Smartwatch Sleep Tracking: How Does It Work?

Sleep tracking devices have become increasingly popular, with many people turning to wearable technology such as smartwatches to gain insights into their sleep patterns. While some people find it uncomfortable to wear a smartwatch to bed, others have found these devices useful for tracking sleep duration, quality, and phases. So, how do smartwatches track sleep?

Heart rate and respiration

Smartwatches use a variety of methods to track sleep, including heart rate and respiration. Heart rate is a key indicator of sleep quality, with a decrease in heart rate indicating that the wearer has entered a deeper sleep stage. The smartwatch uses an optical heart rate sensor to detect blood volume pulse and heart rate variability, which can be used to estimate sleep time and quality. This data is then used to provide the user with a sleep report and recommendations to improve sleep quality.

Respiration rate, or breathing rate, is another important indicator of sleep quality. It is considered the most important parameter of physiological data as it clearly indicates sleep disorders such as snoring and sleep apnea. Some smartwatches use a microphone to capture noise from the room or the wearer's body, which can be used to measure respiration rate and detect snoring or sleep apnea.

In addition to heart rate and respiration rate, smartwatches may also use actigraphy, or movement detection, to track sleep. This method uses an accelerometer and gyroscope to detect wrist movement and orientation, which can be used to record different sleep stages. However, it is important to note that tracking devices may not provide exact data about sleep habits, and a medical sleep study that monitors brain waves may be necessary for a more accurate analysis.

Overall, smartwatches can provide valuable insights into sleep patterns and quality by tracking heart rate, respiration rate, and movement. This information can help users reflect on their sleep habits and make any necessary changes to improve their sleep hygiene and overall health.

Accelerometers and gyroscopes

The Apple Watch, for example, contains a triaxial gyroscope, which allows the force of gravity to be separated from the raw accelerometer measurement via knowledge of device orientation at each sample. The triaxial, gravity-corrected time series is then converted into a time series of vector magnitude. The Oura Ring, on the other hand, uses motion seconds, motions low, and motions high to quantify acceleration and provide sleep insights.

Gyroscopes are sensors that detect the orientation and rotation of the wrist, providing additional data to differentiate between sleep stages. The data from gyroscopes, in conjunction with accelerometers, can be used to determine the atmospheric pressure, which is then displayed on the smartwatch screen. The gyroscope sensor also helps conserve battery life by turning off the display when no movement is detected.

Pulse and oxygen saturation

Smartwatches use a combination of methods to track sleep, including actigraphy, which is movement detection, and heart rate detection. The accelerometer and gyroscope sensors in a smartwatch are used to detect wrist movement and orientation, and to record different stages of sleep. This is because the heart rate lowers while sleeping and varies when the different stages of the sleep cycle are reached.

Oxygen saturation is measured in a similar way to pulse, but with an oximeter, and is less accurate. It is supposed to be an indicator, but it is not as accurate as it should be. This method can be used to indicate sleep apnea, and a doctor may use the data to order a polysomnography to get an accurate reading.

Smartwatches can record and store sleep data through an accompanying app. Users can download the app and follow the instructions for setup and operation. In the app, users can view sleep charts, sleep time, sleep stages, and heart rate changes. Some apps also provide suggestions and guidance on how to improve sleep quality.

Sleep duration and quality

Sleep trackers can be useful for helping you recognise patterns in your sleep habits. They can shine a light on your sleep duration and quality.

Sleep trackers can detect interrupted sleep, letting you know when you’re tossing and turning or waking during the night. They do this by measuring your wrist movement and orientation. Some trackers also measure your heart rate, respiration rate, and oxygen saturation to determine your sleep quality. These measurements can indicate sleep disorders such as snoring and sleep apnea. However, it's important to note that sleep trackers don't directly measure sleep. Instead, they often measure inactivity as a surrogate for estimating sleep.

Sleep trackers can also provide suggestions and guidance on how to improve your sleep quality. For example, they might suggest that you avoid caffeine after lunchtime if it disrupts your sleep. They might also recommend that you sleep in a cooler room or exercise during the day if those factors improve your sleep.

It's important to keep in mind that wearing a sleep tracker may impact your sleep quality. Some people find the light, strap, or notifications from the tracker disturbing. Additionally, some people are concerned about the radiation emitted by smartwatches, although the levels are minimal. If you want to avoid potential disturbances, you can place your tracker on your bedside table or clip it to your pillow instead of wearing it on your wrist.

Apps and algorithms

Smartwatches use a combination of hardware and software to track sleep. The hardware includes sensors such as accelerometers, gyroscopes, microphones, and heart rate monitors. The software includes the accompanying app and its algorithm.

The data collected by the sensors is then analysed by an algorithm to estimate sleep time and quality. The algorithm uses this data to estimate sleep duration, sleep quality, and sleep phases. Sleep duration is estimated by tracking the time of inactivity, while sleep quality is determined by detecting interruptions in sleep, such as tossing and turning or waking up. Sleep phases are estimated by tracking the different stages of the sleep cycle, such as light sleep, deep sleep, and REM sleep.

The accompanying app plays a crucial role in sleep tracking. Users need to download the app that is compatible with their smartwatch brand and model. The app provides a user interface to view and interpret sleep data. It typically displays sleep charts, sleep time, sleep stages, and heart rate changes. Some apps also offer suggestions and guidance on improving sleep quality. These recommendations are based on the analysis of sleep data and the identification of patterns.

While the algorithms and apps can provide valuable insights into sleep patterns and quality, they may not always be 100% accurate. It is important to take the data with a grain of salt and consult a health practitioner if there are concerns about sleep quality. Additionally, it is worth noting that tracking sleep does not necessarily lead to improved sleep. Changing habits and addressing the underlying issues based on the insights from sleep tracking is key to enhancing sleep quality.

Frequently asked questions