I thought I'd give my two cents worth. We've abandoned the use of sleeping
bags, that's tent camping. Cold zippers and feeling trapped is not my idea
of comfort.  We make the bed using a sheepskin on the mattress covered with
a flannel sheet, flannel top sheet and a down duvet. In the coldest of
weather(ice fishing) we put a wool blanket under the sheepskin and bring an
extra duvet. We camp all winter in B.C and  are always comfortable, although
reluctant to get out of bed. Seems bulky but a very inviting bed at the end
of the day. Jim 83.5 westy with no heater.