I don't think it matters much. I turn the light on when I need the light. I do keep a towel draped over the tub at all times so it's somewhat dark though. Like Snizards said, mine is up and about during parts of the nighttime hours no matter what the light situation is. Sometimes she sits with her head poking out of the hide and sometimes she is moving the hides all over the tub.
I tried a towel today too before I read this and like you said, I don't think it matters much. I had the room lights on for a little bit and he came out for a while to roam at night then went back in his hide. He sure uses those hides a lot. Thank you for your help!