A

Adenosine

A brain chemical that builds up while you're awake and creates "sleep pressure". Caffeine works by blocking adenosine receptors — which is why coffee masks tiredness but doesn't remove it. See caffeine and sleep.

Anchor sleep

A protected core sleep block kept at the same time every day, even by shift workers with rotating schedules. Keeping even 4 hours identical each day helps stabilize the body clock.

C

CBT-I (Cognitive Behavioral Therapy for Insomnia)

The gold-standard, drug-free treatment for chronic insomnia. Combines stimulus control, sleep restriction, and cognitive work — and usually starts with a sleep diary.

Chronotype

Your natural preference for mornings or evenings, largely genetic. "Larks" peak early, "owls" peak late. Fighting your chronotype with alarm clocks is possible, but it has costs.

Circadian rhythm

The internal ~24-hour cycle that schedules sleepiness, alertness, hormones and body temperature. Driven by the brain's master clock and reset daily by light. See circadian rhythm explained.

Circadian dip

The natural drop in alertness in the early afternoon (roughly 1–3 p.m.) — the reason lunch makes you sleepy and why the early afternoon is the best nap window.

Coffee nap

Drinking coffee immediately before a short 20-minute nap. Caffeine kicks in just as you wake, doubling the alertness boost. Time one here.

Core sleep

The minimum nightly sleep (often quoted as 4–5 hours) containing all the deep and REM sleep your brain urgently needs. Below that, deficits compound fast.

CPAP

Continuous Positive Airway Pressure — the standard treatment for obstructive sleep apnea. A mask pumps gentle air pressure to keep the airway open during sleep.

D

Deep sleep (slow-wave sleep)

The deepest NREM stage, dominated by slow, large brain waves. It handles physical repair, immune function and growth hormone release, and it dominates the first half of the night. See REM vs. deep sleep.

DST (Daylight Saving Time)

The seasonal clock shift used in much of North America and Europe. The one-hour jump acts like mini jet lag — see our 4-day DST adjustment plan.

DSPS (Delayed Sleep Phase Syndrome)

A circadian rhythm disorder where the body clock runs several hours late: falling asleep at 3 a.m. is easy, waking at 7 a.m. is brutal. See the full DSPS guide.

H

HRV (Heart Rate Variability)

The tiny variations in time between heartbeats. Higher HRV during sleep generally signals better recovery; many trackers use it to score your night.

Homeostatic sleep drive

One half of sleep regulation (the other is the circadian clock): the longer you stay awake, the stronger the pressure to sleep — driven mostly by adenosine buildup.

Hypnogram

A chart showing which sleep stage you were in across the night — the staircase of light, deep and REM sleep produced by a sleep study or tracker.

I

Insomnia

Difficulty falling or staying asleep at least 3 nights a week for 3+ months, with daytime impact. First-line treatment is CBT-I, not pills.

J

Jet lag

The mismatch between your body clock and a new time zone after crossing meridians fast. Eastward travel is usually harder. See our jet lag calculator.

L

Light sleep (N1 and N2)

The early NREM stages where you spend over half the night. N2 contains sleep spindles and K-complexes, which help memory consolidation and protect sleep from noise.

Lucid dream

A dream in which you know you're dreaming — sometimes with the ability to steer events. Most common in REM sleep near morning.

M

Melatonin

The hormone your pineal gland releases after dark to signal "night" to the body. Supplements are a timing signal, not a sedative — dose and timing matter more than strength. See melatonin and sleep.

Micro-sleep

An involuntary sleep episode lasting a few seconds, often unnoticed — a major danger in sleep-deprived driving.

N

Night owl

Colloquial term for a late chronotype whose natural bedtime and wake time run late. See when being a night owl becomes DSPS.

Non-REM (NREM)

All sleep stages except REM: N1 and N2 (light sleep) and N3 (deep sleep). About three quarters of the night.

P

Parasomnia

Odd behaviors during sleep: sleepwalking, night terrors, sleep eating. Most common in deep sleep during childhood, usually harmless and outgrown.

Polysomnography

The full overnight sleep study: brain waves, eye movements, muscle tone, breathing, oxygen and heart rhythm, recorded in a lab.

Power nap

A short nap of 10–20 minutes that stays in light sleep — alertness without grogginess. Plan one or read whether power naps work.

R

REM sleep

Rapid Eye Movement sleep — the stage where most vivid dreaming happens, with a paralyzed body and a brain that looks almost awake. Crucial for emotional processing and creativity, and concentrated in the last third of the night. See REM vs. deep sleep.

Rebound REM

Extra-long REM periods that occur after REM has been cut short (by alcohol, deprivation or some drugs) — the brain paying back its REM debt first.

S

Sleep architecture

The structure of a night: how cycles are organized and how much of each stage you get. A healthy adult cycles through 4–6 rounds of light → deep → REM per night. See sleep cycles explained.

Sleep debt

The cumulative shortfall between the sleep you need and the sleep you got. Two short weeks can impair you like an all-nighter. Calculate yours.

Sleep hygiene

The set of habits and environment choices that support good sleep: consistent times, a cool dark room, caffeine rules, wind-down routines. See all 14 rules.

Sleep inertia

The groggy, disoriented feeling after waking, worst when woken from deep sleep. It fades in 15–60 minutes — and it's why 30–60 minute naps backfire. See sleep inertia.

Sleep latency

How long it takes you to fall asleep after lights out. Healthy range: roughly 10–20 minutes. Instant sleep often signals chronic deprivation. See sleep latency.

Sleep pressure

The drive to sleep that builds the longer you're awake (see adenosine). Naps reduce it — which is why late naps push bedtime back.

Social jet lag

The chronic mismatch between weekday and weekend sleep schedules — sleeping in on Saturday is like flying two time zones west every week. See social jet lag.

W

Wake window

The time a baby or toddler can comfortably stay awake between sleeps, by age. The key unit of baby nap scheduling.

White noise

Static-like sound that masks disruptive noise spikes. Pink and brown noise emphasize lower frequencies; some sleepers find them less harsh.

Wind-down routine

A 30–60 minute pre-bed buffer of calm activities that cues your brain for sleep. One of the highest-yield sleep hygiene habits.