A semi-decent heart rate monitor (even a Fitbit) is all you need to track sleep quality, I think. It's also much more reliable to detect when you fall asleep vs

A semi-decent heart rate monitor (even a Fitbit) is all you need to track sleep quality, I think. It's also much more reliable to detect when you fall asleep vs motion sensors (alone). If I have a good nights' sleep, my heart rate plummets quickly and pretty much flatlines for 7 hours. I can see the difference if I've been drinking alcohol, if I was woken up in the night or if I'm ill. All of these things tend to produce either an elevated heart rate, or a heart rate that slowly declines over the night (rather than a steep initial drop). The study is cool, but is there a comparison to a control instrument that can actually detect REM sleep? My conclusion from that graph was that none of those trackers are any good at telling you at which point you are in your sleep cycle, but most are OK at guessing your bedtime.