If I'm worried about something (anything, ranging from the silly to anything big), then mu chances of sleep have dramatically dropped. I have the bad habit of being obsessively anxious over things, and if I have a problem then good luck getting that off my mind to go to sleep.
If I'm being randomly creative, that will also keep me awake. This isn't so much a problem if I feel I can get out of bed and go write down notes without taking away from my sleep (like if I had to wake up early the next morning). If I don't, though, then the ideas will keep bouncing around in my head, keep me up, and then make me worried I'll forget them in the morning.
Usually it's the first, but yeah. Anxiety and an overactive imagination are what tend to keep me up. <_<