Fitbit's Sleep Profile: Why No Rem Details?

Fitbit devices can be used to track sleep and provide a sleep profile, including sleep stages such as REM sleep. However, it is important to note that the accuracy of these sleep profiles may vary, and there are certain requirements that must be met to access this feature. While Fitbit can estimate sleep stages, it does not provide a full sleep profile that includes all the traditional sleep stages. This raises the question of why Fitbit doesn't give a more comprehensive sleep profile, including all the sleep stages and their durations.

Fitbit estimates sleep stages by tracking movement and heart-rate patterns

Fitbit devices with heart-rate tracking capabilities (excluding the Fitbit Charge HR and Surge models) can estimate sleep stages by tracking movement and heart-rate patterns. When a user hasn't moved for about an hour, the device assumes they are asleep. Additional data, such as the length of time spent in sleep-related movements like rolling over, help confirm the user's sleep status.

While the user sleeps, the device tracks beat-to-beat changes in their heart rate, known as heart rate variability (HRV), which fluctuate as the user transitions between light sleep, deep sleep, and REM sleep stages. In the morning, when the user syncs their device, it uses the movement and heart rate data to estimate their sleep cycles from the previous night.

Fitbit's method of tracking sleep stages involves an accelerometer, which measures bodily accelerations of the wrist, coupled with 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. This allows the device to infer whether the user is awake, in a deep sleep, a light slumber, or dreaming.

While Fitbit's sleep staging is only correct for 30-second blocks slightly more than two-thirds of the time, there is no systematic bias to how much light sleep, deep sleep, or sleeplessness is estimated. This means that with continued use over multiple nights, users can gain an accurate understanding of their typical sleep patterns.

Fitbit devices use an accelerometer and optical photoplethysmography to monitor sleep

The accelerometer measures movement, while the optical photoplethysmography (PPG) measures heart rate and heart rate variability (HRV). These two data streams are combined to estimate sleep stages. During sleep, the autonomic nervous system modulates cardiovascular functions, and HRV can be used to assess cardiovascular autonomic control. This provides physiological changes that correlate with brain activity.

Fitbit devices use movement and heart rate patterns to estimate sleep cycles. When a user doesn't move for about an hour, the device assumes they are asleep. Additional data, such as the length of time when movements indicate sleep behaviour, help confirm the sleep status.

Fitbit's sleep researchers and the National Sleep Foundation describe the following sleep stages: light sleep, deep sleep, and rapid eye movement (REM) sleep. Light sleep is the entry point into sleep each night, and it typically begins within minutes of falling asleep. Deep sleep typically occurs in the first few hours of sleep, and it becomes harder to wake the sleeper. REM sleep is associated with vivid dreams, and it usually occurs after the initial stage of deep sleep.

Fitbit devices can help users understand their sleep patterns and make adjustments to improve sleep quality. The data provided by these devices can also be shared with a doctor if there are any concerns about sleep health.

Fitbit's sleep staging is correct in 30-second blocks slightly more than two-thirds of the time

Fitbit devices can be used to track sleep patterns and quality. They can detect sleep automatically when worn to bed, and they record that a user is asleep when their body is completely at rest and they have not moved for about an hour. Fitbit devices that track heart rate (except Charge HR and Surge) can also track sleep stages.

Fitbit estimates sleep stages by using movement and heart-rate patterns. It assumes the user is asleep when they haven't moved for about an hour, and it uses the length of time when movements indicate sleep behaviour (like rolling over) to confirm the user's sleep status. It tracks beat-to-beat changes in the user's heart rate, known as heart rate variability (HRV), which fluctuate as the user transitions between light sleep, deep sleep, and REM sleep stages.

While Fitbit can be a useful tool for tracking sleep, it is important to note that its accuracy may vary. 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. This means that Fitbit's sleep staging is correct in 30-second blocks slightly more than two-thirds of the time. Therefore, it is recommended to use the app for multiple nights to get a better sense of your sleep patterns.

Fitbit's sleep staging accuracy has been a subject of discussion among users. Some users may expect higher accuracy, especially when using the app for a single night. However, it is important to consider the context and limitations of the technology. Fitbit's estimation of sleep stages is based on movement and heart rate patterns, which may not always accurately represent an individual's sleep stages. Additionally, there are other factors that can affect the accuracy, such as the fit and position of the device on the wrist.

Despite the limitations, Fitbit's sleep tracking technology has improved over time, and the company continues to research and develop more advanced features. In conclusion, while Fitbit's sleep staging may not be correct all the time, it can still provide valuable insights into a user's sleep patterns when used consistently over an extended period.

Fitbit does not over or underestimate the duration of sleep stages

Fitbit's sleep-staging models do not over or underestimate the duration of sleep stages. In fact, they show promising performance, especially in differentiating wake from sleep. However, these models are not a substitute for polysomnography (PSG), which is regarded as the gold standard for diagnosing sleep disorders and conducting sleep research.

Fitbit sleep-staging models showed higher sensitivity and specificity in detecting sleep epochs than non-sleep-staging models and those reported in the literature for regular wrist actigraphy. They also showed no significant difference in measured values of WASO (P=.25; heterogenicity: I2=0%, P=.92), TST (P=.29; heterogenicity: I2=0%, P=.98), and SE (P=.19) when compared to PSG.

The lack of a significant difference in these sleep parameters indicates that Fitbit sleep-staging models provide an accurate estimation of sleep stages without over or underestimating their duration.

Fitbit's SpO2 sensor is not currently used by the app to provide sleep feedback

Fitbit devices with SpO2 sensors can be used to track blood oxygen levels while you sleep. The SpO2 feature is intended for general wellness purposes only and should not be used for medical purposes.

The data is derived from a combination of red and infrared sensors on the back of the device. The sensors shine red and infrared light onto your skin and blood vessels, using the reflected light to estimate how much oxygen is in your blood. The richly oxygenated blood reflects more red light than infrared light, and poorly oxygenated blood reflects more infrared light than red light.

However, the SpO2 sensor is not currently used by the Fitbit app to provide sleep feedback. While the app can provide a generalised level of your blood oxygen levels, it does not provide exact, hour-by-hour levels of oxygen in your bloodstream. Instead, the app presents a graph called "Estimated Oxygen Variation", indicating whether there is a big or small variation in your blood oxygen level. This data is intended to show how your body is distributing oxygen in your bloodstream.

Fitbit has chosen to generalise this data to prevent it from being used for medical diagnoses. While the SpO2 sensor can provide valuable insights into your health, it is not currently used by the app to provide detailed sleep feedback.

Frequently asked questions