Lighting question regarding reptiles....?

I have 2 snakes, an iguana, and a bearded dragon... I understand that the light is required for proper digestion and heath....but can i maybe switch it from 12hrs, to maybe 8 hrs ... the only reason i'd be doing this is because my room is upstairs in my home...and it gets pretty hot in there with all the lights on and stuff.....so i was just wondering if it would be ok to only have them on for about eight hours this summer...any suggestions???? :)