My fitbit will do sleep tracking and use a silent alarm (ie. it vibrates rather than beeps) to wake me up during a window of time where it detects I'm not in a REM cycle. This has worked extremely well for me and it's almost a bit jarring. I wake up, not instantly "alert", but not groggy. I don't get that "smack the phone to turn off the alarm" desire. It's more of a "Huh, I'm awake now. Okay." type of feeling. The only catch is it needs a 30 minute window in order to detect and it can "wake" you at any point during that window.