Dreams, including nightmares, are what's on your brain's mind, so to speak. I'm a combat vet whose also a shrink and have worked with other combat vets for over forty years (yeah, I'm an old fart). The brain is a learning machine - it tries to get something from all of our experiences. Ordinarily that's pretty routine but when things get stuck we start talking about trauma. You're having a talk with some friends, someone walks up and says, "What's Joe's cell number?" No one knows. You go on about your business and 20 minutes later that number pops into your head even though you weren't thinking about it. That's the brain trying to answer the question that little experience presented. Ok, the number's not a big deal and the brain will eventually drop it. But if the questions are the kind that come out of horrible situations like you've been through, the brain doesn't drop it. Even if we try to bury it, for whatever reason, when our guard is dropped (like when we're asleep) or bypassed (like when we're taken by surprise), what the brain is still stuck on comes into view. So while there are things that can be done with repeated bad dreams, usually the fastest way to get them to stop is to get on your own brain's side and go after the experience it needs to return to. This usually means, if it doesn't happen on its own, getting to a pro who knows what they are doing - they all don't - and get some assistance. I hope that makes sense. This is a very studied area in PTSD-related research.