So I have the same problem every night, but I think I finally know why. It is my brain that is to blame, not for being smart (pfft, yeah right, as if I'm smart xD), but for being so extremely cunning.
There are this one part of my brain telling me "Miriam, look at the clock, you have school tomorrow. Go to bed at once". It works pretty good until 10-15 minutes after I've gone to bed to sleep. Then, my second (and cunning) brain-part starts working, determined to make it impossible for me to sleep. That's when all my "oh, I wonder if I got any messages on Facebook" and "perhaps I should draw something" and "Japanese horrormovie-marathon in the middle of the night, great idea" - thouhts appear. Somehow they take over the brain part that tells me to go to bed and make me stay awake by making me do all those things.
I've never tryed to count sheeps, and I'll never do it. Sheeps are scary, therefore I count jellyfishes. Jellyfishes are cool, they regenerates (just like The Doctor) themselves when they die pluss technically they should be able to reflect all the colours of the rainbow :3
Now I'll go to sleep... yeah, right. Going back to rotten on my drawing tablet and giving up on getting some sleep.