Sleep pressure
A chemical that builds the longer you're awake and drains while you sleep. Caffeine blocks the gauge.
From the moment you wake, a substance called adenosine builds up in the brain. The more of it, the sleepier you feel. Sleep clears it. That's the pressure side of sleep; the body clock is the timing side, and you need both to line up. Evidence: solid
Caffeine works by sitting in adenosine's parking spaces. The pressure is still there, you just can't feel it. Evidence: solid When the caffeine wears off, it all arrives at once. A nap drains some pressure, which is why a late one leaves you awake at midnight. Evidence: decent
Why it matters
Going to bed early after a bad night often buys you more time awake in bed, because the clock isn't ready even if the pressure is. Evidence: decent Getting up at the usual time keeps the two lined up for tonight.