I remember hearing somewhere (probably some podcast) that it used to be normal for people to sleep for 3-4 hours, then wake up and do stuff for an hour or so, then go back to sleep for 3-4 hours again. Maybe to help keep watch while you sleep or something similar?

It’s about 50/50 for me. Sometimes getting up and reading for an hour or so will let me fall back to sleep instantly, other times I’m just wide awake at 2AM and then magically get sleepy at 6:30 right before work.