Liquid Ocelot
dead + tired
Mine was my worst fear (being kidnapped and forced into human trafficking). I was scarred for life (well, maybe a month) after that. After I woke up that night, I seriously went to my parents bedroom and acted like a kid, saying that I couldn't sleep.
/pats
The more realistic a nightmare is, the more scarring it can be. I think people underestimate that sort of thing.