Circadian rhythmBody clock
The body's roughly 24-hour internal clock, governing sleep, hormones and metabolism.
Your circadian rhythm is an internal clock running on a cycle of roughly 24 hours, kept in step with the outside world mainly by light. A small region of the hypothalamus acts as the master clock, and nearly every tissue in the body carries a peripheral clock that takes its cue from it.
It governs far more than sleepiness. Core body temperature, cortisol, melatonin, blood pressure, digestive activity, alertness and even peak physical performance all vary predictably across the day, which is why the same effort feels different at seven in the morning and six in the evening.
Light is the dominant signal, and timing matters more than brightness. Bright light in the morning advances the clock and makes earlier sleep easier; bright light late in the evening delays it. This is why the most effective sleep advice is usually about the first hour of the day rather than the last.
Chronotype is real and largely inherited. Genuine morning and evening types exist, they are not a matter of discipline, and school and work schedules built around one of them impose a measurable cost on the other.
That cost has a name. Social jetlag is the gap between your sleep timing on work days and free days, and in survey data it is associated with worse health and metabolic measures. Shifting your clock back and forth every weekend produces something close to a small time zone change each week.
Consistency is therefore the lever with the best return. A wake time you can hold seven days a week, morning light, and dim light in the evening will do more for how you sleep than any device or supplement.
In practice, our sleep calculator works backwards from the wake time you intend to keep, which is the end of the schedule worth fixing first.