically restorative stage. Dominant in the first half of the night.
REM — Rapid Eye Movement sleep. Dreaming stage. Important for emotional and cognitive processing. Dominant in the second half of the night.
Sleep Disorders
Circadian rhythm disorder — A condition where the body's internal clock is significantly misaligned with desired or socially required sleep-wake times.
Delayed sleep phase syndrome (DSPS) — A circadian rhythm disorder where the natural sleep-wake cycle is shifted significantly later than desired. Common in adolescents and young adults.
Insomnia — Difficulty falling asleep, staying asleep, or waking too early, accompanied by daytime impairment. Chronic insomnia persists for three months or more.
Narcolepsy — A neurological disorder causing excessive daytime sleepiness and sometimes sudden loss of muscle tone (cataplexy).
Parasomnias — Abnormal behaviors during sleep, including sleepwalking, sleep talking, night terrors, and REM behavior disorder.
Restless leg syndrome (RLS) — An uncomfortable urge to move the legs, typically worse in the evening and at rest. Can significantly delay sleep onset.
Sleep apnea — A condition where breathing repeatedly stops and starts during sleep. Obstructive sleep apnea (OSA) is the most common type, caused by airway collapse.
Treatments and Techniques
CBT-I — Cognitive Behavioral Therapy for Insomnia. A structured program combining behavioral and cognitive techniques. The gold standard non-drug treatment for chronic insomnia.
Cognitive restructuring — A technique that identifies and challenges unhelpful thought patterns, replacing them with more realistic alternatives.
CPAP — Continuous Positive Airway Pressure. A device used to treat sleep apnea by delivering pressurized air through a mask to keep the airway open.
Light therapy — Use of bright artificial light (typically 10,000 lux) to reset circadian rhythms. Used for delayed sleep phase, seasonal affective disorder, and jet lag.
Paradoxical intention — A CBT-I technique where the patient tries to stay awake rather than trying to fall asleep, reducing performance anxiety.
Progressive muscle relaxation (PMR) — A relaxation technique involving systematically tensing and releasing muscle groups to reduce physical tension.
Sleep hygiene — Behavioral and environmental practices that promote good sleep, including consistent timing, dark rooms, and caffeine management.
Sleep restriction therapy — A CBT-I technique that limits time in bed to match actual sleep time, consolidating sleep and increasing sleep drive.
Stimulus control — A CBT-I technique that strengthens the mental association between bed and sleep by restricting bed use to sleep and sex only.
Measurements
Heart rate variability (HRV) — The variation in time between heartbeats. Higher HRV generally indicates better recovery and lower stress. Often measured during sleep by wearables.
Lux — A unit of light intensity. Indoor lighting is typically 100–500 lux. Outdoor daylight ranges from 10,000 to 100,000 lux. Light therapy lamps provide 10,000 lux.
Sleep efficiency — The percentage of time in bed spent actually sleeping. Calculated as total sleep time ÷ time in bed × 100. Target: 85% or higher.
Sleep onset latency (SOL) — Time from lights off to falling asleep. Normal is under 20 minutes. Over 30 minutes may indicate insomnia.
Wake after sleep onset (WASO) — Total time awake during the night after initially falling asleep. Under 30 minutes is considered normal.
Substances
Blue light — Light in the 450–495nm wavelength range. The most potent signal for circadian alertness. Emitted by screens and cool-toned LED lights.
Caffeine half-life — The time it takes for caffeine concentration in the blood to reduce by half. Approximately 5–6 hours in most adults.
L-theanine — An amino acid found in tea that promotes relaxation without drowsiness. Sometimes used as a sleep supplement.
Magnesium glycinate — A highly bioavailable form of magnesium supplement. Supports muscle relaxation and nervous system function.
Tryptophan — An amino acid and precursor to serotonin and melatonin. Found in turkey, eggs, nuts, and cheese.
Other Terms
Brown noise — A deeper, richer noise spectrum than white noise. Often described as resembling a waterfall or wind. Popular for sleep.
Social jet lag — The circadian disruption caused by different sleep schedules on weekdays versus weekends. Analogous to crossing time zones.
White noise — A consistent sound that contains all frequencies at equal intensity. Used to mask environmental noise disruptions during sleep.