I don't think you need to. I live in CT and it is very cold here. I have Silkies (which are not especially cold hardy) and I have less chickens than you and they seem to be doing fine without a lamp/light/heat, etc. The coop is insulated so that probably helps a little and I put about 6 inches of wood shavings on the floor to help keep them warm. They also keep eachother warm. It sounds like you have enough chickens that they would keep eachother warm. Mine go into the coop when it gets dark and go to sleep until the sun comes back up. There is no need for them to see at night because they just sleep. They wait until the sun comes up to eat, drink, etc.