Sleep glossary

Every term that comes up in sleep research, defined in one or two sentences of plain English.

Adenosine: A neurotransmitter that builds up while you're awake and creates 'sleep pressure.' Caffeine works by blocking adenosine receptors.
Beta-amyloid: A metabolic waste protein that accumulates between brain cells. Deep sleep activates the glymphatic system, which clears it. Chronic sleep deprivation lets it build up — a hallmark of Alzheimer's disease.
CBT-I: Cognitive Behavioral Therapy for Insomnia. The first-line treatment for chronic insomnia — outperforms sleep medications at long-term follow-up.
Chronotype: Your genetically influenced preferred sleep-wake schedule. Roughly 30% morning types, 30% evening types, 40% intermediate. Largely fixed by genetics.
Circadian rhythm: Your body's ~24-hour internal clock, governed by the suprachiasmatic nucleus and entrained by light exposure to your retina.
Deep sleep (N3): Slow-wave sleep, characterized by delta brain waves. Where physical recovery, immune tuning, and memory consolidation occur. Concentrated in the first half of the night.
Glymphatic system: The brain's waste-clearance system. Active during deep sleep — flushes metabolic waste including beta-amyloid.
Melatonin: A hormone produced by the pineal gland in darkness. Signals to your body that it's nighttime. Light exposure (especially blue) suppresses it.
Non-REM (NREM): All sleep that isn't REM. Includes the three NREM stages: N1, N2, and N3.
REM sleep: Rapid Eye Movement sleep. Brain activity nearly matches wakefulness; muscles are paralyzed (REM atonia). Where vivid dreams, memory consolidation, and emotional processing happen.
Sleep architecture: The structural pattern of sleep stages across a night. A healthy night has 4–6 cycles of N1 → N2 → N3 → REM, each ~90 minutes.
Sleep cycle: One full pass through the four sleep stages, lasting ~90 minutes on average (range 80–110 min).
Sleep debt: The cumulative deficit between the sleep you need and the sleep you got. Can be partially repaid in 2–3 nights of catch-up; chronic debt may take weeks to fully resolve.
Sleep efficiency: Time asleep ÷ time in bed. Above 85% is healthy; below 80% suggests insomnia or fragmented sleep.
Sleep inertia: The grogginess after waking from deep sleep. Why naps over 30 min often leave you feeling worse, not better.
Sleep latency: How long it takes to fall asleep. 5–20 min is healthy. Under 5 min often indicates sleep deprivation; over 30 min suggests insomnia or hyperarousal.
Sleep pressure: The biological drive to sleep, increasing with hours awake. Driven primarily by adenosine accumulation.
Sleep spindles: Brief bursts of brain activity during N2 sleep. Protect against waking and play a role in motor-skill consolidation.
Slow-wave sleep (SWS): Another name for deep sleep / N3. Defined by delta-frequency brain waves.
Social jet lag: The biological cost of sleeping on different schedules during weekdays vs. weekends. Equivalent to flying across multiple time zones each week.