Herbie Murray
Sleep-tracking watches are a popular way to monitor sleep patterns and quality. These devices work by monitoring body movements and, in some cases, heart rate changes to determine how much time the wearer spends awake versus asleep. However, their accuracy has been called into question, especially when differentiating between sleep stages. While they may provide insights into sleep habits, they are not as precise as medical sleep studies that analyse brain waves, heart rate, breathing, blood oxygen levels and body movements.
| Characteristics | Values |
|---|---|
| How do watches track sleep | Watches use an algorithm to estimate sleep based on body movements and sometimes heart rate data. |
| How do they track REM sleep | REM sleep is harder to track than light or deep sleep. Some watches use light refraction to measure oxygen levels in the blood, which can indicate REM sleep. |
| Accuracy | Sleep trackers are only accurate 78% of the time when identifying sleep vs. wakefulness. Accuracy drops to 38% when estimating how long it took to fall asleep. |
| Limitations | Watches cannot differentiate between sleep and wakefulness in people with insomnia. |
What You'll Learn
Heart rate and movement
Sleep trackers, such as watches, monitor your heart rate and movement to estimate when you are asleep and the quality of your sleep. They do this by tracking your heart rate and movement to determine when you are inactive and thus asleep. The idea is that as you move through the stages of sleep, your heart rate slows and your body is paralysed during REM sleep. Therefore, the tracker can estimate the stage of sleep you are in.
However, it is important to note that these devices are not always accurate. They often confuse lying down with sleeping, and a stable heart rate and lack of movement with light sleep. This can lead to an overestimation of the amount of sleep an individual is getting. For example, one user reported that their device overestimated their period of sleep by more than 2 hours on several occasions.
In addition, sleep trackers only measure movement and heart rate, which can be similar during sleep and wakefulness. This can make it difficult for the device to differentiate between the two states accurately. One study found that sleep trackers were only accurate 78% of the time when identifying sleep versus wakefulness, and this accuracy dropped to around 38% when estimating how long it took participants to fall asleep.
Furthermore, sleep trackers cannot differentiate between the different stages of sleep based on motion alone. While heart rate data can help to improve the accuracy of sleep duration estimates, the accuracy of these measurements is still uncertain due to limited research and variations between devices. For example, one study showed that two consumer devices underestimated the amount of deep sleep by up to 46 minutes.
Overall, while sleep trackers can provide valuable insights into your sleep patterns, it is important to interpret their data with caution and consult a healthcare professional if you have concerns about your sleep quality.
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Blood oxygen levels
The Apple Watch Series 6 was the first smartwatch to introduce a blood oxygen monitoring app. Since then, blood oxygen monitoring has become a staple in smartwatch technology, with the likes of Samsung, Fitbit, Garmin, and Withings following suit.
The Blood Oxygen app on the Apple Watch allows users to measure the oxygen level of their blood on-demand directly from their wrist, providing insights into their overall wellness. However, it's important to note that these measurements are not intended for medical use and are only designed for general fitness and wellness purposes.
Other sleep trackers, such as the Oura Ring 4, WHOOP 4.0, and Sleepon Go2Sleep 3 Tracker, also offer blood oxygen monitoring as one of their features. These devices can help identify trends in your sleep, monitor your activity and heart rate, and provide insights into your overall health and wellness.
While blood oxygen monitoring in smartwatches has its limitations, medical experts recognize its potential as a useful tool for tracking another vital sign.
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Light sleep vs deep sleep
Sleep is divided into two types: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages: N1, N2, and N3, with N3 being the deepest stage of sleep.
Light sleep occurs during the N1 and N2 stages of NREM sleep. During N1, a person is still hearing things and has a sense of awareness, and their skeletal muscle tone and breathing are the same as when they are awake. N1 usually lasts for about 5 to 10 minutes. During N2, a person is in light sleep but can be easily awakened. Breathing and heart rate typically decrease slightly during this stage, and the length of this stage increases with each cycle. N2 may last for 10 to 25 minutes during the first cycle.
