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Concept

Circadian rhythm

Also known as: biological clock, circadian biology

The ~24-hour internal biological cycle that coordinates virtually every physiological function — sleep-wake cycle, hormone secretion, body temperature, blood pressure, immune function, gene expression patterns, gut motility, and cognitive performance. Driven by a master clock in the hypothalamic suprachiasmatic nucleus that is itself entrained primarily by retinal light input, with secondary peripheral clocks in essentially every tissue of the body. The 2017 Nobel Prize in Physiology or Medicine (Hall, Rosbash, Young) recognized the molecular discovery of the circadian-clock gene network. Misalignment of circadian rhythm — through shift work, late-night light exposure, irregular meal timing, jet lag — is a substantial driver of metabolic dysfunction, immune impairment, cardiovascular disease, certain cancers, and mood disorders.

Scientific

The circadian system has three integrated levels:

Master clock. The suprachiasmatic nucleus (SCN) of the hypothalamus contains ~20,000 neurons that maintain a ~24-hour oscillation in firing pattern through a transcription-translation feedback loop of the BMAL1, CLOCK, PER, and CRY genes. The SCN is entrained primarily by retinal light input through the retinohypothalamic tract — the small population of intrinsically-photosensitive retinal ganglion cells that signal SCN with light-exposure information independent of vision.

Peripheral clocks. Essentially every tissue and organ has its own circadian-clock gene network operating on the same molecular machinery. Liver, gut, muscle, immune cells, and other tissues maintain ~24-hour rhythms that are normally synchronized with the SCN master clock through endocrine signaling (cortisol, melatonin) and feeding-fasting cycles.

Outputs. Circadian-rhythm-coordinated outputs include: sleep-wake cycle (driven primarily by melatonin signaling and homeostatic sleep pressure); cortisol release pattern (peak ~30 minutes after waking, low at sleep onset); body temperature (~1.5°F daily oscillation, low in early morning, high in evening); blood pressure (morning rise, evening trough); immune function (different immune cells active at different times of day); gut motility and digestive enzyme release (anticipatory of expected meal times); cognitive performance (varies by ~20% across the day in tightly-controlled tests).

Practical

Lifestyle inputs that support healthy circadian rhythm:

  • Morning bright light exposure. 10–30 minutes of direct outdoor light within an hour of waking is the strongest circadian-entraining signal available. Indoor light is substantially weaker.
  • Evening light minimization. Bright artificial light in the 2–3 hours before bed (particularly blue-spectrum from screens) suppresses melatonin production and delays sleep onset. Dim warm lighting in evening; screen-blocking glasses or screen color-temperature shifts; ideally no screens 1 hour before bed.
  • Consistent sleep timing. The SCN maintains its rhythm best when bed and wake times are within 30 minutes day-to-day, including weekends. Irregular schedules are circadian-disruptive.
  • Time-restricted eating. Confining food intake to a 8–12 hour daily window aligned with daylight hours supports peripheral-clock synchronization. Late-night eating is one of the most-circadian-disrupting common practices.
  • Temperature. Cool sleeping environment supports the body’s natural overnight temperature drop.
  • Caffeine timing. Caffeine has a 5–6 hour half-life; afternoon caffeine measurably disrupts subsequent night’s sleep architecture even when subjective sleep onset is unaffected.

See also

Auto-generated from this entry’s typed relations: frontmatter, grouped by relation type so the editorial signal isn’t flattened.

  • Shares substrate with: [[sleep]] · [[endocrine-system]] · [[lunar-planting]]

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