Well the thing about sleepwalking, is that the ones does it, actually act out their dreams. The thing about these people, is that they won't get paralysed like everyone else, because they don't produce the hormone which paralyses you.

If Laberge is right, it could also be that they act out their HI's. I was trying to get some sleep in a car here the other day, after about 5 minutes, I had reached some deep HI and I started acting it out, like swinging my arm because I had this baseballdream. Of course, I wasn't at all paralysed because of the short time that had elapsed. It could probably be something like this with sleepwalkers too.