Goes something like this:
Tired. Go to bed. Turn lights off. Eyes *snap* open, brain flashes to life. AWAKE AWAKE AWAKE! No sleep. Fall asleep some hours later. Wake up every 15 minutes or so. Brain: IS IT TIME TO WAKE UP YET? AWAKE! AWAKE! AWAKE! Then, an hour or so before my alarm goes off, I wake up and can't go back to sleep at all. Brain: UP UP UP UP! TIME TO GET UP!
As far as I can tell, I have no diet or habit change between the good sleeping times and the bad. Plus, with fall coming, it's staying nice and dark through all of my sleeping hours.