a woman sleeping with a fitness tracker

How Do Fitness Trackers Know When You Are Asleep and Is the Data Correct?

Juhi

Written by health expert Juhi. Last updated:

Fitness trackers are devices that measure the number of steps you take and the calories you burn daily. Some of them claim to measure sleep, meaning they track your sleep duration and tell you whether you got a good night’s rest. How do fitness trackers track sleep? Is the sleep data they collect accurate?

Activity trackers have sensors that measure things like heart rate variability and body movement. Some trackers measure sleep time and tell you whether you have normal sleep patterns. The data they collect is fed into algorithms and sorted into useful information so that you can make informed decisions about your sleep and health. However, sleep trackers are not accurate enough to track certain metrics like sleep stages. Nonetheless, sleep tracking with fitness trackers does have benefits.

What are the Different Sleep Stages?

a man sleeping soundly in his room

Before we get into fitness trackers and sleep tracking, let’s first understand the various sleep stages. Adults with normal sleep patterns go through different stages of sleep throughout the night.

  • Stage 1 of the sleep cycle occurs as soon as you fall asleep. It lasts only a few minutes and you are easily roused during this stage.
  • Stage 2 consists of fairly light sleep. Your brain waves begin to slow down at this stage.
  • Stages 3 and 4 are characterized by deep sleep and slower brain activity. You will be difficult to wake up during these stages. This is when the body repairs and replenishes itself, and it boosts the immune system.
  • REM sleep is a stage characterized by rapid eye movement. During this final stage of sleep, brain waves become more active again. Your brain stores memories and information during REM sleep. This is also the stage when you dream.

The different sleep stages occur in cycles, repeating every 90 to 110 minutes. With every subsequent cycle, the duration of REM sleep increases.

Types of Devices Used to Track Sleep

There are two major types of fitness trackers that are used for sleep tracking. Wearable sleep trackers are typically worn on the wrist (like a watch) to assess sleep. Besides being a sleep monitor, they also function as an activity tracker. Top fitness trackers often double as a smartwatch.

Less commonly, wearable fitness trackers are worn on the head or clipped to your waistband. These devices contain sensors called accelerometers that detect movement and use it to decide whether you are asleep or awake. They use a process known as actigraphy for sleep tracking. The data collected by the device is translated into useable information like sleep time and other sleep parameters.

Non-wearable fitness trackers are devices that are not in contact with your body. They either go under your mattress, sit on your nightstand, or clip onto your pillow. Devices that go under your mattress use a process called ballistocardiography. This type of tracker works by detecting small movements that are generated by breathing and the pumping of your heart. A nightstand tracker measures things like breathing, movement, noise, and light to give you information about the quality of your sleep.

How Do Specialists Track Sleep?

a woman lying in her bed trying to sleep

To understand the advantages and disadvantages of personal sleep tracking devices, it’s useful to understand what happens in a specialist sleep center or professional sleep lab. If your doctor suspects you have a sleep disorder, they will refer you for a sleep study, which is also called a polysomnogram. This requires you to stay overnight in a sleep lab for one or two nights.

During a sleep study, the staff measure more than just sleep time. You are hooked up to different pieces of equipment, including brain electrodes that measure your brain waves, such as delta waves. This allows them to differentiates between various sleep stages, including deep sleep. Some of the measurements made during polysomnography include:

  • Electroencephalography (EEG) to measure brain activity.
  • Electrooculography (EOG) to measure eye movements.
  • Electrocardiography (ECG) to measure heart rate and blood pressure.

Other important metrics like breathing, body movements, and leg movements are also recorded. A pulse oximeter may be used to measure blood oxygen levels. The staff may observe you while you sleep and make a note of your sleeping positions throughout the night.

Some of this information, such as brain activity, eye movements, and heart parameters is critical to understanding sleep. These things cannot be measured with a personal fitness tracker. Polysomnography is, therefore, considered the gold standard in diagnosing sleep disorders. The information collected during polysomnography is carefully assessed and analyzed. Your doctor can then make a decision about what needs to be done, and whether you need treatment for conditions such as sleep apnea.

