What is the coldest temperature a human can survive in without clothing?

33F. 1C

A healthy person can generate enough heat to survive, in other words maintain core temp, if they have enough calories. Above the freezing point there is no chance of frostbite to the outer skin so it will be uncomfortable to say the least but circulation will be more in the core body and less in the skin. Australian aboriginees often slept mostly nude with minimal shelter in temps not much higher than freezing, sometimes with a little fire sometimes without. It would be a profound mental transformation to learn to be able to sleep not when you are a little cold but completely cold all the time. The stress both mental and physical would probably shorten your life.

When I was a Karate student winter training was a standard thing every year, mediating in the snow or river. I used to joke with people who thought I was mad how relaxing it is, more correctly if your stressed you really feel the cold or heat for that matter, relaxing is part of how you do it. The idea that your enduring or fighting against the cold is totally wrong. As you say it’s simply having enough calories to generate the heat, no need to exercise or shiver.