Your circadian rhythm is a roughly 24-hour internal clock that governs nearly every biological process. When aligned, sleep is effortless. When drifted, every other intervention fights a headwind.
WHERE IT LIVES. The "master clock" is the suprachiasmatic nucleus (SCN), ~20,000 neurons in the hypothalamus. It receives input from melanopsin-containing retinal cells responsive to blue light, and synchronizes peripheral clocks in every organ.
THE CORE PATTERN. 6–8 a.m. cortisol peak. 10 a.m. peak alertness. 2:30 p.m. best motor coordination. 4–6 p.m. peak strength. 7 p.m. highest body temp. 9 p.m. melatonin onset (in dim light). 2–4 a.m. lowest body temp. 4–6 a.m. lowest alertness — the "zone of vulnerability" for shift workers.
WHY IT DRIFTS. Internal rhythm is 24.2 hours on average. Without external cues, you drift later by ~12 minutes per day. Modern life — artificial light at night, irregular meals, weekend shifts — drags the clock later.
THE FOUR LEVERS. (1) LIGHT — by far the most powerful. (2) MEAL TIMING. (3) EXERCISE. (4) MELATONIN — small doses (0.3–0.5 mg) timed correctly shift the clock; large doses (5–10 mg) just sedate.
CHRONOTYPES. Genetic variation in PER and CRY genes makes some people morning types ("larks") and some evening ("owls"). ~40% morning, 30% evening, 30% intermediate. Owls forced into early schedules have higher rates of depression, diabetes, and cardiovascular disease.
COMMON DISRUPTIONS. Eastward jet lag is harder than westward. Shift workers have higher rates of cancer, diabetes, heart disease. Heart attacks spike 24% on the Monday after springing forward.
HOW TO RESET. Bright outdoor light within 30 minutes of waking (10,000+ lux). Dim warm light after sunset. Total darkness during sleep. Consistent meal timing. Exercise morning or afternoon. Use 0.3 mg melatonin 5 hours before desired sleep time, not at bedtime.