I hate being hot in a sleeping bag, The whole too hot, stick a leg out, too cold thing drives me nuts. in cold weather I also wear a large beenie that pulls down to my nose that keeps my head and neck warm, kuiu marion 210 bottoms, 210 top, hoodie, even 210 gloves to keep my hands warm if I poke them out at night. IMO this is way way better than trying to use a zero degree bag.
Given the choice of having to unzip a bag that's too warm to ventilate vs. attempting to sleep in a 20 degree bag in honest-to-goodness zero degree temps, it's not even close.
umm ok, I guess you know more than I do. big game hunts around here are done by mid November, should be the same to the east of me in colorado as well. I hunt longer as I hunt high deserts in nevada into december. upper single digits was the coldest temps I stayed in this year. I have used my gear down to about zero although I am unsure of the exact temp. I have to ask what are you doing camping in Antarctica? if its below zero I am staying in the truck, the tent, whatever that is my threshold. I spend 25+ days a year in the sleeping bag I mentioned. YES give me a 20 degree bag and I will simply wear the appropriate clothes to bed to make up the difference. kuiu merino wool goes with a down sleeping bag like I have like burgers and fries. I often sleep better than I do at home. Also when the days are short its common to just throw up a tent cook dinner and go to bed before 8:00, its common to be asleep for 10 hours before getting up the next morning fully refreshed.
you have your system I have mine, I know what works for me emphatically over 15 years and lots of nights in nearly all temps, you can wear more clothes and adjust to make your sleeping bag warmer, but you can't make a sleeping bag cooler. nothing wakes me up like being hot cold, hot cold,
