I'm in Michigan, too, and we keep the chameleon in the warmest room of the house. Our thermostat is set to 68 daytime, 64 at night; the first few years we had chams, it was 60 at night and when I discussed this with the breeder, they said that was fine and he didn't need supplemental night heat. If your cham is within that temperature range, he's probably fine. If you're talking an active draft - i.e. cool air is liable to be blowing on him - you might put up something to block the airflow to that side of the cage. My guy is free range; his basking space is warmer, but he usually spends less than an hour a day there.