I have nightmares every night of my life (I am 42). Only recently did I find out that not everyone does. But I also noticed that when I sleep on my side, they are much more horrible and vivid. I even feel physical contact and/or pain. Demons & spirits suffocate me, enter my body, hit me, etc. This is so true that on nights when I don't have to get up in the morning, I lie that way on purpose for the adventure of it and to test it out, though I'm usually sorry I did.

I have one theory, though not sure if it has any merit. My best sleeping position is on my stomach with my face on the sheets and pillow under my arm. I think it's possible that these very vivid dreams while lying on my side may have something to do with being in some kind of half-way state between asleep and awake, since it's not my comfort zone where I sleep best. Not sure really, but it's horrible and extremely real. I'm literally afraid to go back to sleep after some of them.