What about giving them a light just at night?
They need their sleep!
IF I were to heat my coop, I think I would create a small "sleeping area" with the 250W red brooder light the girls started with. I have a remote sensing thermostat for reptiles so the light wouldn't NECESSARILY need to be on all the time. It would be better to get something that would only go on if the temps were down where the girls couldn't keep themselves warm though.
Bruce