Saunas are great, and i recommend them for general wellness. I`ll assume you`re talking dry sauna, because 90 Celsius would boil your skin in a wet(steam) sauna. 90 C is excessive. I would not go over 80 C as anything past that will start to burn you up. I use a dry sauna 4-5 times a wk for 30-45mins a session. usually post-workout. It`s great for dropping water weight if that`s a goal.
For me, it`s more for the detoxing effect/recovery/relaxation. Sauna really draws out toxins stored in the body, particularly fat. It's best to get a shower soon following a sauna in order to wash the toxins off the skin. Recovery - it's like an induced fever. you metabolic system doesn't have work on keeping the body warm, thus more resources to repair body. Relaxation - it's like meditation for me. i usually have the lights out and veg, sometimes with ipod. Any way you slice, i think everyone should take advantage of a dry sauna if available.