Fitbit's Heart Rate Tracking: Does It Work While Sleeping?

Fitbit devices with heart-rate tracking capabilities can monitor your heart rate while you sleep. This is done by tracking beat-to-beat changes in your heart rate, also known as heart rate variability (HRV). Your sleeping heart rate is detected continuously throughout the night and is often lower than your resting heart rate. However, some users have reported issues with the heart rate monitor not working properly during sleep. Fitbit offers advanced sleep tools that can help you track your sleep, view trends over time, and improve your rest.

Fitbit Sleep Score

The Fitbit Sleep Score is a personalised score that shows how well you slept. It is based on your heart rate, the time you spend awake or restless, and your sleep stages. The overall sleep score is the sum of your individual scores for sleep duration, sleep quality, and restoration, for a total score of up to 100. Fitbit trackers and watches use your sleeping heart rate, movement, and more to measure your time spent in each sleep stage.

The Fitbit Sleep Score is a quick way to gauge your sleep. It is available on trackers and watches with heart rate tracking. To get your Sleep Score, you need to wear your Fitbit device to bed and sync it when you wake up. You can then check your sleep tile to find your sleep score. On certain devices, you can check your sleep score from your wrist.

The sleep stages that the Fitbit Sleep Score is based on are light sleep, deep sleep, and REM sleep. 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 these sleep stages. 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.

Fitbit Premium offers several tools that may help you get better sleep, including a daily sleep score breakdown that can help you understand where you can improve your sleeping habits and receive personalised guidance. Fitbit Premium also has guided programs developed with sleep experts, including "Habits for Restful Sleep", which is aimed at helping you build better daytime habits to help achieve deeper sleep at night. Another program in Premium is "Get More Sleep", which may help you improve your nighttime routine and work towards longer sleep with goals and relaxation tools.

Fitbit Premium

Additionally, Fitbit Premium offers guided programs developed in collaboration with sleep experts. These programs aim to help you build better daytime and nighttime routines to improve your sleep quality. The "Habits for Restful Sleep" program focuses on daytime habits to promote deeper sleep, while the "Get More Sleep" program assists in extending your sleep duration with goals and relaxation tools. These programs provide a structured approach to improving your sleep habits and overall well-being.

To enhance your sleep tracking experience, Fitbit Premium also provides monthly personalised analyses of 10 key metrics related to your sleep habits and quality. This comprehensive understanding of your sleep patterns empowers you to make informed decisions about your sleep routine. Fitbit Premium also offers sleep tracking for different types of Fitbit devices, including trackers and watches with heart rate tracking capabilities.

It's important to note that the accuracy of your heart rate readings may vary depending on factors such as device placement, personal physiology, and external conditions like air temperature. Fitbit's heart rate tracking technology uses optical sensors and infrared light to detect changes in blood volume in your wrist's capillaries. While generally reliable, factors like wrist position during sleep or a loose-fitting device can impact the consistency of heart rate data.

Fitbit Sleep Stages

Fitbit's sleep-tracking technology uses movement and heart-rate patterns to estimate sleep stages. When a user hasn't moved for about an hour, the Fitbit tracker assumes they are asleep. It also uses the length of time when movements indicate sleep behaviour, such as rolling over, to confirm sleep status. The device tracks beat-to-beat changes in heart rate, known as heart rate variability (HRV), which fluctuate as the user transitions between light sleep, deep sleep, and REM sleep stages.

Sleep scientists recognise three main sleep stages: REM sleep, deep sleep, and light sleep. REM sleep is associated with vivid dreams, and periods of REM sleep tend to be longer towards the end of the night. Deep sleep, which occurs earlier in the night, is a restorative phase for the mind and body. Light sleep is the entry point into sleep each night as the body unwinds and slows down.

Fitbit trackers and watches use sleeping heart rate, movement, and other data to measure the time spent in each sleep stage and give a personalised Sleep Score. This score is based on sleep duration, sleep quality, and restoration, for a total score of up to 100. The Sleep Score provides a quick way to gauge sleep quality and can be useful for understanding sleep patterns and improving sleep habits.

It is important to note that Fitbit's sleep staging data is based on inferences from physical movement, pulse rate, and breathing rather than direct measurements of brain activity. Therefore, it may be less accurate than data obtained from a laboratory sleep study. Factors such as the position of the device and sleeping position can also affect the accuracy of sleep stage recording. For optimal sleep tracking, it is recommended to wear the Fitbit moderately snugly and higher on the wrist, about 2-3 finger widths above the wrist bone.

Fitbit Heart Rate Zones

Fitbit devices with heart-rate tracking use optical heart rate sensors to detect changes in blood volume in the capillaries of your wrist. Light-sensitive photodiodes measure these changes, and your device calculates your heart rate in beats per minute (bpm). Fitbit also uses your movement and heart-rate patterns to estimate your sleep stages. 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.

Heart rate zones on Fitbit are personalized for your age and fitness level. They are percentages of your maximum heart rate and can help you determine the intensity of your workout or activity. The formula Fitbit uses to calculate your heart-rate zones is based on your device. Fitbit personalizes your heart rate zones using your heart rate reserve, which is the difference between your maximum heart rate and your resting heart rate. While your maximum heart rate is generally determined by age, your resting heart rate can be lowered by increasing your fitness level.

You can check your personalized heart rate zones on the Active Zone Minutes screen in the Fitbit app: From the Today tab in the Fitbit app on your phone, tap Active Zone Min. Swipe up. Your personalized zones are listed in the "Your heart rate zones" section. When you're in an exercise mode, your Fitbit tracker or watch buzzes to let you know when you enter and exit a heart rate zone: 1 buzz for the fat burn zone, 2 buzzes for the cardio zone, and 3 buzzes for the peak zone.

The zones cannot be customized on Fitbit. The only thing you can change is the maximum heart rate, which will change the zones' range (based on your resting HR and max HR). If you want to specify each zone individually, you cannot do it. However, instead of using the 3 heart rate zones, you can create one custom zone.

Fitbit Heart Rate Accuracy

Fitbit devices with heart-rate tracking (except Fitbit Charge HR or Fitbit Surge) can be used to track sleep. Fitbit's sleep researchers and the National Sleep Foundation have identified the following sleep stages: Light Sleep, Deep Sleep, and REM Sleep. During sleep, Fitbit devices track the beat-to-beat changes in heart rate, known as heart rate variability (HRV), which fluctuate as the user transitions between sleep stages. The overall sleep score is a sum of individual scores for sleep duration, sleep quality, and restoration, for a total score of up to 100.

The accuracy of Fitbit's heart rate tracking has been questioned by some users, who have noticed discrepancies between their Fitbit data and their actual heart rate. Some users have reported that their Fitbit devices lose their pulse signal during exercise, resulting in inaccurate heart rate averages. However, other users have found their Fitbit devices to be accurate when compared to manual radial pulse checks and blood pressure readings.

A study published in PubMed evaluated the heart rate measurement accuracy of the Fitbit Charge 4 and found that it slightly underestimated the heart rate across all activities, with a mean absolute error of less than 10%. The study concluded that the device produced valid results, but accuracy varied with activity.

To ensure the best accuracy when using a Fitbit to track heart rate and sleep, it is recommended to wear the device correctly and securely, positioned higher on the wrist, and to utilize the automatic sleep tracking feature by simply wearing the device to bed.

Frequently asked questions