The Sad Truth About Sleep-Tracking Devices and Apps

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For some practical tips on how to get more shut-eye, I sought out Raphael Vallat, a postdoctoral researcher at the Center for Human Sleep Science at the University of California, Berkeley. His most important advice: Do not check your sleep data on a regular basis.

“If you look at your data, it can modify the perception of your sleep,” he said. “You may think: ‘Oh, gosh, I didn’t sleep well. Should I be tired? Am I in a bad mood?’”

Here’s what I learned after two weeks of sleep tracking.

How sleep cycles work

To understand sleep-tracking data, I delved into how sleep works. There are three main stages: Light sleep, deep sleep and REM sleep (for rapid eye movement).

The deep sleep stage is beneficial for physical restoration, like muscle reparation and metabolism recovery, researchers said. REM sleep, the stage in which we dream, helps in repairing our psychological and emotional networks.

On average, a person completes a sleep cycle, which includes each of the three main stages, every 90 minutes. To get a good night’s sleep, you need to complete four or five cycles. That’s partly because the cycles are not the same throughout the night: The early cycles have more deep sleep, whereas the later ones have more REM sleep.

But our sleep-tracking tech? It generally can’t accurately measure REM sleep.

In sleep-tracking labs, the gold-standard scientific equipment typically includes sensors that are connected to someone’s face and neck to measure eye and brain activity, among other variables.

But sleep-tracking apps for wearable devices like the Apple Watch or Fitbit primarily look at movement and heart rate to determine when you are asleep or awake — which are generally not precise enough to measure the different sleep stages, Dr. Vallat said. Without a good look at REM sleep, those apps may give an incomplete picture of sleep quality.

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