Why do I only get nightmares when I'm extremely exhausted?

I'm 20 years old, and I only get psychotic nightmares when I'm exhausted. today, I dreamt about death, gore, and murder and woke up with body aches. I understand my dreams are vivid during REM sleep but what could be the explainmation for violent and dark themes, since I am not the kind of person who thinks these things.... ever.