One early physiological study examined how the body responds to sauna heat, cold water immersion, and alternating heat-and-cold exposure. In the experiment, nine participants experienced several conditions including sauna sessions, ice-water immersion, cool showers, and combinations of heat followed by cold. Researchers continuously monitored heart rate and blood pressure to understand how the cardiovascular system responded to each temperature change. The results showed that sauna heat caused blood vessels to relax and widen, lowering resistance to blood flow and increasing heart rate as the body worked to regulate temperature.
Cold exposure produced the opposite response. When participants were immersed in ice water, blood vessels constricted and the body redirected blood flow, increasing the amount of work the heart performs with each beat. When heat and cold were combined—such as moving from a sauna into cold water—the cardiovascular system experienced the most pronounced changes, as blood vessels repeatedly expanded and contracted. Researchers concluded that alternating heat and cold creates a strong circulatory stimulus, which helps explain why practices like sauna followed by cold plunge (contrast therapy) can significantly influence circulation and cardiovascular responses.
