Two Clocks, One Sleep

When you fall asleep is decided by two systems working at once.

The first is sleep pressure: the longer you're awake, the more that built-up adenosine pushes you toward sleep. The second is your circadian clock, a roughly 24-hour internal timer that reads daylight to decide when you should feel awake or sleepy — no matter how long you've been up.

These two systems usually agree. When they don't, you feel it.

Jet lag is the classic clash: after a long flight, your sleep pressure says "rest now," but your circadian clock is still set to the old time zone and insists it's the middle of the day. You lie awake exhausted, or nod off at noon — your body is following two different schedules at once.