In reply to James Gordon:
ME Snowline would be a good bet down to -20C. When it becomes really cold, you can boost it in various ways. First of all: have a really good sleeping mat. Thermarest NeoAir XTherm or the like. Then, before entering the sleeping bag, put in bottle of warm water. Then take off your trousers, but keep the woolen longs and socks on, and enter the bag. Then take off your puffy, but keep on the woolen sweater, zip up the puffy and pull it over the foot end. Then zip up the sleeping bag and cinch up the inner draw cord around your neck and the outer so tight that it will leave only a small breathing hole. Wear a hat and have a neck gaiter to cover your face and breathe through the material. This way you will have a lesser tendency to turn your head and breathe into the sleeping bag building up condensation. A bivy bag will boost the system even more, and so will a tent. Then there is the good old pee bottle. It requires well choreographed moves, but it saves the warmth in two ways.