Resting energy expenditure, often abbreviated REE, is the number of calories the body burns at rest to keep its basic functions running: breathing, circulation, cell maintenance, and body temperature. Measured after an overnight fast while the person lies quietly awake, it accounts for the majority of daily energy use in sedentary people, typically around 60 to 70%. It is closely related to basal metabolic rate, though measured under slightly less strict conditions. The main determinants are fat-free mass, age, sex, and body size, which is why predictive equations use weight, height, age, and sex. Dietitians multiply REE by an activity factor to estimate total energy needs for weight management, clinical feeding, and sports nutrition planning.