Chameleons are incredibly resistant to Hypothermia- its Hyperthermia that is usually the killer. They have all sorts of defenses against the cold, and there are chameleon species that endure nightly frosts (drops below 32F) during some seasons.
Personally I think, unless your house is commonly going below 0C (32F) in the winter I wouldnt bother, and even if it was, I have a feeling you would know about it and wake up- which would give you a chance to solve the problem.