Common wisdom, as I’ve come to understand it, is that if you’re cold, you shouldn’t close your sleeping bag over your head as your breath condenses and you’ll get even colder. However, in my long experience of freezing at night (believe me I am very experienced), this ain’t the way it works. I’m definitely warmer inside, rather than freezing my face off through a little opening.
What say ye, condensation experts?
edit to add: Unless it’s raining heavily, I always sleep under the stars, so that may be a factor to consider. Oh and have only synthetic bags.
All bags in sub freezing weather accumulate a lot of moisture over time as your skin gives off moisture as you sleep. If those bags are down, the down looses insulation value, synthetic also loses some but not as bad. Some experienced winter campers that stayed out in the cold for weeks or longer would use vapor barrier liners to keep the moisture from the skin or clothing inside the bag from getting into the bag insulation.
I first noticed this on a week long snowshoeing trip with two friends several decades ago. Temperatures ranged from the minus 10 to plus 25 degrees (F) range [or minus 23 to minus 4 (C)]. I had a down bag that performed well. But we stopped on the way home to visit family of one of the group, and for that we slept indoors. Thus, after sleeping in our down bags for a week in sub freezing temperatures, when we brought our bags indoors and slept inside in warm conditions, all of us were shocked how much moisture our bags had accumulated.
Years ago I read the book written by Will Steger and Paul Schurke on their trip to the north pole by dog sled, I do not recall how many people in the group, maybe four or six? I recall that their synthetic sleeping bags got quite heavy from accumulated moisture weight. I was surprised that if they had used vapor barrier liners, they did not work or maybe they just did not use them? But the very heavy sleeping bags later during the trip was a logistics problem for them.
In sub freezing conditions, that moisture in your sleeping bag insulation will be ice crystals, sublimation will be very slow and you will just accumulate more and more moisture during the trip as long as it stays below freezing.
If you want to exhale inside your bag in cold conditions, that is your call. I do not do that.