Fitbit's Claim To Measure Rem Sleep: Fact Or Fiction?

Sleep is an essential component of health, and the best way to know if you're getting enough is to track it. Fitbit devices can help you do this. They estimate sleep stages by using a combination of movement and heart-rate patterns. When you haven't moved for about an hour, your Fitbit tracker or smartwatch assumes that you're asleep. While you sleep, your device tracks the beat-to-beat changes in your heart rate, known as heart rate variability (HRV). These numbers fluctuate as you transition between light sleep, deep sleep, and REM sleep stages. However, it's important to note that Fitbit devices are not as accurate as more sophisticated equipment used in sleep labs, which include eye trackers and brain wave monitors.

How does Fitbit know when you're asleep?

Fitbit devices can estimate sleep stages by using an accelerometer to measure bodily accelerations of the wrist, coupled with a technique called optical photoplethysmography, or PPG, which measures blood flow based on how green light from an LED on the back of the watch is reflected by the body.

Fitbit looks at how much you're moving and how often your heart is beating to infer whether you're awake, in a deep sleep, a light slumber, or dreaming. The Fitbit Versa also features an SpO2 sensor for measuring blood oxygenation, but Fitbit is not yet using this feature to tell you about your sleep.

Fitbit estimates your sleep stages by using your movement and heart-rate patterns. When you don't move for about an hour, your tracker or watch assumes you're asleep. Additional data such as the length of time when your movements indicate sleep behaviour (like rolling over), help confirm your sleep status.

While you sleep, your device tracks the beat-to-beat changes in your heart rate, known as heart rate variability (HRV), which fluctuate as you transition between light sleep, deep sleep, and REM sleep stages. When you sync your device in the morning, it uses your movement and heart rate patterns to estimate your sleep cycles from the previous night.

According to a paper published by Fitbit scientists in 2017, the watch's sensors can classify sleep stages with 69% accuracy in any given 30-second time window. If you only use Fitbit for one night, take the app's sleep staging with a few grains of salt. But if you use the app for many nights, you'll eventually get a good sense of how often you tend to be awake, lightly asleep, or deeply asleep on a typical night.

How does Fitbit measure REM sleep?

Fitbit devices can estimate sleep stages by using a combination of movement and heart-rate patterns. When you haven't moved for about an hour, your Fitbit tracker or smartwatch assumes that you're asleep. Additional data, such as the length of time of movements that are indicative of sleep behaviour (such as rolling over), can also help confirm that you're asleep.

While you sleep, your Fitbit device tracks the beat-to-beat changes in your heart rate, known as heart rate variability (HRV). These numbers fluctuate as you transition between light sleep, deep sleep, and REM sleep stages. When you sync your device in the morning, Fitbit uses movement and heart rate patterns to estimate sleep cycles from the previous night.

The Sleep Stages feature uses accelerometer data, heart rate variability (the changes in time between beats) and algorithms to more accurately estimate how long you spend in light, deep and REM sleep stages (as well as time awake) each night.

Fitbit trackers and watches use your sleeping heart rate, movement and more to measure your time spent in each sleep stage and give you a personalised Sleep Score that shows how well you slept.

How accurate is Fitbit's REM sleep measurement?

Fitbit devices can estimate sleep stages by using a combination of movement and heart-rate patterns. When you haven't moved for about an hour, your tracker or smartwatch assumes that you're asleep. Additional data, such as the length of time of movements that are indicative of sleep behaviour (such as rolling over), can also help confirm that you're asleep.

While you sleep, your device tracks the beat-to-beat changes in your heart rate, known as heart rate variability (HRV). These numbers fluctuate as you transition between light sleep, deep sleep, and REM sleep stages. When you sync your device in the morning, Fitbit uses movement and heart rate patterns to estimate sleep cycles from the previous night.

Fitbit's sleep staging is correct in 30-second blocks slightly more than two-thirds of the time. If you only use Fitbit for one night, it is best to take the app's sleep staging with a pinch of salt. However, there is no systematic bias to how much light sleep, deep sleep, or sleeplessness Fitbit estimates. This means that if you use the app for many nights, you will eventually get a good sense of how often you tend to be awake, lightly asleep, or deeply asleep on a typical night.

Fitbit's R&D team conducted extensive internal testing by observing and analysing several hundred nights of heart rate and movement data from volunteer sleepers while simultaneously characterising their sleep with gold-standard lab equipment.

According to a paper published by Fitbit scientists in 2017, the watch's sensors can classify sleep stages with 69% accuracy in any given 30-second time window. While this may seem low, it is important to consider the context. Most of us don't have polysomnogram equipment at home, which uses multiple sensors to determine sleep stages. In this context, Fitbit's accuracy is quite impressive.

It is worth noting that there is a slight overestimation of REM from sensor readings, but the Fitbit algorithm corrects for this. Overall, Fitbit's REM sleep measurement is reasonably accurate, especially when used consistently over multiple nights.

How does Fitbit's sleep measurement compare to other sleep trackers?

Fitbit devices can estimate sleep stages with 69% accuracy in any given 30-second time window. They do this by using an accelerometer to measure bodily accelerations of the wrist, coupled with a technique called optical photoplethysmography (PPG), which measures blood flow based on how green light from an LED on the back of the watch is reflected by the body. Fitbit also uses heart rate and movement to infer whether the wearer is awake, in a deep sleep, a light slumber, or dreaming.

Fitbit's accuracy has been compared to that of other sleep trackers, such as the Jawbone, in several studies. These studies have concluded that Fitbit and other sleep trackers are "pretty accurate" when it comes to sleep tracking. However, it is important to note that sleep trackers like Fitbit cannot accurately tell you about your REM or deep sleep stages. For this, more sophisticated equipment, such as eye trackers and brain wave monitors, are needed.

Despite this limitation, Fitbit's sleep staging is still a useful tool for understanding your sleep patterns and quality. By using the device consistently over many nights, you can gain insights into how often you tend to be awake, lightly asleep, or deeply asleep on a typical night. Additionally, Fitbit provides a personalized Sleep Score based on heart rate, restlessness, and more, allowing you to quickly understand how well you slept.

In summary, while Fitbit's sleep measurement may not be as accurate as medical-grade equipment, it still provides valuable insights into your sleep habits and quality when compared to other sleep trackers.

How can I improve my Fitbit sleep score?

• Exercise earlier in the day or at least 3 to 4 hours before you sleep.
• Maintain a consistent sleep schedule.
• Meet the CDC's recommended 150 active minutes per week.
• Set a sleep schedule in the Fitbit app to maintain a more consistent sleep pattern.
• Explore your sleep habits through sleep insights and set a bedtime reminder in the Fitbit app to help you wind down each night.
• Set your watch to sleep mode so that your rest isn't disturbed by your watch's display or notifications.
• Set bedtime and wake-up targets, then get reminders to help you stick to them.
• Use Smart Wake to set your Fitbit alarm to wake you with a quiet vibration during a lighter sleep stage to help you feel more refreshed.
• Power down your devices an hour before bed to reduce blue light exposure.
• Try unwinding with a book or relaxing music before you go to sleep.

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