Sleep glossary

A neurotransmitter that accumulates while you're awake and creates the sense of sleep pressure.

Caffeine blocks adenosine receptors, which is why coffee makes you feel less sleepy without removing the underlying drive to sleep.

A roughly 24-hour internal biological cycle that governs sleep timing, alertness, hormones, and body temperature.

Set primarily by light exposure. The suprachiasmatic nucleus is the master clock.

Your innate preference for being awake or asleep at certain times — your biological 'morning person' or 'night owl' setting.

Largely genetic, modifiable only at the margins. Strongly affects optimal bedtime.

A hormone that peaks 30–45 minutes after wake (the 'cortisol awakening response') and helps you feel alert.

Elevated cortisol at night is a common cause of middle-of-night waking.

Continuous Positive Airway Pressure — the standard treatment for moderate-to-severe obstructive sleep apnoea.

A machine that delivers pressurised air via mask to keep the airway open during sleep.

The deepest stage of non-REM sleep (also called slow-wave sleep or stage 3), dominated by delta brain waves.

Concentrated in the first third of the night. Crucial for physical recovery and immune function.

A circadian rhythm disorder where your natural sleep window is shifted significantly later than conventional schedules.

Distinct from insomnia — the issue is timing, not the ability to sleep.

Electroencephalography — recording electrical activity of the brain via scalp electrodes.

The primary tool for classifying sleep stages in a sleep study.

The increasing pressure to sleep that builds the longer you've been awake.

Driven mostly by adenosine accumulation. One half of the two-process model of sleep regulation.

An involuntary muscle twitch experienced just as you're falling asleep.

Harmless and very common. Mechanism not fully understood.

A graph showing how sleep stages progress across a night.

What you see in a sleep tracker app, simplified — and what a sleep clinic produces with much higher accuracy.

Difficulty falling asleep, staying asleep, or returning to sleep, occurring at least three nights a week for at least three months, with daytime impact.

Distinct from short-term sleep difficulty. Has its own diagnostic code.

Non-REM stages 1 and 2 — easier to wake from, lighter brain activity, makes up roughly half of total sleep time.

Not 'low quality' sleep. Serves memory consolidation and other functions.

A hormone produced by the pineal gland in response to darkness; signals biological night to the body.

A timing signal, not a sedative. Most effective for phase-shifting (jet lag, shift work).

Brief episodes of sleep lasting a few seconds, occurring during wakefulness when sleep-deprived.

Particularly dangerous behind the wheel; you may not be aware they happened.

Non-rapid eye movement sleep — covers stages 1, 2, and 3.

Distinct from REM sleep in brain activity, physiology, and function.

Abnormal behaviours during sleep — sleepwalking, night terrors, REM behaviour disorder, sleep paralysis.

Some are benign and developmental; others (like RBD) can indicate underlying neurological conditions.

A full overnight sleep study, recording EEG, eye movements, muscle tone, breathing, oxygen, and heart rate.

The gold standard for diagnosing sleep disorders. Vastly more accurate than wearables.

Rapid eye movement sleep — dreaming sleep, characterised by paralysed muscles and active brain activity.

Concentrated in the second half of the night. Vital for memory consolidation and emotional processing.

An increase in REM sleep on subsequent nights after REM has been suppressed (by alcohol, certain medications, or REM deprivation).

Often experienced as vivid dreams after stopping antidepressants or quitting alcohol.

A breathing disorder where the airway repeatedly collapses (obstructive) or the brain fails to signal breathing (central) during sleep.

Severely fragments sleep. Strongly linked to cardiovascular disease and daytime fatigue.

The pattern and proportion of sleep stages across a night.

Changes systematically with age — older adults have less deep sleep and more fragmentation.

The cumulative deficit between sleep you've had and sleep you need.

Accumulates but isn't strictly linear; partial recovery is possible across a few nights.

The percentage of time in bed actually spent asleep.

Above 85% is healthy. Below 80% is the threshold where CBT-I becomes relevant.

The grogginess and impaired performance immediately on waking, especially from deep sleep.

Worse after long naps or waking mid-cycle. Why short naps (under 20 min) outperform long ones.

The time it takes to fall asleep after lights-out.

Healthy range is roughly 10–20 minutes. Falling asleep in under five minutes can signal sleep deprivation.

Synonymous with homeostatic sleep drive — the build-up of pressure to sleep that increases with time awake.

Reduced by sleep, especially deep sleep.

A core CBT-I technique: temporarily reducing time in bed to consolidate sleep and rebuild sleep efficiency.

Counterintuitive but extremely effective. Highly evidence-supported.

Synonymous with deep sleep — stage 3 NREM, dominated by delta waves.

Most concentrated early in the night, decreases with age.

The discrepancy between your weekday and weekend sleep schedules, creating a recurring mini-jetlag effect.

Common and meaningful. Linked to metabolic and mental health outcomes in observational data.

A region of the hypothalamus that serves as the body's master circadian clock.

Receives input from the eyes; sets the rhythm for nearly every other clock in the body.

An external cue that resets the circadian clock — German for 'time-giver'.

Light is the dominant zeitgeber. Meals, exercise, and social cues are weaker but real.