First of all, the millet wristband 2 has built-in accelerometer and photoelectric rhythm sensor. The data monitored by these two sensors is the basis for judging whether my Xiaomi Mi Band track my in deep sleep or not. The acceleration sensor can monitor whether the position of the hand ring is moving great technology. In deep sleep state, the human arm is basically not moving. Studies have shown that the light sleep period and light sleep period, which account for about 55% of the total sleep time, have little effect on fatigue relief.