Why do the weirdest thoughts come when you're trying to sleep?

I'll admit, my thoughts constantly jump from one to the other. But it's always worse when I'm trying to fall asleep. I ask myself the weirdest questions and run through the craziest scenarios. Most of which I end up rambling to my boyfriend about. Just wondering what interesting things other people think about while trying to sleep. I'd like to think I'm not the only one choreographing dances to songs, wondering what it'd be like to be a fish, wondering what I'd be doing during a zombie apocalypse, etc..

And I really feel sorry for my boyfriend when this happens. I'll ramble about my thoughts for hours .__.