The Science Behind Wearables And Sleep Tracking

Sleep trackers are devices that monitor your sleep patterns and habits. They come in several forms, including wearable devices such as smartwatches, bracelets, rings, and headbands, as well as non-wearable sensors that can be placed under your mattress or sheet. While these devices provide insights into sleep quantity and quality, they do not directly measure sleep. Instead, they estimate sleep by tracking body movements, heart rate, and other physiological data. Wearable sleep trackers are convenient tools for those curious about their sleep habits and seeking ways to improve their sleep hygiene. However, it's important to note that excessive use of such devices may lead to increased anxiety and depression, especially for individuals with poor sleep or mental health conditions.

Wearables use an accelerometer to sense motion

However, this method of tracking sleep has its limitations. For example, people with insomnia may remain very still while trying to fall asleep, and so sleep trackers may fail to differentiate sleep from wakefulness in these cases. As such, sleep trackers that incorporate heart rate data tend to be more accurate when measuring sleep duration, as heart rate fluctuates during different sleep stages.

Sleep trackers can be worn as bracelets, rings, or wristbands. They can also be non-wearable sensors that sit on your bedside table or under your mattress. The Oura Ring 4, for example, is a lightweight, stylish wearable sleep tracker that monitors an array of metrics to evaluate your sleep and daytime activities. It measures your heart rate, temperature trends, and blood oxygen levels throughout the day and night. The WHOOP 4.0 is another example of a wearable sleep tracker, designed to be worn as a wristband.

While sleep trackers can collect a lot of information about your sleep habits, they don't measure sleep directly. Instead, they often measure inactivity as a surrogate for estimating sleep. For exact data about your sleep habits, you would need to undergo a medical sleep study, which monitors brain waves to analyse the stages of sleep you cycle through during the night.

They can also measure heart rate and respiration rates

Wearables can also measure heart rate and respiration rates to track sleep. While wearables don't directly measure sleep, heart rate is a good indicator of sleep quality as it fluctuates during different sleep stages. Respiration is also a good indicator of sleep quality and is considered the most important parameter of physiological data as it clearly indicates sleep disorders such as snoring and sleep apnea.

Some wearables, like the Oura Ring 4, measure heart rate, temperature trends, and blood oxygen levels. The Withings Sleep Device, which is a non-wearable that sits under your mattress, also measures heart rate, movement, breathing, and snoring.

It's important to note that some experts are uncertain about the accuracy of wearables that track heart rate due to limited research and differences between devices. For example, one study showed that two consumer devices underestimated the amount of deep sleep by 46 minutes.

While wearables that track heart rate may be slightly more accurate, they are not perfect. If you want to accurately track the stages of sleep, you would need to have EEG leads placed on your head in a sleep lab.

Wearables can't track light or deep sleep well

Wearable sleep trackers are devices that monitor sleep patterns and quality. They are usually in the form of a ring, wristband, or smartwatch, and they can collect a lot of information about sleep habits. However, it is important to note that these devices do not directly measure sleep. Instead, they rely on metrics such as body movement and heart rate to estimate sleep patterns. While these devices can provide valuable insights, they may not always accurately differentiate between light and deep sleep stages.

Sleep is a complex process that involves different stages, including light sleep, deep sleep, and rapid eye movement (REM) sleep. Each stage has a unique purpose and contributes to overall rest and recovery. Light sleep, also known as stages 1 and 2, is characterized by slow brain waves and easy awakenings. Deep sleep, or stages 3 and 4, is a harder state to wake from, and it is during this time that the body repairs itself and boosts immune function. REM sleep is when brain activity increases, dreams occur, and memories are formed.

Wearable sleep trackers typically use actigraphy, which measures movement, to estimate sleep patterns. However, this method has limitations in differentiating between sleep stages. For example, a person can be still while awake, leading the device to assume they are asleep. Additionally, movement during sleep occurs in all stages, providing limited clues about the specific sleep stage an individual is in. As a result, wearables that solely rely on movement data may not accurately distinguish between light and deep sleep.

Some wearable trackers have incorporated heart rate monitoring to enhance sleep tracking accuracy. Heart rate fluctuates during different sleep stages, providing additional data for estimation. However, even with heart rate data, experts remain uncertain about the accuracy of these devices in differentiating sleep stages. One study found that consumer devices underestimated deep sleep duration by up to 46 minutes.

To accurately determine sleep stages, polysomnography tests are considered the gold standard. These tests involve monitoring brain waves, heart rate, breathing, blood oxygen levels, and body movements through electrodes attached to the skin and scalp. While polysomnography provides definitive data, it is more invasive and typically requires medical supervision, unlike wearable devices designed for at-home use.

They can cause sleep-related anxiety

Sleep tracking wearables are becoming increasingly popular, with more people wearing smartwatches and fitness trackers than ever before. These devices can collect a lot of information about your sleep habits, but they don't directly measure sleep. Instead, they often measure inactivity as a way to estimate sleep. While this data can be useful in understanding your sleep patterns, a growing body of evidence finds that an obsession with sleep data can lead to insomnia and sleep-related anxiety.

This phenomenon has been termed "orthosomnia" or "orthoinsomnia", describing the obsessive pursuit of optimal sleep metrics. People with orthosomnia may experience symptoms similar to traditional insomnia, including difficulty falling asleep, frequent night wakings, early morning wakings, daytime tiredness or drowsiness, anger, despair, and anxiety. They may also have trouble with focus and memory and are more prone to making mistakes or having accidents.

The use of sleep tracking wearables can cause sleep-related anxiety in several ways. Firstly, individuals may overestimate the accuracy and reliability of their sleep tracker data, which can lead to a misunderstanding of their sleep quality. This can result in anxiety about not getting enough sleep or achieving the "perfect" sleep. Secondly, the effort to achieve optimal sleep metrics as dictated by the tracker can induce anxiety and arousal, compromising rather than improving sleep.

Additionally, relying solely on sleep tracking wearables may delay individuals from seeking appropriate professional help for sleep issues. They may become so focused on the data provided by their tracker that they disregard the need for formal sleep studies or medical advice. This can be problematic, especially if the data is inaccurate or incomplete, as consumer sleep tracking technology is not as advanced as the devices used by researchers and medical professionals.

Wearables can be useful for recognising patterns in your sleep habits

Wearables can also be used to track other lifestyle factors that may impact your sleep, such as caffeine intake, diet, and stress levels. By recognising patterns, you can make informed decisions about your sleep habits. For example, you may find that you feel more energetic when you sleep from 11 pm to 7 am, or that your bedroom temperature affects your sleep quality. This information can guide you in optimising your sleep schedule and environment to improve your sleep.

Additionally, wearables can be especially useful for athletes, those with irregular work schedules, or women approaching menopause. For athletes, a good night's sleep can enhance performance, while those with irregular work schedules may benefit from monitoring their sleep quantity and quality. As for women approaching menopause, tracking sleep can help identify opportunities to improve mood, memory, and concentration through better sleep.

However, it is important to note that the accuracy of wearables may vary, and they are not a substitute for medical advice or treatment. Some studies have also suggested that sleep trackers could cause unwanted anxiety, especially for those with poor sleep or mental health conditions. Nevertheless, wearables can still provide valuable insights to help you recognise patterns and make adjustments to your sleep habits.

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