The Master Clock

Your circadian rhythm runs from a tiny control center deep in the brain called the suprachiasmatic nucleus — a cluster of about twenty thousand neurons in the hypothalamus.

It acts as the conductor. Nearly every organ, from your liver to your skin, keeps its own local clock, and the master clock keeps them all in sync using hormones and nerve signals so the whole body runs on one schedule.

That schedule is easy to throw off. Your clock takes its main cue from light, so the blue light from screens at night fools it into thinking it's still daytime. It holds back melatonin, the hormone that signals sleep, and can push your sleep onset later by roughly an hour.

When the clock drifts, everything it runs drifts too — your metabolism, immune defenses, repair cycles, and mood all fall out of step.