You dont ever have to worry about hens getting cold at night as they are so well insulated by their feathers and used to living out of doors. So long as they are in a waterproof coop, where the wind doesn't blow in directly and cause a draught, the more ventilation, the better. If you close up the pophole and the vents, they are at risk of condensation, damp conditions, and respiratory infections. Certainly at this time of year, they need all the air they can get at night.