This is definitely a thing for me, but I consider it a part of my crying fetish, because it's the emotional response to the nightmare that interests me most. Nightmares are just a great tool for getting my boyfriend to cry in my fantasies, because they take you to a very emotional (even illogically so) place, even if there's nothing that bad going on in your life for real. They're also a great way to bring things to the surface that you can usually hide from in the light of day. Honestly, I eagerly await the day that my boyfriend wakes up crying from a nightmare in real life. I thought I heard him cry out, "No!" in his sleep once, but whatever I'd heard had woke me up and he seemed perfectly calm when I looked over at him, so either I dreamed it and actually heard something else or it wasn't having quite the effect on him one might have thought. To this day, I still regret not reaching out to him that night, even though he seemed fine, just in case he was trying to hide it, but that was earlier in our relationship, and I was much less brave about such things then.