Why is it that when I go to bed slightly early, I wake up *really* early?

I had a headache last night so I went to bed around 11:30, which is about an hour or so before I normally go to bed. I woke up around 4am and couldn’t fall back asleep. It’s now 5:20am and I feel ready to get up and start my day.

This seems to happen any time I feel exceptionally tired an hour or two before my bedtime and I decide to go to bed. What gives?