Last night I wore two Alta HRs, one on my left wrist and one on my right. If the FitBits were perfectly accurate you'd certainly expect them to have the same results. After all, it's not really possible for my left wrist to be in Deep Sleep and my right wrist to be in REM, is it?
Sadly, the results were pretty disappointing.
According to my LW (left wrist) FitBit - I woke up 13 times. According to my RW (right wrist) - I woke up 19 times. That's dramatically different - a ~50% increase if you look at the RW data instead of the LW data.
The Sleep Stages data had some pretty key differences too. LW logged an hour of deep sleep (16.3%) while my RW logged only 25 minutes (6.7%) Light Sleep was different by nearly 60 minutes! Looking at specific points in the night, I can find examples of one FitBit saying I'm in Deep Sleep while the other say Light or REM, and essentially every possible combination of sleep stage. One says REM, the other says Light. One says Light, the other says Deep. Looking at the charts side-by-side I can say pretty confidently that it isn't purely random, and there are some similarities, but it feels a lot more like a sleep mood ring than advance sleep tracking technology.
It's not all bad news - while the Sleep Stages seem pretty made up to me, and the number of times awake is wildly different, if you look at it from strictly an 'awake' vs. 'sleeping' they are very close to each other. Time Asleep was 4 hours 58 minutes compared to 5 hours and 7 minutes. +/- 10 minutes seems pretty alright to me. I'm not sure that's any better than you'd get by just, ya know, writing down when you think you fell asleep and when you woke up, but I think it's a reasonable margin of error. And the large blocks of awake time are virtually identical - IE - both have me awake until 2:17am and both show me awake again for 18 minutes at 3:24am. There are just a few minutes where one says I was Light Sleeping and the other said I was awake.
Conclusion
Tracking awake vs. asleep seems to work quite well for me, but everything else reported was unreliable.
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