sleep cycles wtf?

I'm not loving this thing that seems to be happening to me about once a week lately, where I wake up in the middle of the night and can't get back to sleep. Last night I woke up at 2:30 in the fucking ack emma, spent over an hour tossing and turning, and finally surrendered and got up at a quarter to four. On the plus side, it meant I got 400 words of Sherlock Holmes fic written before work. On the minus side . . . everything else.

So tired. And tomorrow I have to be up at 4 am in order to be at work at 6. So now to bed. Hopefully not to awaken before the alarm goes off.