Sleep phase via mi band sdk?

Question

Android

Hello!
I would like to know if there are open sdk for miband, through which it would be possible to determine the phase of sleep in real time? The mi fit application has phase tracking, but it is not clear how this happens - through the analysis of the heartbeat or directly in the watch, the function of determining the phase of sleep is sewn in. I found a couple of sdk on github, but there is no necessary function there