Two forces decide when you're tired

Sleepiness is run by two independent systems. The circadian rhythm is an internal ~24-hour clock that rises and falls whether you slept or not. Sleep pressure is simpler: a chemical, adenosine, accumulates every minute you're awake and washes out only during sleep.

Normally the two align — pressure peaks as the clock dips, and you sleep. Jet lag, night shifts, and all-nighters are what misalignment feels like: the clock says noon while the pressure says 3 a.m.

This also explains chronotypes. Owls aren't undisciplined larks; their clocks are genuinely shifted, largely by genetics. A society that starts everything at 8 a.m. isn't testing virtue — it's testing clock inheritance.