Key Terms

The following definitions are provided for quick reference. Full context for each appears in the relevant section of the guide.

Adenosine

A chemical that accumulates in the brain during wakefulness, driving the homeostatic sense of sleepiness (Process S). Cleared during sleep. Caffeine works by blocking the receptors that detect it, masking sleep pressure rather than removing it.

CBT-I (Cognitive Behavioural Therapy for Insomnia)

A structured, multi-session non-drug treatment for chronic insomnia combining stimulus control, sleep restriction therapy, cognitive techniques, and relaxation training. Recommended as first-line treatment ahead of medication by major clinical guidelines.

Chronotype

An individual's natural circadian preference — broadly, "morning lark" versus "night owl." Partly genetically determined, though light exposure and schedule consistency can shift it by an hour or two for most people.

Circadian Rhythm

A roughly 24-hour internal cycle, generated by the suprachiasmatic nucleus and synchronised daily by light, that governs the timing of sleep, alertness, hormone release, and body temperature.

Delayed Sleep Phase Disorder (DSPD)

A circadian rhythm disorder in which the internal clock is persistently shifted later than conventional schedules require, making early sleep onset and early waking genuinely difficult regardless of effort. Distinct from an ordinary night-owl preference.

Glymphatic System

The brain's waste-clearance channel, running through fluid-filled spaces between brain cells. Substantially more active during sleep, when the space between cells expands and cerebrospinal fluid flow increases.

Melatonin

A hormone released by the pineal gland as darkness falls, signalling to the body that biological night has begun. Suppressed by bright light, particularly blue-wavelength light. A messenger of circadian timing, not a sedative that directly forces sleep onset.

NREM Sleep (Non-Rapid Eye Movement)

The stages of sleep (N1, N2, N3) other than REM, encompassing the transition into sleep and slow-wave deep sleep, which is concentrated in the first half of the night and is the most physically restorative stage.

Obstructive Sleep Apnoea (OSA)

A common, frequently undiagnosed disorder involving repeated pauses or reductions in breathing during sleep due to airway collapse, fragmenting sleep and elevating cardiovascular and metabolic risk. Effectively treated with CPAP or oral appliances once diagnosed.

REM Sleep (Rapid Eye Movement)

The sleep stage during which brain activity resembles wakefulness, most dreaming occurs, and emotional memory processing is concentrated. Lengthens across the night and dominates the second half — meaning short nights disproportionately cut into REM.

Sleep Architecture

The structured pattern of sleep stages across a night, cycling roughly every 90 minutes. Changes measurably with age — generally shifting toward lighter, more fragmented sleep with a smaller proportion of REM.

Sleep Debt

The cumulative cost of insufficient sleep, which compounds across consecutive short nights rather than resetting each day. Notably, subjective sleepiness plateaus after a few days of restriction even as objective cognitive impairment continues to worsen — making self-assessment an unreliable guide to whether sleep debt has been repaid.

Sleep Pressure (Process S)

The homeostatic drive to sleep, built by rising adenosine across wakefulness and cleared by sleep. One of the two components of the two-process model, alongside circadian timing (Process C).

Social Jet Lag

The circadian disruption caused by a mismatch between weekday and weekend sleep schedules — functionally similar to crossing time zones. Even a two-hour weekend shift produces a measurable disruption, which is why capping weekend wake-time drift matters.

Suprachiasmatic Nucleus (SCN)

A cluster of roughly 20,000 neurons in the hypothalamus that functions as the body's master circadian clock, receiving light input directly from the retina and coordinating the timing of peripheral clocks throughout the body.

Two-Process Model

The framework describing sleep drive as the interaction of homeostatic sleep pressure (Process S) and circadian alerting timing (Process C). Misalignment between the two — from an irregular schedule, jet lag, or shift work — is behind most real-world sleep-timing problems.

Zeitgeber

German for "time-giver": any external cue that helps synchronise the circadian clock to the 24-hour day. Light is the dominant zeitgeber in humans; meal timing, exercise, and social cues play secondary roles.