Boyfriend is 40. when he is sleeping his muscles twitch moderately-severely. at times i think it's a seizure, but no history. stops immediately when he wakes. what is it? dangerous? can treat home?

Need more data: If you can obtain, get a formal sleep study (polysomnography) so the character of the twitching can be observed and correlated to EEG. If this is not possible then try to get movie of the movement disorder with a smart phone or movie camera. Show the findings to your doctor for further decisions.

Possible sleep apnea: I suggest he sees a doctor to have a sleep study as this can be sign of Sleep Apnea. Classic symptoms are snoring, shifting in bed, stopping breathing during sleep with sudden awakenings sometimes. People usually have fatigue throughout the day even though they get 6-8 hours if sleep at night.

"myoclonus": Periodic limb movements of sleep (FKA nocturnal myoclonus) can be a variant of restless leg syndrome, and may benefit from a low dose dopamine agonist at night time, but otherwise, is annoying but benign. Although seizures can occur at night, this does not sound to be the case. Have him contact his doctor.