🪴 PG Notes

Circadian Rhythm

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Circadian Rhythm

The circadian rhythm is a self-sustaining biological oscillation with a period slightly longer than 24 hours (~24.2 hours in most humans) that governs sleep-wake timing, hormone release (melatonin, cortisol), core body temperature, alertness, and dozens of other physiological processes. It is one of the two components of the two-component-sleep-model.

The master clock: SCN

The circadian system is orchestrated by the suprachiasmatic nucleus (SCN), a tiny cluster of ~20,000 neurons in the hypothalamus. The SCN receives direct light input from the retina via the retinohypothalamic tract and uses this signal to synchronise (entrain) the internal clock to the external 24-hour day.

Light is the primary zeitgeber (time-giver). Morning light advances the clock (shifts it earlier); evening light delays it. This is why:

  • Exposure to bright light in the morning helps maintain a conventional schedule.
  • Blue light from screens at night delays sleep onset by suppressing melatonin and shifting the clock later.
  • Blind individuals with intact retinal ganglion cells can still entrain; those without them often free-run on a >24h cycle.

Other zeitgebers include meal timing, exercise, social cues, and temperature, but light dominates.

Circadian alertness profile

Across a typical day (for someone waking ~7 AM):

  • Morning peak (hours 1–4 after waking): rising alertness, best time for focused learning and demanding cognitive work.
  • Afternoon dip (~7 hours after waking): a circadian trough in alertness independent of lunch. This is the biological basis for the siesta and the optimal napping window.
  • Evening peak (~10–12 hours after waking): a second wave of alertness. Wozniak calls this the “forbidden zone for sleep” — it’s very hard to fall asleep during this period despite moderate homeostatic pressure.
  • Night descent (~14–16 hours after waking): rapid decline in alertness, melatonin rises, body temperature drops. The optimal sleep window opens.

Chronotypes: larks and owls

Individual circadian periods vary genetically. People with shorter natural periods tend toward early chronotypes (“larks”); those with longer periods tend toward late chronotypes (“owls”). This is not a choice or a habit — it is a heritable trait.

The lark-owl distinction is often misunderstood. Wozniak argues that the “lazy owl” stereotype is harmful: owls forced onto lark schedules (via alarm clocks and early school/work) are chronically sleep-deprived, not undisciplined. See delayed-sleep-phase-syndrome.

Circadian disruption

Misaligning behaviour with the circadian clock carries serious consequences:

  • Shift work: rotating shifts force sleep at circadian-inappropriate times, increasing risks of cardiovascular disease, diabetes, obesity, and cancer.
  • Jet lag: temporary desynchronisation; recovery rate ~1 day per time zone crossed.
  • Social jet lag: the gap between biological and social clocks, especially on weekdays vs. weekends. Chronic social jet lag mimics shift-work effects at lower intensity.
  • sleep-deprivation: circadian misalignment compounds the effects of insufficient sleep.

Connections

  • two-component-sleep-model — circadian rhythm is “Process C” in BorbĂ©ly’s framework.
  • free-running-sleep — what happens when you let the circadian clock run without external constraints.
  • sleep-and-learning — circadian timing determines optimal learning windows.
  • delayed-sleep-phase-syndrome — a delayed circadian clock, not a behavioural problem.
  • napping — the afternoon circadian dip creates the optimal nap window.
  • nudge-theory — school start times and work schedules are circadian choice-architecture.

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