Fitness Bands: Tracking Sleep And Your Health

Sleep tracking is a common feature of fitness bands, and they can provide a wealth of health data, from blood oxygen levels to heart rate and sleep quality. The technology behind sleep tracking is complex, but most fitness bands use accelerometers, small motion detectors, to measure sleep quantity and quality. They track your movement to estimate when you are active and when you are inactive (asleep). Some fitness bands also use a microphone to capture noise from the room or your body, and some can even measure the temperature of your room. While fitness bands can provide a lot of data about your sleep, it's important to remember that tracking your sleep doesn't automatically lead to better sleep—changing habits based on what you learn is key.

Accelerometers and gyroscopes

Fitness trackers usually have built-in accelerometers and gyroscopes, which enable them to monitor and track body movement. Accelerometers help track body movement by calculating daily steps and monitoring sleep patterns. They can also detect skin resistance to a specified current transmitted over the skin to monitor parameters like heartbeat and sleeping patterns. The gyroscope notifies the user when they move their wrist to look at their smartwatch and helps save battery life by turning off the display when there is no movement.

The data collected by accelerometers and gyroscopes can be used to infer sleep patterns and provide valuable information about sleep quality. For example, by analysing movement data throughout the day, sensors can estimate when a person is active and inactive (asleep). This process is known as actigraphy. However, it is important to note that the accuracy of these devices can be affected by factors such as the placement of the device and the quality of the sensors.

In addition to accelerometers and gyroscopes, fitness trackers may also use heart rate sensors and photoplethysmography (PPG) to track sleep. PPG involves using a light source and a photodetector to emit light onto the skin and measure the reflected light. This information can be used to calculate heart rate and track sleep patterns.

Some examples of fitness trackers that utilise accelerometers and gyroscopes include the Apple Watch Series 4 and later, the Muse S Gen 2 Headband, and the Biostrap Kairos. These devices can break down sleep into different stages, such as awake, REM, light, and deep sleep, and provide insights into sleep quality.

Microphones

Some fitness bands use microphones to track sleep. These microphones, similar to the ones on smartphones, are placed near the bed to capture noise from the room or the user's body. They can measure respiration and detect snoring, sleep apnea, and how often the user wakes up during the night. Additionally, some sleep trackers with thermometers can measure room temperature, indicating if the user wakes up frequently due to a warm environment.

The microphones in fitness bands are part of a broader set of tools used to track sleep. Accelerometers, for example, are small motion detectors that measure how much a person moves while sleeping. This data is then analysed using an algorithm to estimate sleep time and quality. However, accelerometers alone cannot accurately measure sleep stages since there is little difference in movement between them. Some fitness bands also estimate REM sleep and deep and light sleep stages by measuring heart rate and respiration rates, which vary significantly during sleep.

The Oura Ring, for instance, is a smart ring that tracks steps, heart rate, activity, and caloric burn. While it is not primarily a fitness tracker, it can sometimes recognise activities such as boxing or climbing stairs. The Whoop 4.0 tracker is another device that offers sleep tracking. A 2020 study at the University of Arizona found that this tracker predicted sleep duration within a precision of 17.8 minutes and accurately detected REM and Slow Wave (deep) sleep.

While fitness bands can provide insights into sleep patterns, they do not inherently improve sleep quality. Users must actively make changes to their habits based on the data to see improvements. For example, a user might notice that they wake up frequently when the room temperature is too warm and make adjustments accordingly.

Heart rate and respiration

Fitness trackers can monitor your sleep by tracking your heart rate and blood oxygen levels. The Oura Ring, for example, is a smart ring that tracks steps, heart rate, activity, and caloric burn. It also provides in-depth data on sleep stages and sleep quality. The Amazfit Bip 3 Pro is another fitness tracker that is accurate in tracking heart rate, activity, and sleep.

Some sleep trackers use a microphone to capture noise from the room or your body. They can measure your respiration, detect snoring, and identify sleep apnea and how often you wake up during the night.

Sleep trackers that use accelerometers to measure sleep quantity and quality may not accurately measure sleep stages because there is little difference in movement between the sleep stages. However, some trackers can estimate REM sleep in addition to deep and light sleep stages by measuring heart rate.

The Fitbit, one of the most popular wearable brands, offers impressive accuracy in sleep tracking, although it still falls short of Polysomnography (PSG), the gold standard of sleep tracking. The Whoop 4.0 tracker also showed promising results in a 2020 study at the University of Arizona, accurately predicting sleep duration and detecting REM and Slow Wave (deep) sleep.

Temperature

The Eight Sleep Pod 4 Cover and Hub is another innovative solution that integrates with your mattress. It regulates your sleep environment by pumping temperature-controlled water through the mattress cover.

In addition to tracking body temperature, sleep trackers can also monitor your heart rate, respiratory rate, blood oxygen levels, and movement. These metrics are used to provide tailored insights into your sleep quality and help you optimize your sleep habits.

It is important to note that sleep trackers are not medical devices, and their accuracy may vary. However, they can provide valuable information about your sleep patterns and overall health.

Sleep stages

Sleep is divided into several stages, which can be broadly categorized into two types: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, ranging from N1 to N3.

N1 sleep is the initial sleep stage, where the body starts to relax and may experience twitches. It usually lasts up to five or seven minutes, and the body and brain activities start to slow down with brief movements. During this stage, it is easy to wake someone up, but if uninterrupted, they can quickly move into the next stage.

N2 sleep is the second stage, where the heart rate drops and breathing slows down. This stage can last up to 25 minutes, and it is characterized by slower brain waves with noticeable pauses between short bursts of electrical activity. These bursts are believed to be the brain organizing memories and information from wakeful periods. N2 sleep accounts for about 45% of total sleep time, making it the stage where most time is spent.

N3 sleep, also known as slow-wave sleep (SWS), is the third and deepest stage of NREM sleep. This stage is marked by delta waves in the brain, and it is crucial for growth and recovery. SWS usually lasts up to 40 minutes in early sleep cycles and gradually shortens as more time is spent in REM sleep. N3 sleep is important for repairing injuries and strengthening the immune system, and it is harder to wake someone up during this stage.

REM sleep is the final stage of the sleep cycle, where the body enters a state of temporary paralysis while the eyes move rapidly behind closed eyelids. The brain is highly active during REM sleep, almost as much as when awake, and dreaming occurs.

The duration of each sleep stage varies and can be influenced by factors such as age, gender, recent sleep patterns, and alcohol consumption. On average, a person goes through four to six sleep cycles per night, with each cycle lasting about 90 to 120 minutes. The first cycle is typically the shortest, ranging from 70 to 100 minutes, while later cycles can be longer, ranging from 90 to 120 minutes.

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