I dont really get nightmares. I have had a few in my life and they feel so real. I feel bad that you get them frequently. What happens if you havnt smoked for a couple of days and then sleep? Do you still get them. I know that most dreams occur during REM sleep, which is a lighter stage of sleeping but dont know if nightmares occur in this same stage. Weed should put you in a deeper stage of sleep causing you to dream less. If nightmares do occur in a deeper stage of sleep it could be a problem.

The only advise I give you is try to make sure you are in a good place mentally before you go to sleep. Listen to some chilled music and try not to think about anything that worries you. I usually find if I am thinking about something before bed, a lot of the time if features in my dream.