Oh, and for future reference, not knowing where you are... but the temps here in CT can get below freezing at night during January and February. I do not plan on heating my chickens at all. I specifically bought cold hardy girls, and no matter what you have... you should be fine.
My chickens are dealing with the 20s at night and the 40s during the day right now... I do not heat them. Your chickens are probably enjoying the night time temps! They'll be fine.