Sleep Wearables Do Not Measure Deep and REM Sleep Accurately

Sleep wearables do not accurately measure sleep stages (light, deep and REM sleep). But they can give you an idea about the time you go to bed, wake up and total time asleep.

In a study examining several sleep wearables (below) as compared to a FDA validated EEG device (Sleep Profiler), all of the devices tended to over or underestimate at least one of these variables measured: total sleep time, total time awake and sleep efficiency (how much time you are actually asleep when in bed). However, the Fitbit Ionic and Oura smart ring were the most accurate for measuring total sleep time, total time awake and sleep efficiency.

Sleep wearables used:

  • Apple Watch Series 3 (with SleepWatch app and Sleep+++ app)
  • Beddit Sleep Monitor
  • Fatigue Science Readiband
  • Fitbit Ionic
  • Garmin Vivosmart 4
  • Oura Smart Ring, 2nd generation
  • Polar A370
  • Whoop 2.0

WHOOP did okay measuring light sleep; the other devices didn’t do as well. All of the devices tended to overestimate deep sleep. And they did an even worse job measuring time spent in REM sleep (all tended to overestimate this too). They were not only inaccurate but there was a lot of variability between trials. The mean absolute percentage error for measuring REM sleep ranged from 57.83% with the WHOOP to 79.73% with Oura (higher numbers are worse). 

What You Need to Know Before You Buy a Sleep Wearable:

There are a ton of devices that measure sleep. But few companies actually measure how well their devices work (through 3rd party testing as is done in a university or research institute). Past studies show the same thing as this study – these devices are not good at measuring sleep stages (light, deep and REM sleep).How Should You Use Your Sleep Data?

How Should You Use Your Sleep Data?

1. Use it to establish healthy bed time routines and total time asleep.
2. Don’t fixate on the sleep stage you see on your sleep wearable (light, deep, REM sleep) as it is likely very inaccurate. 
3. If you feel like seeing the data will make you perform worse “I didn’t sleep well and I have a game today” then ditch the device or talk to a sleep expert on how you should use it.

The Apple Watch has the technology to make this watch competitive with other sleep devices. But, third party apps may not make the most of the watch’s capabilities. 

* In this small study, all participants had light skin tone. Previous research shows higher error rates in those with darker skin tones when using PPG technology that uses a green LED.

 Should You Get a Test in a Sleep Lab?

Only a sleep specialist can guide you on whether you should get a test in a sleep lab or wear a FDA-approved at home device to see if you have any trouble sleeping.

If you go to a sleep lab, be aware of the following sources of error:

You are in a strange environment

You are putting the device on immediately before you go to bed

The study is scheduled which may not jive with your typical bed schedule

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