A 2021 study on Scandinavian winter-swimmers highlighted a unique phenomenon where experienced cold-water swimmers demonstrated a distinct peak in between 4:30 a.m. and 5:30 a.m.. While most humans experience a nocturnal reduction in skin temperature to conserve energy, those acclimated to extreme cold show this "night high" peak, which suggests their bodies have adapted to higher levels of cold-induced thermogenesis .