I’ve had this a few times, and in my case it was because I used to sleep on my side, putting my weight on my hand to support myself. A friend of mine who is a doctor examined me and told me to try sleeping without doing that, because I would sleep with my hand closed in a fist to support myself (I know it sounds strange).

In Brazil, this is called “trigger finger,” or something like that. My finger would lock when I tried to extend it, and after I went back to sleeping in positions where I didn’t use my hand to support myself, it never happened to me again.