Well, it's definitely individual. Not everyone responds to the same things in the same way.
That being said, PTSD and cPTSD are prone to causing nightmares that either replay, or are similar to the originating trauma/s. When that's the case, part of the stress on waking is that time it takes to come back to reality. Being in the dark, fumbling around, those seconds cam feel like forever. But when there's already light, you can just see your surroundings and know it's over.
Back in my group therapy and support group days, it really shocked me how many of the others kept lights on for that same reason. Folks that were more depression centric in their symptoms didn't get much benefit from keeping lights on, but the anxiety and panic attack folks often did.
On the flip side though, I'm also very much chilled out by dim lighting or darkness when fully awake.
