Because I do. Just as I am starting to drift off, and after I'm asleep, it's mostly my hands and arms that will twitch a little. Sometimes it will actually wake me up. If I was by myself it wouldn't be that big of a deal, but it can be a pain for my wife, who often has to deal with been woken up by me doing this. And it really sucks because one of the most relaxing things you can do is to fall asleep with your wife in your arms- until you start twitching, and wake both her and you up. I doubt that there is a "cure" for this kind of thing- it's probably just one of those things that you have to live with. But it would be sorta nice to know that I'm not alone in this.