When the body hasn't eaten for a while and is at total rest, the amount of energy it requires to maintain its cellular integrity and total organ function is called its basal metabolic rate (BMR) . Think of the BMR as being like the amount of energy a car uses while idling in traffic. It needs a minimum amount of energy just to keep the engine running before the driver steps on the accelerator. So too, the BMR is a measure of the amount of energy the body uses just to maintain its cellular and organ function while it waits to be put into action. And just like a car, the faster the body moves and the more work it does, the more energy it needs, the more heat it releases, and the higher its internal energy and temperature. So the laws of nature regarding the release of heat when energy is used to do work affects the body's core temperature, not only when it is at complete rest (BMR) but with any level of activity.