Soldato
It doesn't matter how fit or healthy you are prolonged exposure to cold air causes harm. Specially as temperatures drop below 0, constantly breathing in cold/sub zero air can cause respiratory issues. I know I was one of those who would hold off putting the heating on for ages, it would give me a sore throat and nose after a while. You need some level of warmth (around 14-15 degrees) to prevent that happening over time.