What is a feature of the basal metabolic rate?
BMR measures rate of energy usage under very strict conditions.
Basal metabolic rate (BMR) is the rate of body energy expenditure during rest. This can be measured by the amount of heat output from the body over a period of time. This definition is limited to endotherms, but should be sufficient for human studies.
BMR assumes that the person's sympathetic nervous system isn't active. This is hard to do when the person is awake, so BMR is usually measured during sleep. Clinically, resting metabolic rate (RMR) is more often used. RMR is simply a more lenient measure of BMR. The person can be awake, as long as the person stays still.
BMR decreases as the person ages.