The metabolism is all the chemical reactions that occur in the body to sustain life. In the process heat is generated as energy is expended. A measure of the amount of heat generated over time is a direct indication of the energy expenditure and this is known as the metabolic rate. The process of energy production requires nutrients like glucose which are then converted into energy molecule known as ATP (adenosine triphosphate).
These molecules are broken down by the mitochondria in the cell to yield energy that powers the various life-sustaining functions of the cell and the body as a whole. Oxygen and various enzymes are also required for the energy production. The entire process is regulated by various hormones and fluctuates according to the energy demands of any given situation. Simply this means that a person who is sleeping requires less energy than a person that is running.