Circadian Rhythm — Your Master Clock

Your circadian rhythm is a ~24-hour internal clock driven primarily by light. The suprachiasmatic nucleus (SCN) in your hypothalamus is the master clock, synchronised daily by: 1. LIGHT: The most powerful cue. Morning light suppresses melatonin and signals 'day.' Evening light delays your clock. 2. TEMPERATURE: Body temperature drops ~1-2°C to initiate sleep. A cool bedroom (18-19°C) accelerates this. 3. FOOD TIMING: Eating signals 'daytime' to peripheral clocks in your gut, liver, and muscles. 4. EXERCISE: Morning/afternoon exercise advances the clock. Late-night exercise delays it. The clock runs at slightly different speeds for different people — this is your chronotype.

CHRONOTYPE is your genetically determined preference for sleep timing. It's not discipline — it's biology. • MORNING TYPES (~25%): Natural rise before 7am. Peak alertness 9am-12pm. Sleep pressure builds early. • EVENING TYPES (~25%): Natural rise 9am+. Peak alertness 6pm-9pm. Biologically disadvantaged in a 9-5 world. • INTERMEDIATE (~50%): Flexible, with mild tendencies. CHRONOTYPE CHANGES WITH AGE: Teenagers naturally shift to late chronotypes (biological, not laziness). Adults shift back. After ~55, most people naturally become morning types. PRACTICAL: If you have control over your schedule, align your most cognitively demanding work with your chronotype's peak alertness window. Fighting your chronotype burns cognitive resources and increases stress.