After another sleepless night, I woke up to this thread. How appropriate.
My father has a notoriously difficult time sleeping at night, and I'm afraid I've inherited it. For me, I think it's correlated with stress. I have NO problem falling asleep at night (usually out in 5 minutes). The problem is I wake up at night after 2-4 hours of sleep, and then have slept enough that I'm somewhat rested/awake and then it takes me 2-3 hours to fall back asleep. Usually the problem is I can't turn my brain off. I get thinking about things I need to do for work, etc. and it snowballs from there.
Counterintuitive to some of the other suggestions, I find watching a show actually help, as it distracts my mind enough to where my body starts to relax and I can get back to sleep.
Regardless the reason, it's no fun. I know that about once every 2-3 months, I'll get a solid 8 hours of sleep without waking up once at night. It's absolutely amazing how much better I feel after those nights of sleep.