1) I'm lonely. I like to cuddle up next to someone when I'm going to sleep. I *think* this may actually have a lot to do with the following reasons -- when I'm cuddling up to someone, my thoughts are warm and fuzzy, they fade easier and I can get to sleep.
2) I think too much. I think too much most of the time -- there's a fine line of course, the rest of the time I think too little (or so I think). As I'm drifting off, I'm attempting to let go of the day or night, spin my body from its mind (or vice versa or versa vice). That's when things occur to me. Stumble, stumble, thud, convince myself it's worth getting up for and writing down -- no, you dumb shit (speaking to myself), you won't remember it in the morning. You've learned that, learned it well and good, and still it's the first excuse to pop itself into your head. At least it's the first one to pop itself out as well. Poof. I refute you. Next is "it's not that interesting." Well and true, perhaps, but I'm not awake enough to verify. The question is whether I wake up enough to write it down, and verifying is certainly a few levels of consciousness above just writing down a thought. So... eventually, I stumble out, grasping the thought delicately with mental fingers, and find tools to put the thought down -- an xterm or a postit or a scrap of envelope. And then I stumble back to bed. And fifteen minutes later, it happens again.
3) 2 is the worst of it, but another issue that rains down on my skull is -- "just a little longer". Just a little longer and I'll be able to finish whatever I was working on. No major mistakes will be made. I'll be happier. Maybe I'll be able to sleep, even. I can do so much more when I don't sleep, really. But... no, for too much of that and I actually do less, unable to wake up for work, or waking up and unable to work at work (which they prefer, unfortunately, to me simply not showing up) -- and then I stumble around dazed and lose the night as well, where if I simply "missed" work I could go in later than night and make up for it. La la la. They don't really mind that all that much, unless meetings are to be had. Of course, that's worst-case scenario. That doesn't happen all the time. If it happened all the time, it wouldn't happen, for I could convince myself then to go to sleep. Often (most of the time?) a few more hours, and then a few more hours on top of that, really don't hurt anything. But then there's the times it does. For instance, I've been sick for a week. I'm sure my sleeping habits don't help.