Miya Pearson
Sleep trackers have become increasingly popular as more and more people report sleep complaints. Sleep trackers come in several forms, from wearable smartwatches to headbands, rings, and devices that slip under your sheet. They can measure the quantity and quality of your sleep by tracking your body movement and sometimes your heart rate. However, it's important to note that they don't directly measure your sleep and only make estimates based on your body movement and heart rate changes. While they can give you insights into your sleep patterns, they are not as accurate as medical sleep studies that monitor brain waves.
| Characteristics | Values |
|---|---|
| Type of device | Wearable smart watches, headbands, rings, under-sheet devices, apps |
| Method of measurement | Accelerometers, gyroscopes, photoplethysmography (PPG), microphones, thermometers |
| Data measured | Body movement, heart rate, skin temperature, blood-oxygen levels, respiration, environmental factors (light, temperature) |
| Data analysis | Estimates of sleep/wakefulness, sleep duration, sleep quality, sleep phases, sleep trends |
| Accuracy | Varies across devices; generally accurate for healthy individuals, less accurate for people with insomnia |
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What You'll Learn
Heart rate and movement
Heart rate and body movement are two of the most common ways fitness trackers measure sleep. As the body moves frequently during all stages of sleep, movement provides some clues about what sleep stage the wearer is in. However, many sleep devices struggle to differentiate one stage of sleep from another based on motion alone.
Fitness trackers usually have built-in accelerometers and gyroscopes, which allow them to track and monitor movement. By analysing your movement data throughout the day, sensors can estimate when you're active and inactive (asleep). This process is known as actigraphy.
Heart rate fluctuates during different sleep stages, so trackers that incorporate heart rate data tend to be more accurate when measuring sleep duration. When you enter deep sleep, your heart rate drops compared to when you're in a light sleep stage, and it raises when you're in REM sleep.
However, many experts are uncertain about the accuracy of devices that track heart rate due to limited research and differences between each device. For example, one study of heart rate sleep trackers showed that two consumer devices tended to underestimate the amount of deep sleep by as much as 46 minutes.
While trackers can provide insights into your sleep patterns, they don't measure sleep directly. For exact data about your sleep habits, you would need to undergo a medical sleep study that monitors brain waves to analyse the stages of sleep you cycle through during the night.
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Respiration and sleep apnea
Sleep apnea is a condition that causes an individual to stop breathing while asleep. This can be due to a blockage of the airway, known as obstructive sleep apnea, or because the brain fails to send signals to keep the breathing-related muscles active, known as central sleep apnea. The former is more prevalent, affecting approximately 1 billion people worldwide aged between 30 and 69. However, central sleep apnea is not a rare condition. Sleep apnea can be diagnosed through an overnight polysomnogram test, which involves wearing sensors that monitor heart rate, breathing, blood oxygen levels, brain waves, and more. Home sleep apnea testing is also an option, although it does not include brain wave monitoring and cannot diagnose central sleep apnea.
Fitness trackers can play a role in detecting sleep apnea by using a microphone to capture noises, including snoring and breathing patterns. These devices can also measure respiration and detect how often an individual wakes up during the night. Additionally, some sleep trackers with thermometers can gauge the temperature of the room, which may be a contributing factor to disrupted sleep. While fitness trackers provide valuable insights, they should not be solely relied upon for diagnosing sleep apnea. Seeking professional medical advice and undergoing specialised tests are crucial for an accurate diagnosis and effective treatment plan.
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Temperature
Sleep trackers use algorithms to estimate sleep cycles based on data such as body temperature, movement, and heart rate. However, it is important to note that sleep trackers are not perfect and may not always accurately differentiate between sleep stages. Individual differences in movement and heart rate can also make it challenging to measure sleep statistics on a device.
Despite these limitations, tracking temperature and other physiological metrics can provide valuable insights into sleep quality and help individuals understand their energy levels and activity bandwidth for the day. For example, the Oura Ring's body clock feature teaches users about their circadian rhythms and body clock by considering body temperature, sleep-wake cycle, and physical activity.
