Can someone actually have a melatonin addiction?

I have a friend who is an extremely bad insomniac. He has tried everything to get to sleep and through trial and error he is immune to most sleeping pills. So he stumbled across some melatonin and started taking it, and it seemed to not cause any side effects. It seemed a little bit safer I guess to him because melatonin is something that naturally occurs in the body.Anyway now he is started to feel some rather unsettling side effects kick in. Severe dreams/nightmares, extreme dizziness, and pain full stomach aches. He decides to go off of them and finds he can't sleep without them and the side effects became worse without them. Been to the doctors around here and they don't seem to be much help.(we live on a collage campus so we get "campus doctors") Can you be addicted to melatonin and does anybody know anyway to help this guy?