3am is a common time to wake up or be half awake. This is the old "witching hour" (not midnight) when most people claim to sight "ghosts" or "aliens" in their room, and/or suffer sleep paralysis.

Then there is the two sleep theory that suggests we are not supposed to sleep in single block. In more traditional environments, people got up to stoke the fire at this point. I know some folk that get up to urinate or have a drink. I used to turn the radio on for a bit.