I have a coop that has one very small window. I have found that when it is dark outside it is pitch black inside. When mine started laying I was receiving numerous rubber shelled eggs and the early morning layers weren't laying in the nest boxes. I left the light on all night and the rubber shells stopped and they layed in the boxes. When we tried to go without it again the egg problems started again and they started staying out in their run, where the yardlight shines, at night. I now have a 7 1/2 watt bulb (called a nightlight bulb for regular sockets) in the coop and it the chickens are happy.