9 Surprising Causes of Nightmares and How to Prevent Them.
Nightmares are dreams that can be upsetting, scary, or strange, occurring during REM (rapid eye movement) sleep, when your brain is very active. Unlike bad dreams, nightmares wake you up. It’s normal to have nightmares occasionally, but there are several factors that can make them more likely.

References:
https://www.webmd.com/sleep-disorders/ss/slideshow-nightmares-causes