I got a Garmin viviosmart hp+ fitness tracker for Christmas, and last year, my partner teaotter got a Polar loop fitness tracker for Christmas. In addition to tracking steps, level of exercise, and for my own device, heartrate, both devices also track how you sleep. teaotter mentioned that her device divided sleep into restful and non-restful sleep and that she regularly got 6.5 hours of restful sleep a night. My device divides sleep into light and deep sleep and says that I regularly get 1.5-2 hours of deep sleep a night. This concerned me a bit, so I wore both devices for sleeping. My said I got 1.75 hours of deep sleep and 6.25 hours of light sleep, but teaotter's said I got a bit less than 2 hours of non-restful sleep and a bit more than 6 hours of restful sleep. I find that data quite reassuring from a health perspective, but rather less so from the perspective of finding such devices to be remotely accurate.