I agree it's because of the light. Ours haven't started laying but we put in a window on our coop so they have light. We also have two windows by the roosts.
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I use a 40 watt bulb in my 8 by 8 coop. Not much chance of them getting "heat" from that.I don’t use lights either anymore , because I think that the hens could become too reliant on the artificial heat, and would panic if the bulb were to go out in the middle of a cold night, or they would be thrown off by the natural light timing.