Once upon a time I was solo hiking in the NC mountains. I think it was spring or early summer and it was nice weather with a good forecast. The problem was, the higher I hiked up in elevation, the colder it got, which was not the issue nor a surprise. The problem was the rain. There was not supposed to be rain but that is all it did but rain.
I had an old surplus military poncho, but to use it, you had to decide which was worse, just getting rained on, or using the poncho and sweating. Take your pick of poison.
I was wearing cotton shorts and T shirt, and at the lower elevations this was fine but at I got higher, the temperature dropped, and it was not fine. If I kept hiking I was warm but when I stopped for a break, I would start to get cold and shiver. I was stopping every hour to eat and drink water, but after one of the stops, I was shivering too much and it was time to use the poncho to retain heat since I was cold even when hiking. Course using the poncho just made me wet with sweat but I was warmer. as long as I kept moving.
I did have wool clothes with me so I was in no real danger from the cold. I did not want to put on the wool while hiking because the clothes would be wet and I would be miserable when I stopped for the night. Which was the right decision. I eventually made camp, hung a tarp in the underbrush and changed into the nice dry, wool clothes. I was as comfy as a bug in a rug. Even more so when I got some hot drink in me and a hot meal. VERY nice and comfy.
The next day I made it to the top of a mountain, which was one of my planned exit routes if I decided to bail out, which I did. Had to hitch hike a ride to town after I hiked to a paved road but all was good.
If I remember right, most hyperthermia cases are in warmer weather, not freezing weather, but in temperatures where it is warm enough during the day or part of the day, and then the temperature drops and catches people unprepared.
In Pappy Boyington's auto biography he talks about his time in a Japanese POW camp. He was able to work in the camp kitchen, which was run by Japanese civilians who were not vicious like the guards. They would let him steal food to eat, and after the Japanese were done with their hot baths, he could sit in the hot water. He said sitting in the hot water would warm up his internal body temperature so that he would be warm during at least some part of the night because the cells were cold, the POWs were poorly fed, and did not have enough warm clothes or blankets.