Fitbit's Rem Sleep Monitoring: How Does It Work?

Fitbit's sleep-tracking tools provide insights into not just how much sleep users are getting, but also how restful that sleep is. The company worked with sleep scientists to develop the software, which breaks down how much time is spent in light sleep, deep sleep, and REM sleep. While traditional sleep stage measurement is done in a lab using an electroencephalogram to detect brain activity, Fitbit estimates sleep stages using movement and heart-rate patterns. When a user doesn't move for about an hour, the device assumes they are asleep, and it tracks beat-to-beat changes in their heart rate, known as heart rate variability (HRV), to determine which sleep stage they are in.

Fitbit's heart-rate tracking

Fitbit devices with heart-rate tracking (excluding the Fitbit Charge HR and Fitbit Surge) can provide a record of the sleep stages the wearer cycles through. This is achieved by tracking the wearer's movement and heart-rate patterns. When the device senses that the wearer has not moved for about an hour, it assumes they are asleep. Additional data, such as the length of time when movements indicate sleep behaviour (like rolling over), help to confirm the wearer's sleep status.

The Alta HR, for example, constantly monitors the wearer's pulse. It also taps into heart rate variability as a source of information to estimate what stage of sleep the wearer is in. This allows Fitbit to estimate the duration of each sleep phase.

While Fitbit's heart-rate tracking technology provides valuable insights into sleep patterns, it is important to note that there may be discrepancies in the data. For example, if the device is unable to detect the wearer's heart rate or if the battery level is low, the sleep tracking may only include movement-based data. Additionally, the device needs to be worn securely and positioned correctly to obtain an accurate heart-rate reading.

How Fitbit estimates sleep stages

Sleep stages are traditionally measured in a lab using an electroencephalogram to detect brain activity, along with other systems to monitor eye and muscle activity. Fitbit estimates your sleep stages in a more comfortable and convenient way.

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.

Fitbit's sleep researchers and the National Sleep Foundation describe the following sleep stages:

• Light sleep: Your body unwinds and slows down. This stage typically begins within minutes of falling asleep. Your breathing and heart rate usually decrease slightly during this stage.
• Deep sleep: Your body becomes less responsive to outside stimuli. Your breathing slows down, your muscles relax, and your heart rate usually becomes more regular.
• REM sleep: Your brain becomes more active. Dreams mainly occur during REM sleep, and your eyes move quickly in different directions. Your heart rate increases, and your breathing becomes more irregular.

Fitbit's sleep tracking accuracy

Fitbit's sleep tracking technology may sound impressive, but there are some limitations that can impact its accuracy and reliability.

Motion Misinterpretation

Fitbit's reliance on motion tracking can lead to inaccuracies. For example, it might interpret periods of restless sleep as periods of wakefulness or classify periods where you are awake but not moving as sleep.

Heart Rate Variability

Your heart rate can vary wildly, even during sleep. For example, your heart rate is likely to be higher if you exercise, eat a large meal, or drink alcohol before bed. As a result, Fitbit's heart rate sensors may struggle to accurately identify sleep stages.

Subjectivity of Sleep Stages

Determining sleep stages using motion and heart rate is a simplified approach. Sleep experts often use electroencephalography (EEG) and other advanced methods to precisely categorize sleep stages. Fitbit's algorithm lacks the depth of information provided by these methods.

Environmental Factors

Fitbit sleep trackers do not consider external factors such as room temperature, noise, or disturbances from partners or pets that can affect sleep quality.

Research has shown that Fitbit sleep trackers tend to overestimate total sleep time by about 10% and underestimate measures of deep sleep and rapid eye movement (REM) sleep by about 20% each.

Fitbit sleep trackers do a passable job of estimating how long you were asleep, but they struggle to predict how long you were in specific sleep stages.

Fitbit's Sleep Insights feature

Sleep Insights uses the data collected by your device and compares it with data from billions of nights of Fitbit users' sleep. It then offers tailored recommendations based on your unique sleep patterns. For example, if you consistently sleep longer on weekends, Sleep Insights may suggest that you're not getting enough sleep during the workweek.

Sleep Insights also allows you to compare your sleep performance with that of others in your age range and gender. This feature is called "Benchmarks," and it shows a shaded bar laid over each sleep stage, indicating how long people of similar demographics typically spend in each zone.

Over time, the Sleep Insights feature is designed to learn your habits, sleep patterns, and activities to provide more personalized advice. The system is intended to tailor its suggestions based on your individual sleep needs and patterns.

In addition to Sleep Insights, Fitbit also introduced the Sleep Stages tool, which uses the optical heart-rate sensor to estimate the amount of time spent in light, deep, and REM sleep, as well as how frequently you wake up during the night. Together, these tools provide a comprehensive view of your sleep habits and offer guidance to help you improve your sleep quality.

How Fitbit's sleep tracking works

Sleep tracking is a feature available on many Fitbit devices, including the Fitbit Charge 2, Blaze, Alta, Alta HR, One, Flex 2, Charge, Charge HR, and Surge. This feature allows users to gain insights into their sleep habits and guidance on how to improve them.

Fitbit's sleep-tracking tools use a combination of movement and heart-rate data to estimate the user's sleep stages and sleep quality. The device can detect when the user has been asleep, awake, or restless by tracking their movement and heart rate patterns. When the user hasn't moved for about an hour, the device assumes they are asleep. It also tracks the beat-to-beat changes in the user's heart rate, known as heart rate variability (HRV), which fluctuate as the user transitions between different sleep stages.

Fitbit divides sleep into three main stages: light sleep, deep sleep, and REM sleep. Light sleep usually occurs at the beginning of the night and serves as an entry point into sleep. During this stage, the body unwinds and slows down, with breathing and heart rate decreasing slightly. Light sleep promotes mental and physical recovery.

Deep sleep typically occurs in the first few hours of sleep and is characterised by decreased responsiveness to outside stimuli. During this stage, breathing slows down, muscles relax, and heart rate becomes more regular. Deep sleep is important for physical recovery and supports memory, learning, and the immune system.

REM sleep, which is associated with vivid dreaming, usually occurs in longer periods during the second half of the night. During REM sleep, the brain becomes more active, and the eyes move rapidly in different directions. Heart rate increases, and breathing becomes more irregular. REM sleep plays a crucial role in mood regulation, learning, and memory consolidation.

Fitbit's sleep tracking feature provides users with a detailed breakdown of the duration spent in each sleep stage, allowing them to identify and address any sleep-related issues. Additionally, the "Sleep Insights" tool compares the user's data with that of other Fitbit users of the same age and gender, offering personalised recommendations for improving sleep quality.

Frequently asked questions