Deep sleep occurs during the N3 stage of NREM sleep. During this stage, a person becomes less responsive to outside stimuli, and their breathing slows and muscles relax. Heart rate usually becomes more regular during deep sleep. It is harder to wake someone during this stage, and they may feel groggy for 30 to 60 minutes if awakened. N3 makes up about 25% of total sleep time in adults and typically lasts for 105 minutes in total, shared between cycles.
REM sleep is associated with dreaming and occurs when a person's eyes move rapidly in different directions. During this stage, the heart rate increases, and breathing becomes more irregular. The body's skeletal muscles are paralysed during REM sleep. This stage usually follows the other stages, starting around 90 minutes after falling asleep.
While sleep trackers can be useful for monitoring sleep patterns, they may not always be accurate. Polysomnography tests, which track brain waves, heart rate, breathing, blood oxygen levels, and body and eye movements, are considered the most accurate method for diagnosing sleep disorders.
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REM sleep vs non-REM sleep
Sleep is divided into two types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is further divided into four stages, representing a continuum of depth. Each stage has unique characteristics, including brain wave patterns, eye movements, and muscle tone.
NREM Sleep
During NREM sleep, the body "powers down", and most body systems, including the brain, become less active. This stage includes four phases, each with distinct brain activity and physiology. The first stage, NREM 1, is a transitional phase between wakefulness and sleep, lasting 1 to 7 minutes. NREM 2 is a deeper sleep stage, characterised by slow and mixed-frequency brain waves, lasting 10 to 25 minutes. NREM 3 and NREM 4 are collectively referred to as slow-wave sleep (SWS), with NREM 3 being the shortest stage. NREM sleep constitutes about 75 to 80 percent of total sleep time.
REM Sleep
REM sleep, on the other hand, is associated with dreaming and irregular muscle movements, including rapid eye movements. During this stage, the brain activity resembles that of an awake person, but the body is temporarily paralysed. REM sleep usually begins about 90 minutes after falling asleep, with each cycle increasing in duration throughout the night. It constitutes 20 to 25 percent of total sleep time.
Sleep-tracking watches use algorithms that estimate sleep patterns based on body movements and, in some cases, heart rate changes. They infer the sleep stage by detecting periods of low heart rate and minimal movement, followed or preceded by actual sleep. However, these devices are not always accurate, especially in differentiating sleep stages, as movement occurs during all sleep stages.
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Sleep quality
Sleep trackers can also prompt you to enter information about lifestyle factors that can affect sleep quality, such as caffeine intake, when you have eaten, or your stress levels. This can help you to identify patterns and make changes to improve your sleep. For example, you might find that you sleep better on days when you have exercised, or when your bedroom is cooler.
However, it is important to note that sleep trackers do not directly measure sleep. They often measure inactivity as a way of estimating sleep, and make guesstimates about how much you are sleeping. They are not as accurate as a medical sleep study, which monitors brain waves, and they may not be suitable for people with insomnia or other sleep disorders.
While sleep trackers can be a useful tool for recognising patterns in your sleep habits, it is always a good idea to talk to a health practitioner if you have concerns about your sleep quality.
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Frequently asked questions
Watches use an algorithm to estimate how much time you spent asleep based on body movements and sometimes heart rate data.
REM sleep is hard to track without measuring brain waves. However, people whose heartbeats fluctuate with their breathing are easier to monitor, as in REM sleep, breathing fluctuates more than in non-REM sleep. Watches with heart rate sensors can also measure blood oxygen saturation, which can help spot REM sleep.
Sleep-tracking watches 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.
The most accurate way to track sleep is with a polysomnography test, which tracks brain waves, heart rate, breathing, blood oxygen levels, and body and eye movements during sleep.
Sleep-tracking watches could cause anxiety for people who experience poor sleep. A study showed that participants who were told by their watches that they had slept poorly exhibited lower mood, difficulties with daytime thinking processes and increased sleepiness.
