Name: Sauna tracker PRO
Rating: 3.7 (123 reviews)
Author: Stanislav.Bures

An app created to track sauna time and rest time. The different phases are switched by the Back/Lap button. Activity is saved in the Gaming category because only this sport profile allows recording the oxygen saturation and stress scores during activity. 

Manual for use: https://drive.google.com/file/d/1RgO2FPqzqDvmmOeup3tKDntmjppaEZiN/view?usp=share_link

The author recommends using the app only in an infrared sauna, where the watch cannot overheat. 
In the Finnish sauna, the watch must not be removed from the hand! 
The app will alert you when the watch has heated up to more than 45°C. When the watch is heated to this temperature, I recommend covering the watch with your hand for a while.
Always use the watch in accordance with the instructions for use. The author and Garmin disclaim liability for damage to the watch due to improper use.

I strongly recommend uninstalling and reinstalling the app when upgrading from version 3.5.x due to the large menu expansion and possible conflict with the original settings.