Is the sauna after exercise good for your recovery?
Taking the sauna regularly after exercise probably adds some extra benefit for fitness and blood pressure, on top of the gains from exercise itself. The sauna after sport is not harmful.
After exercise, the sauna has a measurable effect on heart function, though it is a short-lived one. Immediately after a sauna session, the resting heart rate is lower than before (68 versus 77 beats per minute), and the part of the nervous system that calms the heart switches into relaxation mode more quickly. However, in a study lasting eight weeks, this short-term effect did not lead to a lasting improvement in heart rate variability. Exercise itself did improve that variability.
More interesting is what was found after eight weeks of regular exercise combined with sauna (15 minutes after each workout), compared with exercise alone. The combination group gained extra fitness (+2.7 ml/kg/min oxygen uptake) and saw an additional drop in systolic blood pressure (-8 mmHg). Cholesterol also fell more sharply. This was a small study with 47 participants, but it was well-designed and the results are concrete.
When it comes to muscle recovery, the picture is less clear. A sauna session after exercise does not cause extra neuromuscular fatigue, as long as your body temperature is back to normal two hours later. The cellular damage that occurs after a workout showed mixed signals: one marker fell in red blood cells but rose in blood plasma. In a small group of elite athletes, the sauna appeared to amplify the body's stress response to exercise compared with cold treatment, but whether that helps or hinders recovery could not be determined from that study.
Heat has shown signs, in animal studies and studies of people with injuries, that it may help preserve muscle mass, but in healthy, actively training people that effect is uncertain and has not been clearly demonstrated. Structured heat acclimatisation (training in a 40-degree environment for ten days) does measurably improve fitness, but that is a different situation from taking a sauna after a workout.
This answer draws on several studies of varying quality. The studies are small (the largest had 47 participants over eight weeks), which limits how confident we can be in the findings. Some results come from studies of people with heart disease risk factors or elite athletes, which do not automatically apply to the average person who exercises. Evidence on muscle recovery is thin and partly based on animal studies.