We measure 24-hour energy metabolism of human subjects using the human calorimeter. Last 15 years of our efforts revealed that 1) rate of substrate oxidation and energy expenditure differ between sleep stages, 2) the time course of sleeping metabolic rate is not simply the result of prolonged fasting. Energy expenditure and carbohydrate oxidation begin to increase prior to awakening despite a prolonged fasting, 3) time course of sleeping metabolism differ between male and female, suggesting that circadian clock of female during sleep is 1-2 h advanced compared to males. Recent findings show that control mechanisms of energy metabolism and sleep are closely related. We believe that it is important to consider the relationship between sleep and metabolism when evaluating the effects of dieting or new sleeping pills.