Is strange. You don't see anything, but the room looks like gets red or darker.
But you have a very strong feel that someone is close to you or wanting to attack you.
I remember very clear that. The last time I had, I felt a very bad presence of someone walking up the stairs of my house, coming to kill me. The feeling was too strong, like the person was the own devil.
They say that since you are paralizyed in the middle of the reality and dream, your mind reacts with a nightmare.