Polar sleep tracking vs. Sleep2 (Nukkuaa)
If you're anyhow interested in sleep tracking on your device, you’ve probably seen at least a few videos of Quantified Scientist on YouTube, where he scientifically tests (to the best of his ability as one person) the accuracy of smart/sports watches or other devices. Fitbit was OK for a long time, then came Oura Ring, Apple, and now the top spots are taken by the app Nukkuaa (now Sleep2), which uses the Polar Verity Sense wristband or the Polar H10 chest strap to track your sleep. It's quite expensive, but even with the FREE version, you can get lots of data to compare it with Polar watch sleep tracking accuracy.
Long story short – Sleep2/Nukkuaa is way more accurate than Polar. I tested it for 16 nights in a row (one night is missing because I forgot to charge the Verity Sense), and those were tough nights for me as I had to take care of a 4-month-old puppy while my friend was abroad. And it's exactly those tough nights that made all the difference – when I slept well, both devices showed similar sleep scores, but when I slept really bad, Sleep2 nailed it every time, while Polar sometimes gave me quite good scores (my average was 75 on Polar when I started this experiment).
Data here is arranged by the score given by the Sleep2 app, and in bold, I selected which app was more accurate when the difference was more than 10 points. As you can see, Polar was more in line with how I felt just once, and even then – it was a terrible night, it’s just that I felt it wasn’t as bad as Sleep2 thought it was. :) Still, my main observation after this experiment is also the biggest criticism of the scientific tests done by Quantified Scientist – for him, accuracy of sleep tracking equals accuracy of sleep stages. But when I look at the data, I don’t see it that way. What I see is that the final sleep score almost always correlates with actual sleep time (be it in minutes or %). For some reason, on the worst nights, Polar didn’t manage to see that for 51-85 minutes I wasn’t sleeping. That’s a lot.
So even though I’m quite happy with sleep tracking by Polar, especially the Nightly Recharge score, there’s a lot of work they need to do to catch up with the best. As for the Sleep2 app... 25 EUR/month or 240 EUR/year for a PRO subscription is crazy (I wouldn’t pay more than 9.99 EUR just to test all the functions), but if someone has BIG problems with sleep and needs accurate data and advice, I’d say testing it for a month or taking their advertised 8-week intensive program (200 EUR) might be a good investment.