I could swear the question a duplicate; even to one of my own old ones! Failing to find it I might just as well ask.
Imagine temperature somewhat/far below freezing; a sleeping bag, bivy bag and ground pad capable of taking it. However I am of the impression that it is a Very Bad IdeaTM to breathe inside the sleeping bag.
One solution is a tent. If it is perfectly sized(not too big) and winter-adapted (no air vents, not single layer) it might raise the temperature with 5°C. Not much help at -20, is it?
There must be some practical solution to this exact problem!
How can I get a good night's sleep while breathing very cold air - it hurts to breathe even when awake?