I don't often have nightmares anymore, but I had one a few nights ago. And I know for a fact it was because of something I wrote before I went to bed.
No spoilers, so no details, but suffice to say it was horrible.
Is that a good thing or a bad thing? I'm not sure. On the one hand, it means the scene evokes some powerful imagery. I imagine Stephen King takes it as a compliment when something he writes provokes nightmares in his readers' slumbering minds.
On the other hand, you risk disturbing the reader to the point where he won't continue reading.
Or perhaps I'm overstating things. Maybe the scene was worse in my mind than it is on paper.
I guess we'll see.