Miya Pearson
Sleep is an essential component of a healthy lifestyle, impacting everything from our daily mood to our appetite and physical activity levels. Fitbit offers several tools to help users understand their sleep patterns and improve their sleeping habits. By tracking heart rate and movement, Fitbit devices can estimate the sleep stages a user cycles through, including light sleep, deep sleep, and REM sleep. While Fitbit's sleep staging is only correct 69% of the time in 30-second blocks, using the app over multiple nights can provide a good sense of a user's typical sleep patterns.
| Characteristics | Values |
|---|---|
| Sleep tracking method | Heart rate and motion sensors |
| Sleep cycle | Alternates between light sleep and deep sleep, then REM sleep |
| Sleep data | Time spent in each sleep stage, sleep score, sleep duration, sleep quality, restoration |
| Sleep score | Based on heart rate, time spent awake or restless, and sleep stages |
| Sleep staging accuracy | 69% accuracy in any given 30-second time window |
| Sleep staging bias | No systematic bias in estimating light sleep, deep sleep, or sleeplessness |
| Sleep staging requirements | At least 3 hours of sleep |
| Additional features | Snore and noise detection, SpO2 sensor for blood oxygenation |
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What You'll Learn
Fitbit's accelerometer and optical photoplethysmography
Fitbit uses an accelerometer and optical photoplethysmography (PPG) to track sleep stages. The accelerometer is a 3-axis accelerometer that detects movement and is used in combination with a PPG sensor to measure heart rate. The PPG sensor uses green light from an LED on the back of the watch, which is reflected by the body to measure blood flow. This is known as optical photoplethysmography.
The accelerometer measures bodily accelerations of the wrist, while the PPG sensor measures heart rate by detecting changes in blood volume in the microvascular bed of tissue volume. This is done through a process called photoplethysmography, which uses light-emitting diodes (LEDs) and photodiodes to detect changes in blood volume. The LEDs emit light into the skin, which is then reflected back to the photodiodes. The changes in blood volume cause a variation in the amount of light absorbed or reflected, which is used to calculate heart rate.
Fitbit's sleep tracking feature uses these two sensors to infer when a person is sleeping and what stage of sleep they are in. It looks at how much the wearer is moving and how often their heart is beating to determine whether they are awake, in a deep sleep, a light slumber, or dreaming (REM sleep). The accelerometer detects movement, while the PPG sensor measures heart rate variability (HRV), which is the beat-to-beat change in heart rate. These two data points allow the Fitbit to estimate sleep cycles and provide a sleep score based on sleep duration, quality, and restoration.
While Fitbit's sleep staging has been found to be correct in 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 using the app for multiple nights will give a good sense of an individual's typical sleep patterns.
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Heart rate variability and movement
Fitbit devices with heart-rate tracking can record the sleep stages you cycle through. 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.
Fitbit uses movement and heart rate patterns to estimate sleep cycles. It 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 features an SpO2 sensor for measuring blood oxygenation, but Fitbit is not yet using this feature to inform you about your sleep.
The Fitbit Versa infers when you're sleeping and what stage of sleep you're in using an accelerometer (an instrument for measuring bodily accelerations of the wrist) coupled with a technique called optical photoplethysmography, or PPG. PPG measures blood flow based on how green light from an LED on the back of the watch is reflected by the body.
While the Fitbit Charge 2 can't detect brain waves, it can detect changes in sleep stages through heart rate and movement. If you turn off the heart rate monitor, the sleep tracking will be based on movement detection alone, which may impact accuracy.
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Fitbit's accuracy in tracking sleep stages
Fitbit devices that track heart rate (except Charge HR and Surge) can also track sleep stages. Fitbit uses movement and heart rate patterns to estimate sleep cycles. It 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.
Fitbit's sleep staging is only correct in 30-second blocks slightly more than two-thirds of the time. 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.
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. However, it is important to note that Fitbit is not a medical device and should not be relied on for medical purposes. If you have any concerns about your sleep health, it is recommended to share the information with your doctor.
