My boy friend is 27 yrs old, whenever he sleep his fingers would twitch and legs would jerk, he is never aware of it. what is the cause/s?

Myoclonic jerks: When we dream, our body becomes paralyzed so we don't act out our dreams and play "superman" off the 3rd story balcony. However, sometimes there can be small muscle fasciculations that get through and there is slight movement. Furthermore, there can be small stimuli to muscle groups causing involuntary jerks of legs or other large muscle groups.