Why can't I sleep?

Hey guys.
I just joined here; a friend linked me to this site.

Well, I guess I’ll just jump right in:
I’m 16 and I can’t sleep. It’s been about 35 hours since I last slept, and I can’t put a finger on what might be causing my constant awareness.

I don’t drink too much caffeine. I do spend a lot of the time on the internet. I’m not depressed. I don’t have any diseases in the family related to restlessness. I’m underweight, just barely, but I don’t think that would have anything to do with it.

My first thought was obviously insomnia but I’ve been researching it and I do not believe I fit the criteria at all.

Do any of you guys know anything about what I may be experiencing?