Many times I've done textures and renderings during late night hours in a dark room and the next day the pictures are so dark I can barely see what's in them.
We even have a name for that; "kellari gamma" = basement gamma... People at remedy started out working in a basement / cellar without windows and all the levels they did come out pretty dark... same happens when you work at night without lights on. I always have ambient lighting on when working because of that (it gets pretty dark in Finland during fall / winter evenings and nights).