I'm not looking for a cure (if one exists), I'm just wondering what causes it.
Basically, as I'm nodding off, I periodically shudder. Maybe shudder isn't the right word; it's more like a quick contraction of my body, if that makes sense. I know this because my girlfriend tells me about it but also because I have woken myself up several times by doing it. I've also seen my dad do it before. Anyone?