Why do I keeping waking up at the same time every night?

I don't have much trouble going to sleep. I usually go to bed around 11-12 to get at least 8 hours in. For the past few weeks, though, I have been waking up pretty much at 4am on the dot every morning. I can usually get back to sleep without much trouble, but this is really annoying. Can anyone shed some light on why this may be happening?