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How does the sleep tracking work?

I picked up a Garmin 645 M yesterday and so far have been very impressed, everything just works, and works well - going to pick up Bluetooth earphones to go for a phone free run for the first time in years. Anyway, while messing around I noticed that it failed to capture my sleep from last night - yet when i entered it manually into the app, stats appeared on deep / light sleep?!

So it went from no sleep data to some questionable data, how did it come up with estimates of deep sleep / light sleep if it didn't know that I was asleep in the first place, until I manually entered the times?

As an aside: I thought the sleep detection algorithms were pretty advanced by now? (I had a Pebble which was excellent for that and even after my short lived relationship with a Fitbit Ionic, about the only thing I can say it was good at was the sleep tracking)!

0 tmk2 over 7 years ago

First important thing to understand is that sleep detection and analysis is not done by the watch, but by Garmin Connect after sync.
Sleep analysis is done solely by assessing amount of wrist movement. Movement above certain threshold is interpreted as light sleep, and below it as deep sleep. Spikes in the movement graph are interpreted as awake time.
Sometimes Garmin Connect does not recognize fall asleep and wake-up times and won't show you any sleep data, but all the movement data is still there, so when you enter your sleep time manually, Garmin Connect will reinterpret these data to show sleep phases.

0 tigamilla over 7 years ago

First important thing to understand is that sleep detection and analysis is not done by the watch, but by Garmin Connect after sync.
Sleep analysis is done solely by assessing amount of wrist movement. Movement above certain threshold is interpreted as light sleep, and below it as deep sleep. Spikes in the movement graph are interpreted as awake time.
Sometimes Garmin Connect does not recognize fall asleep and wake-up times and won't show you any sleep data, but all the movement data is still there, so when you enter your sleep time manually, Garmin Connect will reinterpret these data to show sleep phases.

Thanks for the explanation - makes complete sense, I have noticed now that it processes sleep data a few minutes after the first sync with phone in the morning.