Some users have also reported their personal experiences with Fitbit's accuracy in tracking sleep stages. While some find it accurate, others believe that the "stages stuff is mostly BS". It is important to note that individual experiences may vary, and the accuracy of Fitbit's sleep tracking may depend on various factors, including the specific model and settings used.
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The different sleep stages
Sleep is divided into two phases: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, while REM sleep has only one stage. Each phase and stage of sleep includes variations in muscle tone, brain wave patterns, and eye movements.
The first stage of NREM sleep, N1, is when a person first falls asleep. This stage normally lasts just one to seven minutes. During N1 sleep, the body has not fully relaxed, but the body and brain activities start to slow with periods of brief movements. It is easy to wake someone up during this stage, but if a person is not disturbed, they can move quickly into the second stage, N2. N2 is still light sleep, but deeper than N1. During this stage, brain waves slow down and have noticeable pauses between short, powerful bursts of electrical activity. Experts believe these bursts are the brain organizing memories and information from the time spent awake. N2 accounts for about 45% of your time asleep.
After N2, the body moves into the third stage, N3, which is deep sleep. During this stage, brain waves are slow but strong, and the body takes advantage of this very deep sleep stage to repair injuries and reinforce the immune system. N3 makes up about 25% of total sleep time in adults, but babies and children need more of this stage. Without enough N3 sleep, one might feel tired and drained, even if they slept for a long time.
The fourth and final stage is REM sleep, which is when most dreams occur. REM sleep gets its name from the way the eyes move behind the eyelids during this stage. There is only one stage of REM sleep, but multiple stages of NREM sleep. The pattern then reverses itself until a person either wakes up or enters REM sleep again.
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Fitbit Premium's sleep features
Fitbit Premium offers a range of features to help you understand and improve your sleep. Firstly, it provides a daily sleep score breakdown, allowing you to see how well you slept and identify areas for improvement. This score is based on your heart rate, the time spent awake or restless, and your sleep stages. Fitbit Premium also offers personalised guidance and recommendations to help you enhance your sleep quality.
Another feature of Fitbit Premium is its guided programs, developed in collaboration with sleep experts. One such program is 'Habits for Restful Sleep', which focuses on building better daytime habits to promote deeper sleep at night. It offers advice and tips to improve your nighttime routine and achieve longer, more restful sleep. Additionally, the 'Get More Sleep' program helps you set sleep goals and provides relaxation tools to support you in reaching those goals.
Fitbit devices with heart-rate tracking can monitor your sleep stages, including light sleep, deep sleep, and REM sleep. They do this by tracking changes in your heart rate and movement while you sleep. Fitbit's sleep tracking uses an accelerometer to measure bodily accelerations of the wrist, along with optical photoplethysmography (PPG) to measure blood flow based on reflected green light from an LED on the back of the watch. This technology allows Fitbit to estimate your sleep stages and provide insights into your sleep patterns.
It's important to note that Fitbit's sleep staging accuracy is based on 30-second blocks, with a 69% accuracy rate in any given 30-second window, according to a 2017 paper by Fitbit scientists. While this may seem low, using the app over multiple nights can give you a better understanding of your typical sleep patterns. Additionally, Fitbit does not over or underestimate the duration of sleep stages, providing a relatively consistent picture of your sleep behaviour.
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Frequently asked questions
Fitbit uses an accelerometer to measure bodily accelerations of the wrist. It also uses optical photoplethysmography (PPG) to measure blood flow based on how green light from an LED on the back of the watch is reflected by the body. It also tracks heart rate variability (HRV) and movement to determine sleep stages.
Fitbit's sleep staging is only correct in 30-second blocks slightly more than two-thirds of the time. There is a slight overestimation of REM sleep from sensor readings, but the Fitbit algorithm corrects for this. If you use the app for many nights, you will get a good sense of your sleep patterns.
For best results, wear your device higher on your wrist (2-3 finger widths above your wrist bone). If you sleep in a position that prevents your device from getting a consistent heart rate reading or wear it too loosely, you might not get a sleep log.
Your sleep stages data helps you track your patterns and notice variations. If you have any concerns about your sleep health, it is recommended that you share the information with your doctor.
