Basal metabolic rate (BMR) is a measure of the rate the body uses energy, for basic processes such as breathing, circulation, and tissue repair, when at rest. It is calculated by oxygen consumption, or the amounts of oxygen and carbon dioxide exchanged, in a certain time. Metabolic rate (see metabolism) increases well above basal metabolic rate (BMR) during vigorous physical activity, fever, or under the influence of certain drugs, such as caffeine. It falls below BMR during sleep, general anesthesia, or starvation. BMR is higher in childhood and decreases with age. It also differs widely between different species of animal.