I helped design the two bag, goretexbivy bag system used by Marines. It is a great bag system for the weight. A great ground pad makes it work
I am amazed to watch people use bags to only partial potential. Take your bag a shake it vigorously to get air between indulators. Of course all of us roll our bags twice- once forward snd once backwards- to get the air out to compress it better, right? That compression stays-shake that bag. All my bags have stocking caps in them. I have a small towel to wrap around my face. Never breathe in the bag, towel prewarms the air. Bring chocolate candy your bedside. If you shiver, eat that tootsie roll to give your body the sugar to burn . take today's sox off and put on tomorrow's. Winters in Norway and Korea teach conservation of warmth and energy. Insulation from the cold ground is a key for long term winter camping/ hunting. All of know a good night 's sleep and balanced meals make the hunt enjoyable