Definition
- Circadian rhythm
- The circadian rhythm is the roughly twenty-four-hour internal cycle that governs sleep timing, alertness, body temperature and hormone release. It is set primarily by light exposure, which is why morning daylight advances it and late-night screens delay it.
Why light matters most
Light reaching the eye is the dominant signal for setting circadian timing. Bright light in the morning shifts the rhythm earlier, making it easier to fall asleep and wake earlier. Bright light late in the evening does the opposite.
This is why getting outside shortly after waking does more for tomorrow morning than for today, and why it is a more durable intervention than any single alarm.
Why fighting it is hard
You cannot simply decide to be a morning person. Circadian timing has a substantial genetic component and shifts naturally with age. What you can do is move it gradually — fifteen to thirty minutes at a time, held consistently including at weekends.