Can anyone recommend a wearable (or frankly anything) that offers a reliable "smart" alarm function?

Background:

I previously owned (and loved) the Microsoft Band 2, and aside from it's terrible durability, I found it to be an excellent fitness/health wearable.

One of the features that I loved the most was the smart alarm. The gimmick here was that you'd set the latest time you'd like to wake, and the band would decide the optimum time to trigger the alarm within a time window leading up to the time you'd set.

It would use input from the various sensors to detect when you were in a light sleep phase and avoid yanking you out of a deeper sleep.

In my experience it worked incredibly well, and I haven't really been able to find an alternative (I've owned a few Fitbit devices since - they just don't come close IMO).

Anyone know of anything that does this job well?