If you want actual fitness, only physical activity will help you (sport, aerobic, physical work, etc).
If you want to lose weight, you usually need to lose fat. Losing fat means burning energy. You can burn energy by doing physical activity or producing heat. Sleeping involves very little activity of any kind and your body actually reduces its temperature while you sleep. Producing a relevant amount of heat while you sleep, if it were possible, would most likely involve sweating, which is generally considered uncomfortable in bed.
So no matter how well you sleep, with or without underpants on, you will not gain fitness or lose weight from just that. Sorry to disappoint...
That being said, when people say they want to lost weight, what they actually want is to lost fat, not weight. (as to if they are aware of this, I'm not sure)
If you do an extreme diet where you don't eat anything, your body will detect that food intake is rapidly going down and go into 'survival' mode where it cannibalize muscle in an effort to reduce energy consumption.
If you are fat, losing muscle means you lose definition and become more of a blob of fat than you actually are.
So the key is 1. Eat enough (preferably 500 calories below maintenance) 2. Exercise enough (I recommend weight lifting for men. Increase muscle, testosterone and overall manliness. Go with StrongLift 5x5 if you are just starting out) Also, weightlifting maintains the muscle. 3. Sleep enough (I need 8 hours a day. Anything lower than that I will start feeling really sluggish, lazy and unproductive)
Go to 4chan /fit/ and read the sticky. It will tell you everything you need to know.