> The Fitbit Inspire, Fitbit Versa, and Oura Ring estimated light sleep with less bias than deep sleep. Conversely, the Withings Mat and Garmin Vivosmart estimated deep sleep with less bias than light sleep. Across devices, estimates for REM sleep were less biased than estimates of light and deep sleep. Despite low absolute biases, it should be noted that some estimates were still up to 250 min different than their PSG derived measures.

In other words: these devices are all terrible, and shouldn't be trusted. Pretty different from the final conclusion, which is probably "soft" to get additional grants from the commercial entities selling these devices.