Yeah, I've definitely dreamed of getting a concussion. I wake up during REM sleep almost every night (not sure whether it's psychological or physiological). Last night I dreamt that I was playing basketball –something I have never done seriously, and haven't done recreationally since grade school– when the person guarding me somehow tackled me. I told him to watch out for my head! Definitely an anxiety dream in this case. But I also wake up from dreams without such manifestly anxious content.