- Circadian rhythm
- A biological cycle of approximately 24 hours generated by an autonomous molecular clock (the CLOCK/BMAL1–PER/CRY feedback loop). Present in virtually every cell of the body. In the absence of external time cues, the human clock free-runs with a period of approximately 24.2 hours.
- Suprachiasmatic nucleus (SCN)
- The master circadian pacemaker — a paired cluster of ~20,000 neurons in the anterior hypothalamus directly above the optic chiasm. Receives photic input from the retina via the retinohypothalamic tract and coordinates peripheral clocks throughout the body.
- Zeitgeber
- German for "time-giver." Any environmental cue that entrains (synchronises) the circadian clock to the 24-hour day. Light — detected by melanopsin-containing ipRGCs in the retina — is the primary zeitgeber. Secondary zeitgebers include meal timing, physical activity, social schedules, and temperature.
- Melatonin
- A hormone synthesised from serotonin in the pineal gland and released into the bloodstream during darkness under SCN control. Signals night to the brain and body; promotes sleep onset; regulates seasonal reproductive cycles. Suppressed by light, especially short-wavelength (blue) light at ~480 nm.
- Sleep pressure (Process S)
- The homeostatic drive for sleep that accumulates progressively during waking hours, largely driven by adenosine build-up in the basal forebrain. Component of Borbély's two-process model. Caffeine works by blocking adenosine A1 and A2A receptors, reducing subjective sleepiness without reducing actual sleep need.
- Two-process model (Borbély)
- A model of sleep regulation proposing two interacting processes: Process S (homeostatic sleep pressure, increasing during waking and dissipating during sleep) and Process C (circadian alerting signal from the SCN, opposing sleepiness during the day). Sleep occurs when S exceeds C.
- NREM sleep
- Non-rapid eye movement sleep, divided into three stages: N1 (light sleep, theta waves, hypnic jerks), N2 (sleep spindles and K-complexes, ~50% of total sleep time), and N3 (slow-wave sleep, delta waves >20% of epoch, most restorative stage, growth hormone release, declarative memory consolidation).
- REM sleep
- Rapid eye movement sleep — paradoxical sleep with a desynchronised EEG resembling waking, rapid conjugate eye movements, and virtual paralysis (atonia) of voluntary muscles. Associated with vivid dreaming. Rich in acetylcholine; noradrenaline and serotonin are suppressed. REM periods lengthen across the night; important for procedural learning and emotional memory processing.
- Sleep architecture
- The overall structure of sleep across a night: alternating NREM–REM cycles lasting ~90 minutes each (4–6 cycles per night). N3 predominates in early cycles; REM lengthens in later cycles. Cutting sleep short preferentially reduces late-night REM.
- Chronotype
- An individual's biological preference for the timing of sleep and wakefulness ("morning lark" vs "night owl"). Partly heritable (~50%), influenced by clock gene variants (PER3, CLOCK). Adolescents shift strongly toward eveningness (peaking ~age 20) before gradually shifting toward morningness in adulthood.