For your reference, please check the following details:
1. First of all, make sure the breathing score function for sleep is enabled in the APP (Path: [Mi Fitness] APP >> [Device] >> [Sleep] >> turn on [Breathing score]);
2. If the sleeping time is too short, the breathing score or monitoring data will not be displayed (usually the night sleeping time is more than 3 hours);
3. Inappropriately wearing the bracelet will result in a failure to monitor the breathing score. It is recommended not to wear the band loosely (the band should be one-finger's width away from the wrist bone).
4. If the above information does not help, please check whether the APP and the band firmware are the latest version. Meanwhile, it is recommended to restart the band or re-bind the band and try again.