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Blood volume and oxygen saturation
Fitness trackers use a variety of methods to monitor sleep. One of these methods involves measuring blood volume and oxygen saturation.
Some fitness trackers, like the Fitbit, use red and infrared sensors on the back of the device to measure SpO2 levels while the user is sleeping. The SpO2 app or clock face must be installed on the device to collect SpO2 data. The Fitbit Charge 6, for example, offers SpO2 technology at a lower price point than other models. The Fitbit Premium subscription service provides additional sleep insights.
The Oura Ring Gen3 and Gen4 are also able to measure blood oxygen saturation, as well as other vitals like heart rate, heart rate variability, and body temperature. The ring-shaped device shines red and green LED light beams through the skin and uses sensors to measure these vitals. The data is then synthesised to provide the user with Sleep, Activity, and Readiness scores.
Other notable fitness trackers that can measure blood oxygen saturation include the Apple Watch Series 4 and later, the Withings ScanWatch 2, the Ultrahuman Ring Air, and the Garmin Epix Pro.
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Sleep cycles and sleep staging
Sleep is divided into several stages, including "deep", "light", and "rapid eye movement" (REM) sleep. The deep sleep stage is when the brain "cleanses" itself, flushing out toxins, and this sleep stage helps you feel refreshed the next day. The REM stage is when your brain is more active, dreams occur, and your brain processes information and stores long-term memories. The sleep cycle repeats every 90 to 110 minutes, with REM cycles increasing in length as sleep progresses.
Sleep trackers come in several forms, from wearable smartwatches and rings to headbands, devices slipped under a sheet, or apps that use motion detection and microphones. While sleep trackers can collect a lot of information about sleep habits, they don't directly measure sleep. Instead, they often measure inactivity as a surrogate for estimating sleep. Most sleep tracking devices make some guesstimates to determine how much you're actually sleeping. For exact data about your sleep habits, a medical sleep study is required, which monitors brain waves to analyze the stages of sleep.
Sleep trackers that are worn on the wrist usually have built-in accelerometers and gyroscopes, which are small motion detectors that allow the device to track and monitor your movement. By analyzing your movement data throughout the day, sensors can estimate when you're active and inactive (asleep). This process is known as actigraphy. Some devices combine actigraphy with photoplethysmography (PPG) to get a more accurate 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 take certain measurements from these reflections, such as heart rate and heart rate variability.
Some sleep trackers also use a microphone to capture noise from the room or the user's body. They can measure respiration, detect snoring, sleep apnea, and how often the user wakes up during the night. Some trackers with thermometers can measure the temperature of the room, which may be a factor in sleep quality.
While sleep trackers can provide a lot of data about sleep habits, it's important to note that they may not always be completely accurate. They make estimates based on movement and, in some cases, heart rate data. However, as people move frequently during all stages of sleep, movement provides limited clues about the sleep stage. Additionally, different devices use different algorithms to make predictions about sleep, and these algorithms are often unknown, making it difficult to validate the assumptions made by the sleep devices.
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Frequently asked questions
Fitness trackers usually have in-built accelerometers and gyroscopes, which allow them to track and monitor body movement. Some trackers also monitor heart rate, blood oxygen levels, and skin temperature.
Deep sleep is harder to wake from, and the body moves less during this stage. Trackers can measure the length of time spent in deep sleep by monitoring the lack of body movement and the lower heart rate, blood oxygen levels, and skin temperature.
The accuracy of fitness trackers varies. In a 2019 study, Fitbit was shown to offer impressive accuracy given its entry-level cost, but it was still less accurate than Polysomnography (PSG), which is the gold standard of sleep tracking. Watches that incorporate heart rate data tend to be more accurate than those that only measure body movement.
Yes, 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 you cycle through during the night.
