Why can't I stop thinking?

I keep trying to go to bed to sleep but then my brain starts thinking. This has happened frequently enough in the last couple months that I don't remember the last time I got eight hours of uninterurpted sleep. (Between thinking, telemarketers and wierd dreams that is.) What am I thinking about you ask? Well sometimes it is fic that won't leave me alone. And sometimes like now it is scenes on the spec script I'm writting.