We would recommend using caution when using your device in an environment such as a sauna. Please reference some of the device specshere for more information.
I have been using mine in a Sauna and steam room for 3 months. 15 mins every other day at 90C maximum. No problems except that the screen saver may be beginning to peel. I suggest using a suitable watchface to show temperature and heart rate.
Once i had a Vivoactive HR! Sauna 90 degree celsius was ok, but It didn't survive 15 minutes in the steam chamber! Now, i never would try this with my vivoactive3!
I take mine into my gym's steam room regularly. It works fine except the screen isn't as sensitive to touch when wet but you wouldn't have that issue in a sauna.
Using Vivoactive 3 in sauna almost every week, tried in many saunas, even in extremely hot ones - no problems so far. I just cooling it down every 3-5 minutes by putting and holdin palm until you feel watch cooled down(not feeling it to be hot).