Fitbit's Deep Sleep Tracking: How Does It Work?

Sleep is an essential component of health, and its timing, duration, and quality are critical determinants of a person's overall health, playing a role in metabolic and emotional regulation, performance, and memory. Fitbit offers a way to track your sleep and uses your movement and heart rate patterns to estimate your sleep cycles. 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. So, how does Fitbit track deep sleep?

Heart rate variability

Fitbit devices use movement and heart-rate patterns to estimate your sleep stages. When you haven't moved for about an hour, your Fitbit tracker or watch 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.

The heart rate is typically the slowest during deep sleep and faster and more erratic during REM sleep. While you sleep, your Fitbit device tracks the beat-to-beat changes in your heart rate, which fluctuate as you transition between the different sleep stages.

Fitbit's sleep researchers and the National Sleep Foundation describe the various sleep stages. Light sleep serves as your entry point into sleep each night as your body unwinds and slows down. Deep sleep enables physical recovery, and REM sleep helps with strategic thinking and creativity.

The overall sleep score provided by Fitbit is a sum of individual scores in three components: sleep duration, sleep quality, and restoration, for a total score of up to 100. The restoration component takes into account the estimated oxygen variation graph, where high variations in blood oxygen saturation levels can indicate disturbances in breathing during sleep.

Movement and heart rate patterns

Fitbit devices track sleep by using movement and heart-rate patterns. The device assumes that you are asleep when it detects no movement for about an hour. It also uses the length of time of movements that indicate sleep behaviour, such as rolling over, to confirm your sleep status.

While you sleep, the 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. 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 feature estimates sleep stages using a combination of movement and heart-rate patterns. The accelerometer, an instrument for measuring bodily accelerations of the wrist, is used to measure movement. The device also uses 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.

Heart rate is slowest during deep sleep and faster and more variable during REM sleep. While deep sleep is associated with less brain activity than REM sleep, it is critical for physical recovery. REM sleep, on the other hand, aids in strategic thinking and creativity.

It is important to note that Fitbit's sleep staging accuracy is slightly over two-thirds for 30-second blocks, and there is a slight overestimation of REM sleep from sensor readings. However, by using the device for multiple nights, you can gain a better understanding of your sleep patterns and the proportion of time spent awake, in light sleep, deep sleep, or REM sleep.

Sleep cycles

Light sleep is the entry point into sleep each night as the body unwinds and slows down. Deep sleep, which occurs earlier in the night, enables physical recovery. REM sleep, which occurs more towards the end of the night, is associated with vivid dreams and helps with strategic thinking and creativity.

Fitbit uses movement and heart-rate patterns to estimate an individual's sleep cycles and stages. When a person hasn't moved for about an hour, the Fitbit tracker assumes they are asleep. It also tracks the beat-to-beat changes in heart rate, known as heart rate variability (HRV), which fluctuate as one transitions between the different sleep stages.

While Fitbit's sleep staging is only 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 it estimates. Therefore, using the app over multiple nights can provide a good sense of one's typical sleep patterns.

The Fitbit app also provides a Sleep Score, which is a personalized score based on heart rate, restlessness, and sleep stages. It also offers tools to help individuals sleep better, such as the ability to set a sleep schedule and bedtime reminders.

Sleep stages

Sleep is an essential component of health, and its timing, duration, and quality are critical determinants of a person's overall health, playing a role in metabolic and emotional regulation, performance, and memory. The best way to know if you are getting enough sleep is to track it. Fitbit devices can help you do this by tracking your sleep stages.

Fitbit devices use movement and heart-rate patterns to estimate your sleep stages. When you haven't moved for about an hour, your Fitbit tracker or watch assumes that you're asleep. Additional data, such as the length of time of movements that are indicative of sleep behavior (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.

Light sleep serves as your entry point into sleep each night as your body unwinds and slows down. Deep sleep enables physical recovery, and periods of deep sleep are typically longer earlier in the night. REM sleep is associated with vivid dreams, and these periods are typically longer towards the end of the night. During a sleep cycle, you commonly switch from light sleep to deep sleep, then back to light sleep, and into REM sleep, and then the cycle generally repeats, but sleep patterns vary naturally.

Fitbit's sleep feature provides a Sleep Score based on your heart rate, the time spent awake or restless, and sleep stages. The overall sleep score is a sum of individual scores in three components: sleep duration, sleep quality, and restoration, for a total score of up to 100. Fitbit also offers tools to help you sleep better, such as setting a sleep schedule and bedtime reminders to help you wind down each night.

Sleep quality

Firstly, Fitbit devices track your sleep by monitoring your movement and heart rate patterns. When you haven't moved for about an hour, your Fitbit assumes you're asleep. It then 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 this data to estimate your sleep cycles from the previous night.

The Fitbit Sleep Score is a quick way to gauge your sleep quality. This score is based on your heart rate, the time spent awake or restless, and your sleep stages. The score is calculated using three components: sleep duration, sleep quality, and restoration, for a total score of up to 100. The restoration component takes into account estimated oxygen variation, as high variations in blood oxygen saturation levels can indicate disturbances in breathing during sleep.

Additionally, Fitbit provides a monthly personalized analysis of 10 key metrics to help improve your sleep habits and quality. This includes insights into how your sleep is related to your activity, mood, and other factors, as well as tips to enhance your sleep quality. Fitbit also offers guided programs developed with sleep experts, such as "Habits for Restful Sleep" and "Get More Sleep," which provide relaxation tools and help you build better daytime habits for deeper sleep at night.

It's important to note that while Fitbit's sleep tracking can provide valuable insights, it may not always be completely accurate. According to a 2017 paper by Fitbit scientists, the watch's sensors can classify sleep stages with 69% accuracy in any given 30-second time window. Therefore, it's recommended to use the device for multiple nights to get a more accurate understanding of your sleep patterns.

Frequently asked questions