Gina Johnson
Smartwatches have become increasingly popular for their ability to track health and activity data, including sleep patterns. Sleep tracking in smartwatches is done through different methods, including actigraphy, which is movement detection, or heart rate detection. Actigraphy uses a gyroscope or accelerometer to track the user's movements and detect the sleep cycle stage. Heart rate sensors monitor the user's heart rate, which lowers during sleep and varies depending on the sleep cycle stage. While these methods provide insights into sleep patterns, the accuracy of sleep tracking in smartwatches is estimated to be around 60-78%.
| Characteristics | Values |
|---|---|
| How do smartwatches track sleep? | Smartwatches use different methods to track sleep, including actigraphy (movement detection) and heart rate detection. |
| Actigraphy | Actigraphy is a non-invasive technique used to assess cycles of activity and rest by detecting movement through a gyroscope or accelerometer. |
| Heart rate detection | Smartwatches can monitor heart rate readings, which lower during sleep and vary across different sleep stages. |
| Photoplethysmography (PPG) | PPG involves using a light source and a photodetector to measure reflected light from the tissue. By analysing different wavelengths of light, certain measurements can be taken, such as blood volume, oxygen saturation, heart rate and heart rate variability. |
| Sleep quality | Smartwatches can determine sleep quality by measuring the time spent in each sleep cycle and the number of interruptions during sleep. |
| Sleep duration | Smartwatches can record the time of falling asleep and waking up to help determine the total sleep duration. |
| Sleep phases | Smartwatches can track sleep cycles, including REM and non-REM sleep phases. |
| Sleep environment | Some smartwatches can record environmental factors such as light and temperature in the bedroom. |
| Lifestyle factors | Smartwatches may prompt users to input lifestyle factors, such as caffeine intake, meal times, and stress levels, which can impact sleep. |
What You'll Learn
Heart rate and movement
Smartwatches use a combination of movement detection and heart rate detection to track sleep. This process is known as actigraphy, a non-invasive technique used to assess cycles of activity and rest. Actigraphy allows the smartwatch to detect your movement and track your sleep patterns. It uses a gyroscope or accelerometer to track your movements and detect which stage of the sleep cycle you are in.
During sleep, people move between cycles of "deep", "light", and "rapid eye movement" (REM) sleep. The "deep" portion of sleep is what makes people feel refreshed the next day. As people move frequently during all stages of sleep, movement provides few clues about what sleep stage they are in. Many sleep devices also fail to differentiate one stage of sleep from another based on motion alone.
Heart rate detection is another method used by smartwatches to monitor sleep. During sleep, the heart rate lowers and varies when a person reaches different stages of the sleep cycle. This data helps the watch track and monitor the sleep cycle closely. For example, the heart rate slows as a person moves into stage 2 (deep sleep) of the sleep cycle, and the body is paralysed during REM sleep. However, heart rate detection has its limitations, as it can be challenging to accurately differentiate between sleep stages based solely on heart rate data.
While smartwatches use these methods to estimate sleep patterns, they are not always accurate. In comparison to polysomnography tests, which are considered the gold standard for sleep tracking, smartwatches may provide less precise data. Polysomnography tests track brain waves, heart rate, breathing, blood oxygen levels, and body and eye movements during sleep, providing a more comprehensive analysis of sleep stages.
Despite their limitations, smartwatches can still provide valuable insights into sleep patterns. They can help individuals recognize trends and patterns in their sleep habits, such as optimal sleep times and the impact of various factors on sleep quality. However, it is essential to interpret the data from smartwatches with a degree of flexibility and not solely rely on them for sleep analysis.
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Actigraphy
Overall, actigraphy is a useful method for smartwatches to track sleep patterns by detecting wrist movements. While it has some limitations in distinguishing between sleep stages, it is widely used and accepted for sleep research and monitoring.
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Photoplethysmography (PPG)
Most PPG sensors use an infrared light-emitting diode (IR-LED) or a green LED as the main light source. IR-LEDs are used for measuring the flow of blood that is more deeply concentrated in certain parts of the body, such as muscles. On the other hand, green light is used for calculating the absorption of oxygen in oxyhemoglobin (oxygenated blood) and deoxyhemoglobin (blood without oxygen present).
To identify if your tracker uses PPG, you can look for a couple of small LEDs on the back of the device. While PPG provides more accurate data, sleep tracking overall is estimated to have an accuracy of about 60%.
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Sleep duration
Sleep tracking devices can record when you fall asleep and when you wake up in the morning by tracking your period of inactivity. They can also detect interrupted sleep, allowing you to know when you are tossing and turning or waking up during the night.
Sleep tracking devices use an algorithm to estimate how much time you spent asleep based on body movements. They also sometimes monitor heart rate changes during sleep to estimate how much time you spent in each sleep cycle.
Some devices combine actigraphy with photoplethysmography (PPG) to get a more accurate picture. PPG involves using a light source and a photodetector. The light source emits light to a tissue, such as the wrist, and the photodetector measures the reflected light from the tissue. As blood absorbs and reflects different wavelengths of light, the photodetector can take certain measurements from these reflections. It can measure blood volume and oxygen saturation and estimate heart rate and heart rate variability by analysing how much blood is passing through the illuminated spot per second.
However, sleep tracking devices are not very accurate. Compared to polysomnography tests, which are used to diagnose sleep disorders, sleep trackers are only accurate 78% of the time when identifying sleep versus wakefulness. This accuracy drops to around 38% when estimating how long it took participants to fall asleep.
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Sleep quality
Smartwatches use different methods to track sleep quality, including actigraphy, which is movement detection, and heart rate detection. Actigraphy uses a gyroscope or accelerometer to track your movements and detect which stage of the sleep cycle you are in. Heart rate sensors monitor your heart rate reading, which lowers while sleeping and varies depending on the sleep cycle stage.
Some smartwatches combine actigraphy with photoplethysmography (PPG), which uses a light source and a photodetector to emit light to the tissue and measure the reflected light. This allows the device to measure blood volume, oxygen saturation, heart rate, and heart rate variability. However, sleep tracking accuracy varies, and it is difficult to determine the exact sleep stage a person is in based on movement or heart rate alone.
While smartwatches can provide insights into sleep quality, they may not always be accurate. It is important to consider other factors that may impact sleep quality, such as environmental and lifestyle factors. Additionally, the comfort and fit of the smartwatch can also affect sleep quality, as constant notifications or a tight strap may cause disturbances.
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Frequently asked questions
Smartwatches use different methods to track sleep, including actigraphy, which is movement detection, and heart rate detection. They can also use photoplethysmography (PPG), which involves using a light source and a photodetector to measure blood volume, oxygen saturation, heart rate, and heart rate variability. These data points can be used to estimate what stage of sleep a person is in.
Sleep tracking with smartwatches is estimated to be around 60-78% accurate. While they can be useful for recognizing patterns in your sleep habits, they are not as accurate as medical sleep studies that monitor brain waves.
Smartwatches can be useful for helping you recognize patterns in your sleep habits and improving your sleep quality. However, they might not be accurate in detecting different sleep stages and can even overestimate your sleep duration. Additionally, the light or notifications from the watch might disturb your sleep.
