Lesson 3
The Morning Light Anchor
See how daylight resets your body clock each morning, which way light at different times pushes it, and how to use that.
A clock that needs setting every day
Sleep pressure explains why a long day makes you sleepy. It doesn't explain why you feel brighter at 10 am than at 3 am after the same night. That's the body clock (Process C): a roughly 24-hour rhythm in alertness, body temperature and hormones that runs whether or not you're tired.
"Roughly" matters. In careful laboratory studies with no daylight to go on, the human body clock runs slightly long, about 24 hours and 11 minutes on average. Left to itself it drifts a little later each day, so it has to be reset every morning. Think of a watch that runs a few minutes slow and gets set each morning against the station clock. Daylight is the station clock.