Most smart watches track sleep, great. But I am in the market for one that did what the Microsoft Band (1 and 2) did where you set a wake alarm (say 5:45 AM) and if you come out of a deep sleep within 30 minutes of that time, the watch would vibrate and wake you. This is far better than an alarm (watch or other) just going off at 5:45 AM when you may have dipped back into deep sleep again and thus are shocked awake. Of course, if you are in deep sleep for the entire 30 minutes and come 5:45 AM, it will vibrate then.
(1) Samsung Support - Posts | Facebook