I did regular saunas for 2 years, 10~20 mins at 80 degrees. Which is testing
Hrv was typically around 25(rmssd) before sauna, after /during sauna is would be lower 12~20 and frequently followed by headache.
More often than not this results in redness that lasts several hours and joint pain and strong fatigue that lasts 24 hrs
I'm assuming we all have very low Hrv