Hmm, I have gotten this too in the past, but never thought much of it. For me, it usually happened when I was fighting the urge to fall asleep. My reasons for fighting the urge to fall asleep sometimes included me having homework that needed done, but most times it was because I would wake up from a frightening dream and be forced back into it. So fear is definitely a factor for me. I don't know if that means it is universal, though.