The human body burns calories at all times, even while sleeping. Energy is constantly expended to sustain basic life support functions. The amount of energy required is represented as the person's basal metabolic rate.
Basal metabolic rate represents the energy required for vital functions, including digestion, respiration, regulating temperature and circulation. It is described as the energy required for life support if a person simply slept for 24 hours. Each person's rate is different and depends on his age, weight, body fat percentage and activity level. Men burn more calories at rest than women. Increasing muscle mass and activity level increase daily energy needs.