It's been a long time since I used my Pebble Time Steel, but I remember the alarm feature to wake you up when you were in a light phase of sleep worked very well. I didn't do any sleep tracking personally, but the watch seemed to be able to tell.
From an old Kickstarter:
"Pebble Health tracks when you fall asleep, wake up, and how much deep sleep you’re getting (that’s the really good stuff). Smart Alarms determine your optimal wake-up time based on your sleep cycle, so you’re less groggy and more energized to tackle the day."
