A sauna is an enclosed room with a high heat source that can produce either a dry heat or a damp, moist atmosphere if steam is used. The entire purpose of a sauna (besides relaxation) is to promote sweating. Many people feel that this is a cleansing regimen.
When a person sweats, their body loses both fluids and electrolytes, or dissolved ions in solution. It is about 99% water, but it can contain up to 1% of electrolytes found in the body like the sodium, calcium, potassium, and magnesium ions, in addition to trace amounts of other metals like copper and nickel. As the water and electrolytes are lost from the body due to heavy sweating, the cells in the body tend to become hypertonic, or they shrink slightly and lose mass as water exits the cells to compensate for the sweating. This in turn activates the thirst mechanism in the body so that it can obtain more water. When Nadine drinks more water, the body's water supply is replenished and the cells absorb the water that they lost and become isotonic, or normally shaped with a proper fluid balance.
Some people say that you should drink water with electrolytes added (like a sports drink) to replenish the electrolytes lost during sweating. But this claim is dubious, however, since such a tiny percentage of sweat is composed of electrolytes and most of these ions are obtained anyway through regular eating and drinking.