Sauna is a dry heat that heats the body up by several degrees creating a false fever.  The body will then try to reduce its temperature by moving blood to the surface of the skin and begin sweating.  This has been known to improve circulation, relieve internal congestion, destroy bacteria, viruses and tumors.  It will also eliminate toxins and will do this in 2 phases.  The 1st phase is the first 10 mins of your sauna session at this time the Autonomic Nervous System which will begin to try to balance itself.  The 2nd phase from 10-30 mins hastens the death of weaker cells, produces heat shock proteins, improves genetics, increases heart rate and enhances circulation, resolves infiltrates, edema and exudates and normalizes enzymatic activity.  However there is a sweet (sweat, as well) spot, and if you go under and not get hot enough you won't stimulate the ANS to start and if you go over and get to hot you will send your ANS into a stress response.  So work with your body and listen to it.