I also do not provide winter light. However, I do run a 250 watt bulb, a red one, for heat. I hang it low so it does not light the place up. I have to heat the hen house otherwise the eggs are frozen solid by the time I gather them.

Despite some unusually cold temps and no extra light aside from a glowing red heat bulb, I have almost 100% fertility in the 15 eggs I put in the bator. I just hope it warms up by the time these eggs hatch or I'm in trouble!