I did use with a partner, a number of years ago, and that version of it seemed to detect my sleep as reasonably well as one could hope based on heartbeat vs. EEG.
A couple nights ago I happened to talk on the phone for 30 minutes while lying in bed. While we know that sleep trackers can confuse sleeping with not moving, I was surprised to see that even the Oura Ring (worn, I think, on the hand I was holding the phone with), thought I was in light sleep:
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Other than that, the stages were somewhat aligned, though nothing like in the polysomnography comparison that the Oura team produced (which could mean that the Emfit just isn’t that accurate). A…