Sweating is a way for the body to control its temperature. This water weight loss is unfortunately not the kind of long term weight control you're looking for. Real weight loss, the kind that's involved when the body burns excess fat, is caused by eating less and exercising, not by sitting in a sauna for an hour.
What is sweat? Why do we sweat?
Sweat is basically water mixed with some electrolytes, mostly sodium. Our body secretes sweat from sweat glands to prevent itself from overheating which would otherwise cause damage to your organs.
(If you don't like chemistry, skip the next paragraph and move onto Does sweating result in weight loss?) Water can be found in three phases: solid, liquid and gas. Sweat is secreted as a liquid and cools us down by evaporating and turning into a gas. Heating water takes a certain amount of energy. When water is heated up to its boiling point, the amount of energy required remains constant. When liquid water reaches 100 degrees Celsius and is ready to make the jump to becoming a gas, it needs extra energy to do so. This requirement of extra energy when going from a liquid to a gas is the cooling effect we feel when we sweat. When the sweat evaporates off of our skin, that extra energy comes from the heat in surrounding tissue. The water on our skin evaporates and takes with it some heat which decreases the temperature.
Does sweating result in weight loss?
Since sweat is made mostly of water, it weighs a lot. If you sweat enough, you will notice an immediate drop in weight because your body has lost a lot of water. Many athletes (wrestling, MMA) use sweating as a way to quickly drop weight to be eligible to fight in a lighter weight class. This weight loss however is temporary.
The main function of the kidneys is to filter the blood. The kidneys also work to regulate your body's water balance. When you drink too much water, your kidneys take the extra water out of your blood and put it into your urine to maintain a proper water balance in your body. This makes your urine very diluted and clear in color. The opposite happens after you sweat a lot and become dehydrated. The kidneys start to hold onto more water and your urine becomes more concentrated and very dark in color.
If you were to sit in a sauna for a while, step on a scale and be satisfied with your new, lighter weight, you'd suddenly get thirsty. You would drink a lot of water over the next few hours and because you were dehydrated, your kidneys would hold onto that extra water to return your body's water balance to normal. The next time you would go weigh yourself, you'd notice that you've gained some weight. This is why sweating simply to lose water weight doesn't accomplish anything permanent. Your body is constantly working to keep a certain amount of water. Your weight will fluctuate with hydration levels. Losing weight requires that you increase exercise levels and decrease your calorie intake. Sitting in a sauna for an hour will do nothing for a long term weight loss goal.
Losing Weight Permanently
Losing weight requires a calorie deficit, a state in which you are burning more calories than you eat. When you burn more than you eat, your body is forced to use fat stores for energy which causes permanent and long term weight loss. There are two ways to create a calorie deficit; you can exercise or you can eat less. For the fastest, most permanent and healthiest results, you need to combine an exercise routine with a calorie restricted diet.
The Bottom Line
Rather than spend your time sitting in a sauna and trying to "sweat your weight out", take a trip to your local track and run a few miles. This will cause you to sweat, but more importantly, help you burn some serious calories. Only by burning calories and eating less will you lose real weight, look better and improve your overall health.