Sort:  

Nightmare can only affect your health when you allow it affect your mind and emotion. Nightmare is just a scary dream and can have no effect on us unless we dwell on them too much. When you start thinking about the nightmare to an extent where it starts affecting your daily activities or mental state, then it may start affecting your health.

Nightmare in itself can't affect your health unless there's a spiritual undertone to it. It's when you allow the nightmare to control your thought and emotion that it may result to a problem.