If you are dreaming and in your dream it is daylight then you suddenly wake up in your dark room, are your eyes adjusted to light or dark? (i.e. pupil dilation)
I would have to guess and say, that your eyes are adjusted to dark. The light is in a dream. The dream is happenening in your mind, but what your EYES are really seeing while dreaming is dark.
thats a good question. abn has the right answer though. there are some people whos eyes remain open while they sleep. if you shine a light into their eyes the pupils will get smaller without neccessarily effecting the persons dream.