How Do Fitness Trackers Measure Sleep?

Fitness trackers can give information about several aspects of sleep, including sleep duration and sleep quality. Some fitness trackers can even tell you about your sleep environment – for example, your bedroom temperature (which should ideally be around 60 degrees Fahrenheit).

The most common way sleep trackers monitor sleep is through a process called actigraphy, which is based on movement. It involves recording the speed and direction of movement using a sensor called an axis accelerometer. The assumption is that the amount of movement corresponds to your being asleep or awake. Periods of inactivity are taken to indicate deep sleep. Periods of movements correspond to wakefulness. In general, when you haven’t moved for about an hour, the device assumes you are sleeping. Some devices also measure the amount of time a movement lasts to exclude behaviors like rolling over in your sleep.

Tracking devices can work in other ways besides actigraphy. Some of them capture the amount of light and noise (a dark room and the lack of noise are assumed to mean you’re asleep). Some devices can even record snoring, or gasping sounds that could indicate serious sleep disorders like sleep apnea. Heart rate tracking is another way for sleep trackers to decide whether you are asleep or awake. A wrist heart rate monitor can evaluate your heart rate in different sleep stages (the heart rate typically increases during REM sleep).

Commercial Sleep Trackers vs. Professional Sleep Studies

a woman sleeping wearing a fitness tracker

The key difference between a professional sleep evaluation (polysomnogram) and a commercial sleep tracker is that even the most accurate running watch cannot measure brain waves and eye movements. Without these important metrics, it is impossible to track the various sleep stages. Therefore, polysomnography is undoubtedly the most accurate method of measuring sleep parameters. What’s more, other body functions are also measured during a sleep study. This means the study can detect things like an irregular heartbeat, which may require further evaluation for heart disease.

Sleep tracker actigraphy is generally accurate in recording movements. However, there can be a high margin of error in using this movement-based information to make assumptions about sleep patterns. Nonetheless, actigraphy does have some benefits.

During a professional sleep study, for instance, the person is in a strange place (the sleep lab), hooked up to all kinds of equipment, including electrodes on their head. These environmental factors mean they are less likely to fall asleep and stay asleep normally. A sleep tracker, on the other hand, is a personal device that is worn at home and can be used for a prolonged period. It, therefore, gives a more accurate reading about a person’s typical sleep pattern in a familiar environment.

Limitations of Fitness Trackers in Measuring Sleep

Studies have found that sleep trackers perform well when a person has reasonably good sleep quality. However, in people with disrupted sleep patterns, there is a high margin of error. In other words, the more disrupted a person’s sleep, the less accurate the sleep data provided by the tracker is. This is especially true if it is a low cost fitness tracker. Nonetheless, personal sleep trackers are a convenient, affordable, and reasonably accurate way to collect data about sleep.

Fitness trackers are best at measuring total sleep time and the amount of time spent in bed. However, there are limitations in their ability to measure the number of times a person wakes up after falling asleep. For instance, if a person is awake and lying in bed without moving, the tracking device will record that as sleep time. Also, errors can creep in if a person forgets to put the device in sleep mode when going to bed, or take it out of sleep mode when getting out of bed.

The Last Word

You now know the answer to the question – how do fitness trackers monitor sleep? But you’re probably still wondering – is it worth buying a tracking device for sleep? It all boils down to a question of accuracy. Commercial activity trackers cannot match the accuracy of sleep tracking performed in a professional sleep lab. Even in sleep mode, they cannot tell you much about the different stages of sleep. Therefore, personal tracking devices should not be used as a substitute for a professional medical opinion and diagnosis of sleep disorders. However, fitness trackers can provide a lot of useful information about many sleep metrics in your natural sleep environment, thus allowing you to modify it and maximize your health.

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