Originally, it is normal to use the sauna unclothed. This was not invented at random. Your skin can absorb heat better when you go into the sauna naked. Your veins dilate and blood circulation to the brain, heart and lungs is stimulated. With swimwear, some of the heat is blocked, which can slow down efficient blood circulation. As a result, you don't experience the health effects of high temperatures as well. But there are other reasons.