If the chickens get real' cold at night you could try a heated roost.. Most of the time when people put heat lamps in coops it'll ended up burning down the chicken coop.
While heat lamps pose a risk, and are not recommended in the majority of situations, the above statement is an exaggeration. Yes, heat lamps are a huge risk. But, they do not cause a coop to burn down MOST OF THE TIME. But, even one coop burning down is one coop too many.
What are the exact dimensions of your coop? L x W x H? The smaller the coop, the greater the risk of adding heat. Especially if you are talking about a heat lamp.