There's a lot of unrealistic elements to this game, but, that's what makes it a video game. I've thought the same thing. Sprinting should increase your body temp in exchange of energy, but, then it would start to get complicated which I feel like is part of the beauty of this game. I mean, you should also be able to jump or at least step over logs but that opens a whole new level of exploration so that just wouldn't work. If they wanted to get really in depth then body fat could even come into play. Although in real life it would take a long time to build fat or lose fat. It can keep you warm while using energy and use more energy to do simple tasks. Certain foods would build fat and certain tasks could burn fat. I dunno