Chronotype is the natural preference for morning, evening, or intermediate sleep-wake timing. Roughly 30-50% genetically determined; PER, CRY, and CLOCK gene variants are major contributors.
Mechanism — Determines optimal cortisol awakening response timing, peak cognitive performance window, exercise tolerance curve, and sleep onset. Mismatch with social schedule produces 'social jet lag' — chronic circadian misalignment with measurable metabolic and mood costs.
Use case — Assess via MEQ (Morningness-Eveningness Questionnaire) or MCTQ. Work with your type — extreme owls forcing 6am wake-ups pay a real cost. Light exposure, exercise timing, and meal timing can shift chronotype within a 1-2 hour window.
Caveats — Chronotype shifts with age — adolescents trend later, then progressively earlier across adulthood. Don't fight biology unnecessarily; align schedule when possible.