Feet, hands, and top of the head are thermal windows.
Dogs don’t perspire, but instead use their tongues to release excess heat. We are very different in that most of our bodies sweat. But when humans are usually clothed, that shifts mostly to feet, hands, and head. Even when we are not clothed, there are more capillaries in these body parts to allow for thermal exchange.
As a barefoot lifestylist, I can attest that I’m especially in-tuned with this idea. I get claustrophobic when I have anything on my feet. If I do wear something, I wear flip-flops, huaraches, or sandal shoes, like Keens. This allows my feet to breathe.
Truth be told, I don’t think I was meant to wear shoes. Fungi and bacteria love my feet because of the heat I put out. When I was in the Army, I was once napping on my bunk and my buddies came in and put foot powder on my feet. That is how bad it got with the smell. I was prone to Athlete’s foot and other issues. I formed blisters easily. Boots and running shoes have never been my friends.
About a month ago, I went backpacking through the Red River Gorge of Kentucky. My footwear was Xero Genesis sandals, which are like beautiful huaraches. The soles are super thin and there is 550 cord for straps. So its super lightweight and feels very bare. What it offers is some slight protection for the soles, which allows me to move just a bit faster. Speed doesn’t usually matter to me, but I was hiking with 4 buddies. So this worked better.
Now back to Bivies—when I sleep in bed at home, I prefer to have my bare feet poking out of my covers. I fall asleep this way. For me, a tucked in bedsheet is overwhelmingly mummifying. I honestly cannot do it. This goes the same for sleepy bags. I totally cannot tolerate them. So I use a sleeping quilt. I can always poke my feet out and tuck in only when they begin to cool. So, like for sleeping bags, a bivy would never work for me. In warmer conditions, I cannot possibly imagine being cooped up in a bivy.
Even for a tent, I use a hammock. It allows me to be more open-air and aware of my surroundings. I’ve been in intense storms with my hammock and have managed to stay very dry. Yes, I’ve been cool at times in cold weather, which I’ve resolved with an underquilt. But having the ability for my body, feet included, open to my environment is better for me.
I think shoe-borne folks are probably more able to use sleeping bags and bivys. But for those of us who spend more time on our bare feet, it works better to be free of having them enclosed.