[slobotron]

Amazfit watch gives a "score" that can be seen on device screen, in addition to the total hours and and breakdown per awake / REM / deep / light time.

[RecycledEle]

Yes, many activity trackers do this.

It is interesting to wear 2 of the same trackers, one in each wrist, and see if they are close to agreeing.

I do that with 2 Mi Bands.

The variance of a single point estimate is infinite.