Fitness Watches: Tracking Sleep, How Does It Work?

Sleep tracking is a common feature of fitness watches and other wearable devices. These devices can monitor sleep duration, quality, and phases by tracking movement, heart rate, blood oxygen levels, and other health metrics. While the accuracy of sleep tracking technology has improved in recent years, it is still estimated to be around 60%. Sleep tracking can be a useful tool for recognizing patterns in sleep habits and making changes to improve sleep quality. However, it is important to note that these devices do not directly measure sleep and are not a substitute for medical sleep studies.

Heart rate, blood oxygen levels, and other health data

Heart rate is a critical indicator of sleep quality. During sleep, your heart rate slows and becomes more regular. Fitness trackers use technologies such as photoplethysmography (PPG) to estimate heart rate and heart rate variability. PPG uses a light source and a photodetector to measure reflected light from the tissue. Since blood absorbs and reflects different wavelengths of light, the photodetector can measure blood volume, oxygen saturation, and heart rate.

Blood oxygen saturation levels provide additional insights into your sleep quality. During sleep, oxygen levels tend to dip at various stages. By tracking blood oxygen levels, fitness watches can help identify potential issues such as sleep apnea or breathing disturbances.

Other health data collected by fitness trackers include sleep phases and sleep duration. Some trackers can differentiate between light sleep, deep sleep, and REM sleep. They can also detect interrupted sleep by monitoring movement and restlessness. Additionally, some trackers allow you to input lifestyle factors such as caffeine intake, meal times, and stress levels, which can impact your sleep.

While fitness trackers provide valuable insights, it's important to remember that they don't directly measure sleep. Instead, they estimate sleep by tracking inactivity and movement. For more precise data, a medical sleep study that monitors brain waves is necessary. However, fitness trackers can help you recognize patterns and make adjustments to improve your sleep habits.

Sleep duration and quality

Sleep tracking devices can help you understand your sleep patterns and the quality of your sleep. They can track the time you are inactive and record when you fall asleep and when you wake up. Some devices can also detect interrupted sleep, letting you know when you are tossing and turning or waking during the night. This can be achieved through tracking your movement, which is done using accelerometers and gyroscopes. These devices can also measure your heart rate and heart rate variability, as well as your blood oxygen levels, as these are indicators of the various stages of sleep. For example, your heart rate tends to slow and your blood oxygen levels dip during REM sleep.

Some trackers also prompt you to enter information about your lifestyle choices, such as how much caffeine you've consumed, when you've eaten, or whether your stress levels are high, as these factors can also impact your sleep.

While sleep trackers can collect a lot of information about your sleep habits, it's important to remember that they don't measure sleep directly. Instead, they often measure inactivity as a way to estimate sleep. The data from sleep trackers can be useful for helping you recognize patterns in your sleep habits and make changes to improve your sleep quality. However, if you have concerns about the quality of your sleep, it's recommended to consult a health practitioner.

There are a variety of sleep trackers available, including wearable devices such as smartwatches, smart rings, and headbands, as well as devices that clip to your pillow or sit on your bedside table. Some people may find it uncomfortable to wear a device while sleeping, and there are also concerns about the radiation emitted by these devices. It's important to consider your personal preferences and comfort when choosing a sleep tracker.

Sleep phases and patterns

Sleep is divided into several distinct phases and patterns, which are defined by different physiological characteristics. These include brain waves, heart rate, breathing, eye movement, and muscle activity. Sleep tracking devices can monitor these patterns to provide insights into sleep quality and duration.

The first stage of sleep is typically characterised by light sleep, where individuals can be easily awakened. This stage usually lasts only a few minutes. As an individual progresses into the second stage of sleep, brain waves begin to slow down, and breathing becomes more regular. This stage can last up to 25 minutes.

The third and fourth stages of sleep are marked by deeper sleep, where it becomes more difficult to wake the sleeper. These stages are crucial for physical growth, repair, and immune function. During this period, the brain produces delta waves, which are associated with slow-wave sleep (SWS) or deep sleep. This stage typically lasts up to 40 minutes in early sleep cycles and gradually shortens as the individual spends more time in the REM stage.

