[frederikvs]

I am less skeptical about an EEG-based device like the Dreem than I am about a wristband.

Most wristbands have a heart rate monitor and an accelerometer. Is there a pattern in the heart rate that shows REM sleep versus deep or light sleep? Or do they use the accelerometer to detect twitches you make during REM sleep? How do they discriminate that from my cat jumping onto the bed?

Compare that to EEG-based devices : I think it is well known that sleep phases can be read in an EEG. The Dreem has fewer channels than a medical EEG, and maybe a little less accuracy, but that is beside the point : it is measuring the relevant signals for sleep phase detection.