From what I have read it has to do with light, and the shorter days causes them to not lay as well as they do during longer days. Some people put a light bulb in the coop to encourage hens to lay during these shorter days in December and January. I don't do this, as I don't mind them taking time off from laying, is just how nature works! I am not sure about the heat though, have never done any research on it.
Nope, do not live in the city, but in the country. This ia a great place to learn all about chicken keeping, Tons of info here.