The rate at which energy (calories) is used for the essential life functions is called Basal Metabolic Rate (BMR). Basal metabolism includes most of the involuntary things the human body does to support life – such as breathing, blood circulation, body temperature regulation, nervous system operations, etc., but not the extra energy needed for any additional physical activity such as gym exercise. In other words, if you laid in bed all day doing nothing else, then you would need to eat at least the number of calories roughly equal to your BMR in order to maintain your normal body functions.