Body clock (circadian rhythm)

A small cluster of cells behind the eyes runs a roughly 24-hour cycle and tells the rest of the body what time it is: when to release cortisol, when to make melatonin, when to be hungry. Its main input is light hitting the eyes: morning light sets it earlier, late-evening light pushes it later. Evidence: solid

It drifts if you let it. Late nights, late mornings, screens in bed and no daylight all push it later, and then you're wide awake at midnight and dead at nine. Evidence: decent