The reason why Apple Watch can calculate resting energy in the middle of the night is because it adopts real-time heart rate monitoring technology, which can monitor the user's resting heart rate around the clock, thus calculating the user's resting energy consumption. In addition, Apple Watch can combine the basic information of the user's height, weight, age, gender, as well as the user's activity level and basal metabolic rate to calculate the resting energy consumption more accurately.