Physical activity increases body temperature; increased body temperature raises metabolism. A person who is exercising burns more energy (or calories) than someone sitting at a desk.
To burn this energy, the person's body has to work harder. Body temperature will rise, and, in an effort to keep the body moving, the metabolism will increase.
Conversely, when a person is at rest, metabolism slows and body temperature lowers, returning to the normal 98.6 degrees Fahrenheit.
Other factors influence metabolism and temperature. When the body thinks it is starving, either by actual starvation or by extreme dieting, metabolism can drop as much as 50 percent, and body temperature lowers accordingly. Additionally, fever increases body temperature, which also raises metabolism.