Having a nightmare is no fun, but is it actually unhealthy to have them? If you have at least one a night, then yes, they can cause health issues. Michael Nordoff, an assistant professor of psychology at Mississippi State University says having roughly one nightmare per night “can lead to stress and insomnia.” He adds that people who have nightmares regularly may avoid sleeping, or may not go back to sleep after having the scary dream, and lost sleep is known to cause a whole range of mental and physical health issues including depression and heart disease. However, nightmares aren’t always a bad thing. Tore Nielsen, a professor of psychiatry at the University of Montreal and director of the Dream and Nightmare Laboratory there says studies have found nightmares may help reliever the dreamer’s daytime anxieties, and even help some people manage stress.