Circadian rhythmBody clock

The circadian rhythm is an internal cycle of close to 24 hours, run by a master clock in the hypothalamus and by peripheral clocks in nearly every tissue. It sets the timing of melatonin and cortisol release, core body temperature, alertness, and much of metabolic function.

It matters because it is entrained principally by light. Morning light advances the clock and evening light delays it, which is why light exposure — not willpower — is the main lever on sleep timing. Meal timing and exercise timing entrain the peripheral clocks in liver and muscle.

In practice, disruption has real metabolic consequences: shift work is associated with higher rates of obesity, type 2 diabetes and cardiovascular disease. Consistency is the practical instruction — regular sleep and wake times, bright light early, dim light late.