Sometimes, if remembered, I dream very complicated, & 'plot twisting' dreams. Have no idea why. My brain must be 'throwing out' some guff from years' gone past!
It's rare for me to have a nightmare, but if I do, it's usually really horrendous, involving my family, or my previous work!
Whatever goes through my brain at night, I rarely remember when I wake up, & that, probably is a good thing.
We humans are of a pretty intricate make up, aren't we!