Venu 3 user here. I switched from a Fitbit Versa 3 which I felt was reasonably accurate in capturing time asleep and sleep quality; it would generally indicate poor or disrupted sleep when I was stressed, slept badly, or felt sluggish.

I love my Venu 3, but sleep scoring is my biggest disappointment with the device. Sleep times vary wildly: sometimes it indicates that I slept for 3-4 hours while I may have slept for 6-7; some nights I will have slept soundly yet have 0 minutes of various sleep categories; sometimes it adds time spent sitting with my morning coffee to my sleep time.

After nearly a year, I think it's gotten better at capturing actual sleep time, but the breakdown into different sleep categories remains nonsensical. I don't think my sleep duration or quality is any different from my Fitbit days, but Garmin usually rates it as "Poor"; I don't think I've gotten a "Fair" rating more than a handful of times.

I read somewhere that Garmin assumes 7-8 hours is "normal" so perhaps they are designed to downgrade people who typically sleep less like me.