Sleep glossary
Circadian rhythm
The body clock behind your sleepy and alert hours, why light is the signal that sets it, and what happens when it disagrees with your calendar.
In short
Your circadian rhythm is the roughly 24 hour internal cycle that decides when you feel sleepy and when you feel alert. It is run by a master clock in the brain, and light is the strongest signal keeping it lined up with the real day.
Circadian rhythm is also called body clock, internal clock or circadian clock.
The clock itself is a small cluster of cells in the hypothalamus, and it keeps time on its own even in a cave with no daylight. Left alone it does not run at exactly 24 hours, which is why it needs resetting every day. Light hitting the eye is the reset signal, and everything else you might try is weak by comparison.
It governs more than sleep. Core body temperature, hormone release, digestion and alertness all follow the same cycle, which is why the afternoon dip is a real feature of the day and not a character flaw. Sleepiness at any moment is the product of two things pulling against each other: this circadian signal and the sleep pressure that has been building since you woke. That pairing is the two process model of sleep regulation, and it explains far more of your day than either half alone.
What actually moves it
Timing beats intensity. The general finding is that light in the morning advances the clock, pulling your natural bedtime and wake time earlier, while light in the evening delays it, pushing them later. Outdoor morning light is far brighter than anything indoors, so ten minutes outside after waking does more than an hour under a ceiling lamp. Meal timing and exercise nudge the clock too, but they are secondary.
Consistency is the other lever, and the one people underrate. A clock that gets the same signal at the same time every day settles. A clock that gets a different signal every day never does, which is the mechanism behind social jetlag, and it is felt most sharply by people whose chronotype already runs late.
The hormone in the middle of all this is melatonin, which rises in the evening to mark biological night and is suppressed by light. If your clock and your alarm are currently arguing, how to fix your sleep schedule sets out the order to change things in: wake time first, light second, bedtime last.