No sleep till Brooklyn (or 3am)

Why is it that around 4 or 5 o'clock this afternoon the only thing I wanted to do was collapse on my bed & fall asleep, and now that it's 1 am I'm more interested in sitting in front of my computer than sleeping? I've got to straighten my ass out pretty soon here, I'm slipping into a bad sleep schedule & I almost feel a bit of a funk hovering around here some-where...