I do not think you need to leave a light on in there! They are nocturnal and on the best of nights they have moonlight. You gave them a huge window and so they will get some light from there at night as well. What I would give for a glider room like yours! LOL
I worry about flashing them as well, so far I have not noticed them acting differently after but goodness some flash photography makes me blind for a minute so I have to think it is not fun for them either... to say the least. During our play time we have a few wall lamps we light and it is not bright.