after midnight confessions

Sometimes (basically every night) I have a hard time going to sleep. Not falling asleep, but going to sleep. I can be perfectly exhausted (like right now) and still not be able to pull myself away from my computer, my study notes, a good book...I finish the task on my to-do list that I've told myself I'd finish and then go to bed, but then I look at the clock (or not, whatever) and think "Oh I'll just do this one more" or I'll just wander aimlessly through my computer, organizing folders and rearranging icons. Pointless hubbub. But I feel like while I'm awake I should do something and as long as my body's not shutting down on me (which it'll do, no matter how hard I try sometimes I can't even finish a thought before my body's said ENOUGH and overruns my harddrive to turn me off) I should be productive. If my body says there's some energy left, then I just sap it til it's dry. So it's not insomnia, because I can sleep- but I just won't. What's that called?