Why do I keep waking up at 3am?

Surfacing in the small hours is not a malfunction. Sleep runs in roughly 90-minute cycles, and everyone briefly wakes between them, usually rolling over and forgetting it. Around 3am you are transitioning out of the deepest sleep into lighter, dream-heavier stages, so a wake-up there registers more. The actual problem is not the waking but the failure to fall back, and the villain is almost always the response: checking the clock, doing mental arithmetic on remaining sleep, and lighting up your phone, all of which spike alertness and anxiety.

Common physiological triggers include alcohol, which fragments the second half of the night as it wears off, a warm room, a full bladder, and the cortisol rhythm that naturally begins rising before dawn. Stress is the frequent driver of the lie-awake version, since a quiet dark room is where an anxious mind finds the floor. The clinical fix is counterintuitive: if you are awake more than about 20 minutes, get up, keep lights dim, do something boring, and return when sleepy, rather than training your brain that bed is a place for lying awake.