How does a smartwatch measure rem sleep?

For the health-conscious, myriad apps and devices are dedicated to improving one’s lifestyle. Fitness apps are available for smartphones and wearables like fitness trackers, sleep trackers, smartwatches, and even specialized devices for people with chronic health issues. All these gadgets operate using methods that detect human activity, movement, or other biological activity.

Smartwatches and fitness trackers are equipped with accelerometers and gyroscopes, which can track the wearers’ movement and provide them with stats such as distance traveled, steps taken, sleep time, and sleep quality. Smartwatches measure the level of their users’ activity and inactivity during the day and later feed this data to an algorithm to gauge their sleep quality by calculating the time they spend in REM (Rapid Eye Movement) and NREM (Non-Rapid Eye Movement) stages in a sleep cycle.

While the data provided by these devices is not accurate, the technology behind them has significantly improved over the past few years. If you own a smartwatch or are planning to buy one, you might be curious about the proficiency and workings of one. Keep reading to know how these sleek and trendy devices accomplish such a feat!

How does a smartwatch measure rem sleep?

Smartwatches can measure sleep quality by calculating the number of minutes a person has spent in the two sleep stages: NREM (which is further divided into four stages: N1, N2, N3, and N4) and REM cycles. They do so by relying on actigraphy—a non-invasive method that assesses a person’s average motor activity over days to weeks using an accelerometer. An actigraph is worn around a person’s non-dominant hand and evaluates the wearer’s cycles of activity and rest through light and movement. Actigraphy is said to be an objective assessment of sleep, meaning it can provide sleep parameters that are more accurate than a person’s self-reported approximations. Smartwatches can thus compute a person’s total sleep time (TST), sleep onset latency (SOL), and wakefulness after sleep onset (WASO) and use these parameters to guesstimate the time taken up by REM and NREM cycles.

Actigraphs were initially used to diagnose ailments like sleep-disordered breathing and circadian rhythm disorders such as advanced or delayed sleep phase disorder and insomnia. Smartwatches use this technology to steer users towards healthier and better sleep cycles.

How accurate is my smartwatch?

Smartwatches have apps that require you to enter information, such as when you go to bed and wake up. Using these timestamps, your smartwatch will ascertain your sleep stages by comparing them to the average person’s sleep cycle data. Results from such data can be erroneous. To improve the precision of smartwatches, they sometimes come with embedded PPG (photoplethysmography) sensors.

Photoplethysmography is a non-invasive technology that utilizes the reflective properties of blood to measure variations in blood volume and oxygen saturation. It is an optical measuring method that uses a light source and a photodetector. Blood volume changes can be detected by shining a light on a skin patch because blood absorbs and reflects light at different wavelengths. These measurements can, in turn, be used to compute heart rate. A person’s heart rate can then be used to study a person’s sleep, as different stages of sleep have other effects on the body, such as slowed heartbeat, muscle atonia, and lowered oxygen levels. For instance, during the NREM stages, your heart rate slows down significantly; during the REM stages, your heart rate speeds up to match the state of wakefulness. This information is exploited by the device to distinguish your sleep stages. The addition of a PPG sensor thus raises the accuracy of tracking devices.

The accuracy of such devices has proven to be around 78%, which is good enough if you are not suffering from any sleep disorders and want to maintain a healthy circadian rhythm.

What else can a smartwatch measure?

Smartwatches can be modeled with various sensors that can be used to evaluate multiple parameters. We have already discussed the functions of the accelerometer (measures acceleration and movement) and gyroscope (measures angular velocity). A gyroscope will also help save your smartwatch’s battery life by turning off the display when there is no movement.

  • The newest models of smartwatches in the market can compute data such as body temperature, atmospheric pressure, and ambient temperature.
  • Some devices also include a skin conductance sensor which measures the skin’s electrical conductivity. Changes in the level of sweat can indicate increased stress or an injury.
  • A PPG or an oximetry sensor is used to measure a person’s oxygen saturation level or, indirectly, a person’s heart rate.
  • Most smartwatches allow you to make voice calls with a microphone.
  • A GPS is also a typical feature of a smartwatch.
  • Some other features that most are familiar with are that smartwatches can measure workout routes (with the help of a GPS), pace (using a pedometer), and floors climbed (with an altimeter).

This is only the tip of the iceberg, and smartwatches will only continue to add more tools with better accuracy for a satisfactory user experience.


Key Takeaways

Smartwatches and fitness trackers have allowed the common folk to become more aware of their general health, which includes their overall sleep quality. If left untreated, sleep disorders like sleep-disordered breathing can lead to potential cardiovascular, neurocognitive, and metabolic complications, which is why an early diagnosis is recommended. With smartwatches, a person can quickly notice abnormalities or deviations in their data and schedule a visit to a doctor or a sleep specialist for a polysomnography test.

A polysomnography or a clinical sleep study is a technique that measures brain activity and is used by experts to get a detailed record of a person’s sleep cycle. A polysomnograph yields results that are far superior to any tracker available to consumers can produce. Compared to this, a sleep tracker or a smartwatch can only estimate the time spent in each stage of a sleep cycle.

In conclusion, smartwatches are a semi-reliable way of measuring sleep parameters. Most smartwatches use an actigraph and a PPG sensor to evaluate sleep quality and can make an educated guess about the wearer’s REM and NREM stages. Technology, as it is now, has not come up with a way to precisely compute a person’s sleep parameters through a fitness wearable or a smartwatch. A polysomnography (clinical sleep study) test remains the gold standard for sleep studies and is used by experts worldwide.


Ryan Lenett
At his core, Ryan’s true passion is helping others achieve their own independent goals in life. His skill sets consist of Scientific research, Gadget Reviews and Technical testing. Year over year, Ryan has consistently amassed revenue streams that exceed seven figures in value.