I'm no expert, but I'm one who thinks that they can't regulate their internal clocks without a true day and night cycle. Mine have been without light since their first night out in the coop when they were 6 weeks old, and average nighttime temps were in the mid-20s. I turned off the heat lamp in the brooder when they were almost fully feathered and left a window cracked open. They did beautifully and they have thrived. I have electricity in the coop, but it's more for me if I have to do my chicken chores later for some reason, and to run the exhaust fan. It's been almost 4 weeks with absolutely no problems, and we haven't hit a night in the 40s yet.