Disclaimer: I won't be able to find all my sources. I'm sorry for that. I just looked into this 2 days ago! While I'm not the OP, this submission happens to be

Disclaimer: I won't be able to find all my sources. I'm sorry for that. I just looked into this 2 days ago! While I'm not the OP, this submission happens to be one of the most exciting articles I found (I should've submitted it :P). I was asking myself whether I should update my Fitbit Flex 2 to a Fitbit Versa or an Apple Watch or anything else really. The answer is: don't upgrade, keep the Fitbit Flex 2, it's about as good as journalling your own sleep times and that's all I need really, a very rough estimate. More importantly, the newer wearable devices are not good enough at giving a reliable breakdown of your sleep that polysomnography (the gold standard would give). With that said: the Basic Health Tracker (doesn't exist anymore, I think), Oura Ring, Philips Actiwatch Spectrum were all able to do things that trackers that the Fitbit and other wearables couldn't. Specifically: > Only mean values of sleep efficiency (time asleep/time in bed) from Actiwatch correlated with PSG, yet this correlation was weak. [2] > Light sleep time differed from PSG (nREM1 + nREM2) for all devices. Measures of Deep sleep time did not differ from PSG (SWS + REM) for Basis. [2] > From EBE analysis, ŌURA ring had a 96% sensitivity to detect sleep, and agreement of 65%, 51%, and 61%, in detecting "light sleep" (N1), "deep sleep" (N2 + N3), and REM sleep, respectively. [4] A good literature review on it is [3]. Some facts (which can be found in [3] or its cited papers): * All trackers overestimate sleep by about 30 minutes. * The biggest problem for all these wearable trackers are: estimating when you're simply lying awake in bed but you're not moving (it thinks you're asleep). * Devices like the Fitbit Flex are about as accurate as sleep journalling with regards to total sleep time. Dreem has the following issues. It's hard to see whether their scientific papers are peer-reviewed. Yes, they produce science articles in PDF formats but not in journals. Yes, they collaborate with universities but it isn't clear how. Nothing is peer reviewed and almost nothing seems to be cited. This was echoed in a Gizmodo article [1], but I figured it out myself first. I'm not saying they aren't legit, because another thing I have noticed is that the scientific community is a bit but not too keen on testing these devices with the golden standard (polysomnography/PSG). There is still space for a startup who is clearly capable of demonstrating this and other research institutes confirming it independently for their device. Edit: I hope I gave enough sources to make my comment a bit credible. [1] https://gizmodo.com/this-smart-headband-was-supposed-to-help-me-sleep-bette-1822568894 https://gizmodo.com/this-smart-headband-was-supposed-to-help... [2] obtained from abstract @ https://www.ncbi.nlm.nih.gov/pubmed/27164110 https://www.ncbi.nlm.nih.gov/pubmed/27164110 [3] obtained from abstract @ https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8648428 https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8648428 [4] https://www.ncbi.nlm.nih.gov/pubmed/28323455 https://www.ncbi.nlm.nih.gov/pubmed/28323455