Colin Snow
Fitness trackers are devices usually worn around the wrist or waist that monitor specific body parameters like heart rate and motion. They can detect interrupted sleep, allowing you to know when you're moving around or waking up during the night. Most fitness trackers use accelerometers, small motion detectors that measure how much you're moving while you sleep. This data is then analysed using an algorithm to estimate sleep time and quality. Some trackers also use a microphone to capture noise from the room or your body, and some use a light source and a photodetector to measure blood volume and oxygen saturation.
| Characteristics | Values |
|---|---|
| Method | Accelerometers, gyroscopes, photoplethysmography, microphones, thermometers, and heart rate tracking |
| Data | Sleep quantity, sleep quality, sleep duration, sleep phases, sleep patterns, sleep staging, blood oxygen levels, atrial fibrillation, respiration, snoring, sleep apnea, temperature, light, caffeine intake, stress levels, etc. |
| Form Factor | Wristband, smartphone app, under-sheet device, ring, head-worn device |
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What You'll Learn
Accelerometers and gyroscopes
Fitness trackers use accelerometers and gyroscopes to monitor sleep. Accelerometers track and monitor movement by analysing movement data throughout the day, allowing the device to estimate when the wearer is active or inactive (asleep). This process is known as actigraphy, a non-invasive technique used to assess activity and rest cycles. Actigraphy is often used in sleep studies, with devices usually worn on the wrist to track movement while sleeping. Software then translates that movement into periods of sleep and wakefulness.
Actigraphy is a convenient way to study the sleep patterns of people with sleep disorders or disruptions without the need for a lab. However, it is worth noting that the accuracy of actigraphy is not perfect. The placement of the device, the sleep variations of the wearer, and the quality of the sensors can impact the reliability of the data.
Accelerometers can be used in conjunction with other sensors to improve accuracy. For example, the Oura Ring 4 uses a 3D accelerometer for movement tracking alongside infrared photoplethysmography sensors for heart rate and respiration, and a negative temperature coefficient (NTC) sensor for body temperature. The Apple Watch Series 4 and newer models also use accelerometers with heart rate sensors to break sleep down into four stages: awake, REM, light, and deep sleep.
Gyroscopes are another type of sensor used in fitness trackers to detect sleep. Gyroscopes measure angular velocity and orientation, which can be used to track movement and sleep patterns. The Google Pixel Watch 2, for example, uses a gyroscope alongside an accelerometer, altimeter, compass, multi-path optical heart rate sensor, and other features to track sleep stages.
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Heart rate and respiration
Heart rate fluctuates during different sleep stages, with a notable decrease during the second stage of light sleep and a further drop during the third stage of deep sleep. In the final REM stage, the heart rate increases, and breathing becomes more irregular. By analyzing these fluctuations, fitness trackers can estimate the duration and quality of sleep, including the amount of time spent in each sleep stage.
Photoplethysmography (PPG) is a technique used by some trackers to measure heart rate and heart rate variability. PPG involves emitting light onto the skin and measuring the reflected light using a photodetector. As blood absorbs and reflects different wavelengths of light, this method can determine blood volume and oxygen saturation, providing insights into heart rate and sleep patterns.
Respiration rate is considered a critical parameter in sleep tracking as it clearly indicates sleep disorders such as snoring and sleep apnea. Some trackers use microphones to capture noise, including respiration and snoring, to detect interrupted sleep and sleep disorders. Additionally, tracking systems can monitor the phases of sleep and time alarms to coincide with periods of lighter sleep, making it easier for individuals to wake up.
While fitness trackers provide valuable insights into sleep patterns, it is important to note that they do not directly measure sleep. Instead, they estimate sleep patterns by measuring inactivity and body movements. For more precise data, individuals can undergo a medical sleep study, which involves monitoring brain waves and other physiological parameters to comprehensively analyze sleep stages and identify potential sleep disorders.
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Environmental factors
These environmental factors are essential to consider when optimising your sleep. For instance, dimming the lights a few hours before bed and creating a cool bedroom environment can improve your sleep schedule. Similarly, limiting screen time can help you fall asleep more easily.
Sleep trackers can also help you identify changes in trends by recognising patterns in your sleep habits. For example, you may discover that your sleep is disrupted when the bedroom temperature is higher or that you sleep better on days you exercise. By tracking these environmental factors, you can make adjustments to improve your sleep quality.
Additionally, some sleep trackers allow you to manually input lifestyle factors that can influence your sleep, such as caffeine consumption, stress levels, and exercise habits. By considering both environmental and lifestyle factors, you can gain a comprehensive understanding of your sleep patterns and make informed decisions to optimise your sleep routine.
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Sleep quality
Some fitness trackers also monitor heart rate and heart rate variability, which can be used to estimate the amount of time spent in each sleep cycle. This is because heart rate tends to slow during certain sleep stages. Additionally, some trackers use a microphone to capture noise from the room or your body, which can help determine if you're moving frequently and not sleeping well. These microphones can also measure respiration, detecting issues like snoring and sleep apnea.
It's worth mentioning that while fitness trackers can provide insights into your sleep quality, they don't directly measure sleep. Instead, they often estimate sleep by measuring inactivity. The accuracy of these devices can vary, and they may not differentiate between sleep stages accurately, especially in individuals with insomnia who remain very still while trying to fall asleep.
If you're concerned about your sleep quality, it's recommended to consult a health practitioner. While fitness trackers can provide useful insights, they should be used as a tool to help you reflect on and understand your sleep patterns, rather than becoming a source of anxiety or obsession.
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Sleep phases
Sleep is a complex and variable process that is still not fully understood by experts. However, it is known that a good night's sleep is essential for the body and brain to function properly. Sleep can be divided into two main phases: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, from N1 to N3.
N1 sleep is the initial sleep stage, where the body begins to relax and muscle twitches may occur. This stage usually lasts up to five minutes. As you progress to N2 sleep, your heart rate drops, and your breathing slows down. The final stage, N3, is also known as slow-wave sleep (SWS) and is the deepest stage of sleep. It is characterised by lower-frequency, higher-amplitude brain waves known as delta waves. This stage is the most difficult to wake up from, and people awakened during N3 tend to experience a transient phase of mental fogginess, or sleep inertia, for up to an hour after waking. N3 is when the body repairs and regenerates tissues, builds bone and muscle, and strengthens the immune system.
After progressing through the three stages of NREM sleep, the body enters REM sleep. During REM sleep, the body experiences atonia, or temporary paralysis, while the eyes move rapidly behind closed eyelids. The brain is highly active during this stage, almost as much as when awake. The duration of REM sleep increases with each sleep cycle, and it can last for around an hour.
The duration of each sleep stage can vary, and a person typically goes through four to six sleep cycles per night. Sleep quality and the time spent in each sleep stage can be affected by various factors, including depression, aging, traumatic brain injuries, medications, and circadian rhythm disorders.
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Frequently asked questions
Fitness trackers use accelerometers and gyroscopes to track and monitor body movement and sometimes heart rate data. This data is then analysed using an algorithm to estimate sleep time and quality.
Fitness trackers are usually worn around the wrist or waist but can also be smartphone apps or tech products that can be slipped under your sheet.
The accuracy of fitness trackers varies. Compared to polysomnography tests, which are used to diagnose sleep disorders, fitness trackers are only accurate 78% of the time when identifying sleep versus wakefulness. This accuracy drops to around 38% when estimating how long it takes for a person to fall asleep.
