I have the vivoactive 3 (not music) and it seems to track my REM as the main an only option showing virtually no deep sleep. On my older Vivosmart HR it tracked deep sleep perfectly but no REM sleep. I would kinda prefer that to this sleep tracking as I feel it just doesn't work. Last night I slept for about 8 hours and it said 2hrs+ REM and 34 mins deep which I find hard to believe considering I consistently got 2hrs with my old garmin tracker in deep sleep.
Hopefully Garmin can have a software update to fix this. It seems to track when I go to bed and wakeup properly though which is nice.
Have been trying to monitor sleep with the fenix 6s saffire, I see the same issues as described here.
Somehow inaccuracy could be expected with only movement and heart rate as an input, but the garmin connect app is buggy, as some other people here notices, the deep sleep part might just disappear during the day, while it is there in the morning.
Other things like how many stairs one walks up or down, or the breath rate are very inaccurate. As a sports watch it's ok, but collecting inaccurate data during the day/night and then presenting it with software which just changes and throws away data at random seems rather useless to me.
expected more accuracy for 800 euro's.....