The final stage of the sleep cycle is known as rapid eye movement (REM) sleep. During REM sleep, the body enters a state of temporary paralysis, while the eyes move rapidly behind closed eyelids. This stage is associated with increased brain activity, including dreaming and the processing and storage of long-term memories. The cycle then repeats itself, with the duration of each cycle ranging from 90 to 110 minutes.

Fitness watches and sleep trackers use various methods to monitor sleep patterns and phases. Some devices rely on accelerometers and gyroscopes to track movement and detect restlessness or tossing and turning. Others utilise photoplethysmography (PPG), which measures blood volume, oxygen saturation, and heart rate variability by analysing reflected light from the tissue. Additionally, some trackers use microphones to capture noise and measure respiration, detecting issues such as snoring and sleep apnea.

Accuracy and reliability

The accuracy of fitness watches in tracking sleep varies. In a 2019 study, Fitbit was found to offer impressive accuracy given its entry-level cost, but it was still outperformed by Polysomnography (PSG), which is considered the gold standard of sleep tracking. Another study in 2020 showed promising results for the Whoop 4.0 tracker, which predicted sleep duration within a precision of 17.8 minutes and accurately detected REM and Slow Wave (deep) sleep.

Fitness watches use different methods to track sleep. Some use accelerometers and gyroscopes to monitor movement, while others combine actigraphy with photoplethysmography (PPG) to get a more detailed picture. PPG involves using a light source and a photodetector to measure reflected light from the tissue. Since blood absorbs and reflects different wavelengths of light, the photodetector can measure blood volume, oxygen saturation, heart rate, and heart rate variability. This data is then used to make observations about a person's sleep, as these factors change at various sleep stages.

Sleep tracking features on smartwatches can also monitor sleep patterns and sleep quality by detecting interrupted sleep and measuring sleep duration. Some smartwatches can also monitor sleep phases and time alarms to go off during lighter sleep. However, it is important to note that these devices do not directly measure sleep. Instead, they often estimate sleep by tracking inactivity.

The accuracy of sleep tracking can be affected by factors such as the device's ability to distinguish between resting and sleeping, as well as the user's lifestyle choices, such as caffeine intake, stress levels, and eating habits. Additionally, some users have expressed concerns about the potential impact of electromagnetic fields (EMF) emitted by smartwatches on their health, although there is no direct correlation between headaches and EMF radiation.

While fitness watches can provide valuable insights into sleep patterns, it is recommended to consult a health practitioner for concerns about sleep quality. These devices can help users recognize patterns and make informed decisions to improve their sleep habits.

Alternative sleep tracking methods

Sleep tracking devices can be useful for helping you recognize patterns in your sleep habits. However, they don't measure sleep directly but instead often measure inactivity as a way to estimate sleep. For exact data about your sleep habits, you would need to undergo a medical sleep study, which monitors brain waves to analyze the stages of sleep.

Polysomnography (PSG) is the gold standard of sleep tracking. It records brain waves, heart rate, blood oxygen level, respiration, and movement. However, this method is typically only available in sleep labs.

There are a variety of alternative sleep tracking methods available for home use, including:

• Smartphone apps: Apps such as SleepScore and Sleep Cycle can help you track your sleep patterns and provide recommendations for improvement. SleepScore is particularly good for setting sleep goals and offers detailed sleep stage data. However, it currently only works with iPhone 6 and higher and a limited number of Android phones.
• Wearables: In addition to smartwatches and fitness trackers, there are also sleep-tracking rings, headbands, and bedside devices. These devices can track sleep duration, quality, and phases. Some can even detect breathing problems that may indicate sleep apnea.
• Under-mattress sensors: One example is the Withings sensor-packed mat that slips underneath your mattress to track your movements, breathing, and heart rate throughout the night. Another is the Eight Sleep Pod 4 Cover and Hub, a mattress cover that regulates your temperature.

Frequently asked questions