Sleep tracking not accurate ...

I don't understand why you have to declare your sleep hours....Other devices recognize it automatically and accurately most of the times without having to specify sleep hours.

Neither do I but since that is how Garmin built their algorithms I think it is wise to do so...

I will add that, in my own case, most of the bad data seems to be in the first hour (when I'm awake in bed) and the last couple of hours (mostly when I wake early and can't get back to sleep or don't bother trying). Sure, I could manually edit the start and stop times to clean *some* of that up, but I really shouldn't have to do that. My FitBit Versa and Charge 3 ::cringe:: were quite good at detecting those scenarios and my current Beautyrest SleepTracker nails them pretty much every time.

I've only compared to a FitBit Charge 3 and for me the F6 (and F3) feels more accurate in proportion to how I experienced the sleep.
Sleep tracking wasn't one of the use cases that made me buy the Fenix, it's more of a bonus feature so I don't really require it to be accurate.

For fun I will wear both my Charge 3 and F6 tonight and compare tomorrow.

Do tell :)

FitBit:

F6:

A few notes; I was not awake for 1½ hours as FitBit says, Garmin says almost 4 hours of deep sleep which is questionable but I don't know if 1½h is closer to the truth. Sleep time and wake up time is about the same on both.

Last night, for the first time in... I don't even know how long, I had a decent night's sleep, filled with vivid dreams. I did wake up from a dream before my alarm went off, but only about 10 minutes before. Garmin's sleep data for the night looks fairly "normal" for a change, tho I don't feel that it was particularly accurate as far as the deep sleep is concerned. However, if I slept like this regularly and didn't have another sleep tracker that I KNOW works better, I might be otherwise inclined to actually believe Garmin's data.

The last time that I looked at the clock was ~ 9:20pm and I fell asleep pretty quickly after that, so the 9:35pm start time doesn't seem unreasonable. But I have a hard time believing that I fell into a deep sleep so quickly, especially considering that my heart rate was still elevated with no sign of dropping off until after 10:00pm and there was still quite a bit of motion until that time as I was apparently tossing and turning a bit.

I did wake from a dream at 1:27am, after which I got up briefly to use the bathroom and it actually shows both the REM sleep and me being awake at that time. However, I also woke from a dream at 5:20 and while it does show a little awake blip *around* that time, it shows me going back into a light sleep until 5:27. By that time, I was already feeding the cats. Still... the start and stop times are pretty close for a change and it did get my mid-sleep wake-up. For this night, I'd probably give its accuracy a score of 6 on a 1-10 scale.It did pretty well with the start and stop times, but I don't really believe most of that first deep sleep period.

For comparison, here's the graph from my Beautyrest SleepTracker:

It was slightly more accurate on the start & stop times (usually it's significantly more accurate) and doesn't show me going into a deep sleep as quickly. My HR was noticeably lower and there was almost no motion during the periods where it shows me in a deep sleep, so I'm more inclined to trust its data. Like the Garmin, it shows the 1:27am wakeup with REM sleep just before it, But it also shows me in REM sleep immediately before waking for the morning and I definitely woke from a dream at that point, as well.

I don't normally put much stock into a tracker's ability to track REM sleep since, without doing brain wave analysis, they're really just making educated (hopefully) guesses, but the SleepTracker almost always shows REM sleep just before I wake from a dream (and can remember the time). I suspect dark sorcery!  Tho I do often see a correlation between the periods of REM sleep and it's separate graph of HR and respiration rate; both are often elevated during those periods.

Edit: Garmin's sleep graph changed at some point in the last 40 minutes, so I've uploaded it's latest version, which seems to have removed a second block of deep sleep. If it changes again (which I suspect it will, I'll update the image... again).

Not that it really matters, since you're using the Garmin, but the Fitbit's motion sensitivity may be high enough that it thinks your light sleep tossing and turning is actually you being awake. There is an option to lower that. I found it to be more accurate after doing that, but it probably varies a lot from person to person.

This^^  Makes no sense that you have to set a time.  I set mine for 24 hours, works good that way.  Seems to detect sleep fine.