I've been having similar problems with the nightmares and the description of the physical symptoms you're having are similar to my own as well. I wake up with a headache that's always on my left temple, I feel dehydrated, and I'm also sweating. The only things that helps the headache is a migraine pill or else it'll be there all day.

So far I haven't found a cure to stop all the nightmares, but before bed I try to repeat inside my head, "none of it is real, no nightmares tonight." Things like that. And I've found it helpful to make sure the room temperature is comfortable, not too cold or hot. If it's too cold, my nightmares feel worse and I'm shivering uncontrollably. Too hot, and there's that headache and night sweats.

Have you made any progress?