A sauna is not a passive activity. Your heart works about as hard as it does during light cardio, and that changes how to read the survival data.
Sauna gets treated as relaxation, which makes the longevity findings look like magic, but the mechanism is plain. When you sit in heat that hot, your body's main job is to get rid of the heat before your core temperature rises, and it does this by opening the blood vessels in your skin and sending a large share of your blood to the surface where heat can escape. That drop in resistance forces your heart to pump faster and harder to keep your blood pressure steady, and your heart rate climbs and stays up the whole time.
One study tracked heart rate and blood pressure through a 25-minute sauna session and compared it to a treadmill-style exercise test in the same people. The strain on the heart matched light to moderate exercise, somewhere around 60 to 100 watts of effort. Sitting still in heat was doing real cardiovascular work.
102 adults who each had a heart-disease risk factor did a single 30-minute session in another study. Afterward their arteries were measurably less stiff, and their top blood pressure number dropped from 137 to 130. One session moved both in a healthy direction.
The survival data lines up with that. In a study of 2,315 Finnish men followed for about 20 years, the more often someone used the sauna, the less likely they were to die during the study. Death rates dropped step by step: highest in once-a-week users, lower at two to three times, lowest at four to seven times a week. Heart-related deaths followed the same pattern. The benefit scaled with how often, not just whether.
This was an observation of one group of middle-aged Finnish men, not a controlled experiment, so it cannot prove cause, and the frequent users may have been healthier to start. No trial has tested this directly against lifespan. What the evidence does show: heat is a real workout for your heart and blood vessels, and the dose appears to be how often you do it.
References:
Laukkanen et al., JAMA Internal Medicine, 2015
Laukkanen et al., Journal of Human Hypertension, 2018
Ketelhut and Ketelhut, Complementary Therapies in Medicine, 2019