Circadian Rhythm Calculator

The circadian rhythm is an approximately 24-hour internal biological clock driven by the suprachiasmatic nucleus (SCN) in the hypothalamus. It regulates sleep-wake timing, body temperature, cortisol, melatonin, hunger hormones, immune function, and dozens of other physiological processes. It is entrained primarily by light exposure.

The most accurate method: take 2 weeks of vacation with no alarm, no obligations, and free access to natural light. Your natural sleep and wake times in week 2 reflect your true circadian preference. The MCTQ (Munich Chronotype Questionnaire) uses your free-day sleep midpoint as a chronotype proxy — the same method used in population-scale chronobiology research.

Primary disruptors are: artificial light after dark (especially blue-wavelength from screens), irregular sleep and wake times, late eating (food is a secondary zeitgeber that shifts peripheral clocks), shift work, trans-meridian travel (jet lag), and social jet lag (weekend schedule shifts). Alcohol and stimulants also alter the phasing of circadian markers.

Yes. Chronobiology research shows that consistent behavioral interventions — bright light exposure in the morning, darkness in the evening, and fixed wake times — can permanently shift the circadian clock by 1–2 hours within 2 weeks. More dramatic shifts (delayed or advanced sleep phase disorder) sometimes require melatonin therapy, light therapy protocols, and in rare cases chronotherapy under medical supervision.

Circadian misalignment is the state of attempting to sleep, work, or eat at times that conflict with your biological clock's preferred schedule. It is the underlying mechanism of social jet lag, shift work disorder, and jet lag. Chronic misalignment is associated with metabolic syndrome, cardiovascular disease, cancer risk, and mood disorders independent of total sleep loss.

Your circadian rhythm is the biological mechanism — the internal 24-hour oscillator. Your chronotype is the expression of that rhythm — whether you naturally gravitate toward morning or evening timing. Chronotype is about 50% heritable, shifts from morning-type (children) to evening-type (teenagers) and back toward morning-type (older adults), and varies across the population in a roughly normal distribution from extreme larks to extreme owls.