The circadian rhythm is the ~24-hour internal cycle that governs sleep-wake, hormone release (cortisol, melatonin, testosterone), body temperature, digestion, and gene expression.
Mechanism: Master clock in the suprachiasmatic nucleus, synchronized by light exposure (especially morning sun) and entrained by feeding, exercise, and social cues. Disruption is linked to metabolic, cognitive, and cardiovascular disease.
Use case: Get bright light within 30 min of waking. Stop bright light 2 hours before bed. Anchor wake time within a 30-min window every day. Time eating mostly during daylight.
Caveats: Shift workers and frequent travelers fight a losing battle. Mitigate via aggressive light timing, melatonin (low dose, timed), and consistent sleep-wake anchors.