Some info:
http://www.sleepsex.org
http://www.sleep-masturbation.com
I have this. I was curious to know if anyone else on TFP has it.
I masturbate in my sleep from time to time and I can never predict when or where it will happen. When I was younger, I would go to sleepovers at my friends' houses and sometimes I would wake up to myself masturbating. It was really embarrassing and I have no idea if anyone noticed me doing it. Sometimes, my friend would wake me up and tell me I was making moaning noises. I would tell her I was having a scary dream and probably trying to scream in my sleep. I remembered my dreams were sexual though, and I was lying. It was believable because I have also been known to talk and laugh in my sleep.
My evil ex also had/has it. I never experienced it with him, but he warned me that he will initiate sex while he's still sleeping, but very rarely.