How accurate are the most accurate sleep trackers?

2017-Sep-01

With so many wearable and bedroom sleep trackers in a $15B wearable medical device market, you'd think that one or two of them will have reached the point of decent accuracy. Maybe some have, but unfortunately not these self-touted "medical-grade" sleep monitors that I've recently tested.

Emfit QS

The Emfit QS is a contact-less sleep sensor that you place under the mattress. Super convenient in that you don't need to tell it you're going to sleep, or charge it; it just tracks when you're in bed and sends the data to your cloud account. From sleeptrackers.io:

Emfit is a technology company based in Finland that has been designing and manufacturing bio-sensors for decades in a range of different health applications. The company has a solid research history with nearly 100 published scientific research articles to their credit.

I've been using the Emfit QS since 2015 and it has reliably detected me as asleep when I was in fact lying in bed watching TV. Not surprising - this problem affects all sleep monitors, it seems. Worse, last night I was allegedly in "deep sleep" while talking on the phone. The other party has assured me that was not the case.

Oura ring

The Oura ring tracks activity and sleep, and the team behind it has conducted peer-reviewed studies of its accuracy, leading to glowing reviews for the device:

Overall, these results are very impressive for a wearable ring, and far more accurate than other wearables currently on the market. You can read the full peer-reviewed study at "Massimiliano de Zambotti, Leonardo Rosas, Ian M. Colrain & Fiona C. Baker (2017): The Sleep of the Ring: Comparison of the OURA Sleep Tracker Against Polysomnography, Behavioral Sleep Medicine"

The ring is more annoying to wear than contactless sleep trackers, but it's portable so you can see how different environments affect your sleep. For example, I found that sleeping very cold in a tent after walking all day during a festival was followed by a record in the amount of deep sleep - 25% (at home, typically I reach 17% at best).

I've been using the ring for about a month and today I took the time to compare its sleep data to that of the Emfit QS.

Emfit QS vs. Oura Ring

Below is my sleep data from the Emfit QS sleep sensor and the Oura Ring. Note how both sleep trackers detected me as asleep while I was on the phone. The Oura was a bit better - it thought I was in light sleep. The QS Emfit though believed I was in deep sleep.

Poor accuracy in both the Emfit QS and Oura Ring

As you can see, both devices signaled a false positive while I was clearly awake. In theory this shouldn't be too worrying - you can later adjust the sleep period in the Emfit QS web app (though not with the Oura) and remove the falsely detected sleep. However, what if the device claims you've been in deep sleep when you were actually in light sleep? Deep sleep is crucial for physical renewal, hormonal regulation, and growth and healing. Trying to optimize deep sleep based on incorrect data is an exercise in futility.

Is there hope?

There are other sleep trackers out there that I haven't tested but seem promising:

  • Beddit Smart 3 - works just like the Emfit QS, but Apple acquired them, so maybe they're more accurate
  • S+ by ResMed was deemed "reasonably accurate" by NoSleeplessNights, a blog that acknowledges the deficiencies in personal sleep trackers.
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