So I tried the light there's no monsters but she still waking up from nightmares I think it's associated with the fear of the dark but she even has fear of the dark during the daytime which is weird.
By having the sim be confident yes it's just she always gave that mood let of fear of dark then I guess when she has nightmares every night so Im guessing it's associated with her going to sleep scared.
Please add your me too to the main bug report:
Does your Sim still get the want to conquer the fear by being confident outside after dark? If so, you could try again to conquer it while that want is active and see if it sticks the second time.
