Do u have nightmares when u sleep on your back. if so why?

Nightmares: can come in your dream at any sleeping positions.

Sleeping on back: should not cause nightmares per say. If assuming a posture in bed causes vertigo, pain or difficulty breathing the symptoms my become " incorporated" into a dream, there by modifying the dream to take on content to account for the somatic symptoms.For example you might dream you are on a roller coaster or in a space capsule if you have benign positional vertigo