I think Garmin, like most sleep trackers, uses movement to decide where you are in your sleep cycle. I know that Fitbit never really showed "movement" as a metric it tracked, but you can see that in Garmin and it has mine as generally "low." But I think if you're a more restless sleeper than other people it can mess with the results. Even when deeply asleep I tend to shift around more than some people who just turn into bricks.