I practically never have nightmares. Oh, my dreams may contain all sorts of action and monsters and whatnot (though mostly my memories of dreams tend to be very vague), but I'm not affected by them, in any negative way. (Actually, I think the dreams most likely to have any kind of negative effect on me are those that touch upon the topic of relationships, or the lack of them. Sometimes a mild melancholy may linger after such dreams – but I would not call them nightmares.)
I sometimes feel this is almost a flaw in my being, although I'm sure people who truly suffer from nightmares will be quick to disagree. The trouble is, when I hear people complain about sleeping badly due to nightmares, I simply have no way of relating to the situation.
I only have memories of two dreams that I'd class as actual nightmares. I guess I should record them here for posterity.
In the first one I felt a strange tickling in my throat. Probing in my mouth with my fingers I found the end of a strange tube. I started pulling it out, and yard after yard of this tube came out. Then suddenly I realised it was my own intestines I was pulling out. At the shock of that realisation I awoke.
In the second dream I was watching Sarah Michelle Gellar perform live on a stage. Suddenly, out of the blue, she turned into a pile of dust, just like a staked vampire from Buffy. I realised that she was dead, and the shock awoke me. Once awake the whole thing seemed remarkably silly, of course, but in the dream the tragedy had been intense enough to actually wake me up.
And that's about it. Sweet dreams. ;-)