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

Sleep is an essential component of health, impacting everything from a person's daily mood to their physical activity and memory. Fitbit devices can help users understand their sleep patterns and cycles, and recognise when they need more rest. Fitbit's sleep-tracking function works by using an accelerometer to measure bodily accelerations of the wrist, along with a technique called optical photoplethysmography (PPG) to measure blood flow. This, along with heart rate and movement data, is used to estimate sleep cycles and assign a sleep score.

Fitbit's sleep tracking functions

Sleep Tracking

Fitbit devices automatically detect your sleep when worn on the wrist during sleep. They use an accelerometer to measure bodily accelerations of the wrist, coupled with a technique called optical photoplethysmography (PPG) to measure blood flow based on how green light from an LED on the back of the device is reflected by the body. When your body is completely at rest and hasn't moved for about an hour, your device records that you're asleep. It also tracks your heart rate and movement patterns to determine your sleep stages, including light sleep, deep sleep, and REM sleep.

Sleep Stages

Fitbit estimates your sleep stages by analysing your movement and heart rate patterns. It tracks beat-to-beat changes in your heart rate, known as heart rate variability (HRV), which fluctuate as you transition between different sleep stages. When you sync your device in the morning, it uses this data to estimate your sleep cycles from the previous night. Fitbit categorises sleep into light sleep, deep sleep, and REM sleep, with REM sleep associated with vivid dreams and occurring more frequently as the night progresses.

Sleep Score

Fitbit provides a personalised Sleep Score based on various metrics, including heart rate, time spent awake or restless, and sleep stages. The score ranges from 0 to 100, with most users scoring between 72 and 83. This quick assessment helps you understand your sleep quality and identify areas for improvement.

Additional Features

Fitbit offers advanced sleep tools and features to enhance your sleep experience. This includes the Smart Wake feature, which monitors your sleep stages and wakes you during a light sleep stage within a specified time window. You can also set silent alarms to wake up with a quiet vibration on your wrist. Fitbit Premium provides additional tools, such as guided programs developed with sleep experts to help you achieve deeper sleep and improve your nighttime routine.

How Fitbit estimates sleep stages

Fitbit devices use a combination of movement and heart-rate patterns to estimate sleep stages. When the device doesn't detect any movement for about an hour, it assumes that you're asleep. Additional data, such as the length of time of movements that indicate sleep behaviour (like rolling over), can also help confirm that you're asleep.

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. 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: This serves as your entry point into sleep each night as your body unwinds and slows down. Light sleep also strengthens memory and learning.
• Deep sleep: This enables physical recovery. Periods of deep sleep are typically longer earlier in the night.
• REM sleep: The stage associated with vivid dreams, REM sleep periods are typically longer as the night goes on. REM helps with strategic thinking and creativity.

It's important to note that sleep cycles can vary naturally, so you may notice your sleep data occasionally fall outside the typical ranges. To gain a comprehensive understanding of your sleep patterns, you can compare your data from the previous night to your 30-day average.

The Fitbit Sleep Score

Fitbit devices that track heart rate (except Charge HR and Surge) also track sleep stages. While you're asleep, your body typically goes through several sleep cycles that last on average 90 minutes. Each cycle alternates between two types of sleep: light sleep and deep sleep. Periods of deep sleep are usually longer earlier in the night, while REM sleep periods tend to be longer towards the end of the night.

Fitbit's sleep researchers and the National Sleep Foundation describe the following 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.
• REM sleep: Helps with strategic thinking and creativity.

Fitbit Premium offers several tools that can 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" and "Get More Sleep".

The Fitbit Smart Wake feature

The Smart Wake feature is designed to wake the user anytime within a 30-minute window before the actual alarm time, depending on the sleep cycle they are in. It is based on the sleep cycle of the user, and it aims to wake them up during the lightest part of their sleep cycle, not during REM sleep. This means that the feature may not always work perfectly, as it can be difficult to distinguish between light sleep and REM sleep with the limited sensors of the Fitbit.

The Smart Wake feature does not rely on a Bluetooth connection between the Fitbit and the phone. However, some users have reported issues with the feature not appearing on their tracker or not working as expected.

Overall, the Fitbit Smart Wake feature has received positive reviews from users who appreciate its ability to wake them up gently and improve their morning mood.

The Fitbit Sleep Profile

Sleep is an essential component of health, playing a role in metabolic and emotional regulation, performance, and memory. Fitbit's sleep tracking function helps users understand their sleep patterns and recognise when they need more rest.

The Sleep Profile assigns users an animal avatar, such as an animal match, to give them a comprehensive understanding of their sleep style. It also provides a daily sleep score breakdown, helping users understand where they can improve their sleeping habits and receive personalised guidance. Fitbit Premium also offers guided programs developed with sleep experts, such as "Habits for Restful Sleep" and "Get More Sleep".

Fitbit devices track sleep by using movement and heart rate patterns. When the device senses that the user hasn't moved for about an hour, it assumes they are asleep. It also takes into account the length of time when movements indicate sleep behaviour, such as rolling over. While sleeping, the device 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. In the morning, the device syncs movement and heart rate patterns to estimate sleep cycles from the previous night.

Frequently asked questions