We live in the rocky mountains, where the temps have hovered in the teens to zero many nights in a row, and dipped to minus temps a few times. My girls have done just fine with no supplemental heat or lighting. As already said, keep the coop clean and dry, and feed them prior to closing them in for the night. Always keep their water clean and in these temps watch that it doesn't freeze. Proper ventilation will remove the moisture from the air, leaving them cozy and able to keep themselves warm. Also, make sure they are able to completely cover their feet with their bodies. Roosting bars that are too narrow can leave parts of their feet exposed, and that can lead to frostbite. Watch out for any drafts at roost bar level. If you coop is very large, you may want to make it smaller by creating a barrier wall with tarps to help them hold more heat in. If it is an average size coop, they will be fine.
I understand your worrying about it, and none of us thinks you are being silly. I worried too, and was only convinced that my girls didn't need any extra heat source as I watched them come through this nasty winter we've had without any problems.
