How Accurate Are Sleep Trackers On Smart Watches And Smart Rings?

Quick Summary

• More Than Just Movement: Modern sleep trackers from brands like Fitbit, Apple Watch, and Oura use heart rate data in addition to motion. This makes them significantly better at telling the difference between sleep and wakefulness compared to older, movement-only devices.

• Sleep Stages Are an Guesstimate: While trackers provide data on light, deep, and REM sleep, this information is an educated guess. No consumer wearable can perfectly replicate the accuracy of a clinical sleep study, which measures brain waves directly.

• Not a Diagnostic Tool: These devices often struggle to accurately measure sleep in people with conditions like insomnia or sleep apnea. They tend to overestimate sleep time in these groups and should not be used to diagnose a sleep disorder.

• How the Tech Works: Most trackers use a technology called photoplethysmography (PPG), which involves shining a green light onto your skin to measure changes in blood flow. The accuracy of this technology can sometimes be affected by factors like darker skin tones or wrist tattoos.

Sleep Trackers & Sleep Measurement

Have you ever looked at your sleep tracker data on your smart watch or ring and wondered how your device knows the difference between deep sleep and REM? Is your device even accurate?

We get asked these questions about sleep and smart watches and devices all the time, by clients, workshop participants, and pretty much anyone that hears we work in the sleep field. So we’re going to break down the science here in a bit more detail.

Let's start with a bit of a background.

How Did We Track Sleep Before Smartwatches?

Before the rise of consumer smartwatches, researchers and doctors used medical-grade devices called actigraphs, which are essentially sophisticated movement trackers worn on the wrist. For decades, actigraphy has been a reliable way to objectively measure sleep patterns in a person's natural environment, avoiding the cost and inconvenience of a formal overnight sleep study in a lab, known as polysomnography (Lujan et al., 2021).

How Do Actigraphs Work?

Actigraphs use an internal sensor called an accelerometer, a tiny piece of hardware that detects motion. Algorithms then translate this movement data into estimates of sleep and wakefulness.

Actigraphs are great for getting general sleep/wake/movement data over time, especially helpful for understanding circadian rhythms. But they have traditionally had a key weakness.

What Are the Weaknesses of Using Actigraphs?

While useful for tracking general sleep patterns over weeks or months, traditional actigraphy has one persistent weakness. It struggles to distinguish between sleep and periods of "quiet wakefulness" (Hauri & Wisbey, 1992). If you are lying in bed awake but not moving (like someone with insomnia might do), the device is likely to incorrectly score that time as sleep.

In technical terms, these devices have high sensitivity, meaning they are very good at correctly identifying when you are asleep.

However, they have low specificity, which is the ability to correctly identify when you are awake.

This imbalance means that movement-only trackers almost always overestimate total sleep time.

To get a clearer picture, developers needed to add more data. So, what other signals from the body could a wrist-worn device measure?

Enter heart rate and HRV.

What Do Heart Rate and HRV Tell Us About Sleep?

Your heart rate and its variability (HRV) change in predictable patterns across different sleep stages, giving trackers important clues about whether you're awake or in light, deep, or REM sleep. Your body’s autonomic nervous system, the system that controls involuntary functions like breathing and heart rate, operates differently depending on your sleep stage, and your heart activity reflects these changes (Lujan et al., 2021).

So how exactly does heart rate change during sleep?

How Does Heart Rate Change During Sleep?

Your heart rate changes significantly as you move through the sleep cycle. Generally, your heart rate is highest when you are awake. As you fall asleep and progress from light sleep into deep sleep, your heart rate gradually slows down. It then tends to increase and become more variable during REM sleep, the stage associated with dreaming (Snyder et al., 1963). Importantly, your heart rate will also briefly spike when you have an awakening during the night. By tracking these patterns, a device can make a much more informed guess about your sleep stage and better detect disruptive awakenings.

Okay, so that is heart rate. What is heart rate variability and what does this have to do with sleep?

What is Heart Rate Variability (HRV)?

Heart Rate Variability, or HRV, is a measure of the variation in time between each of your heartbeats. Contrary to what you might think, a healthy heart doesn't beat with the perfect regularity of a metronome. A high HRV, meaning there is more variation between beats, is generally a sign of good health and resilience. It indicates a healthy balance in your autonomic nervous system, particularly the influence of the parasympathetic nervous system, your body's "rest-and-digest" system. A low HRV, on the other hand, can be a sign of stress, illness, or fatigue, reflecting a dominance of the sympathetic nervous system, your body's "fight-or-flight" system (Shaffer & Ginsberg, 2017).

How Does Heart Rate Variability Change During Sleep?

During the night, heart rate variability patterns also align with sleep stages. Parasympathetic activity and HRV tend to increase during deep sleep, while sympathetic activity increases and HRV decreases during REM sleep and periods of wakefulness (Busek et al., 2005). This provides another layer of data for sleep-tracking algorithms.

This extra layer of data is how your smart watch or smart ring differs from a traditional actigraph in measuring your sleep.

But how exactly does your device do this?

How Does a Smart Watch Measure my Heart Rate?

Most smartwatches and fitness trackers measure your heart rate using an optical technique called photoplethysmography, or PPG. This is the technology behind the flashing green lights on the underside of your device. The process is straightforward: LEDs shine light through your skin, and a sensor measures how much of that light is reflected back (Lujan et al., 2021).

When your heart beats, it pumps blood through the arteries in your wrist. Since blood is red, it absorbs green light. Therefore, when there is more blood flow during a heartbeat, more green light is absorbed, and less is reflected. Between beats, less blood is flowing, so less light is absorbed, and more is reflected. By flashing these LEDs hundreds of times per second, the device can detect these tiny changes in light reflection and calculate your heart rate and HRV.

This all sounds good in theory, but how well do the most popular devices actually perform when put to the test?

How Accurate Are the Top Consumer Sleep Trackers?

When compared to the gold standard of sleep measurement, a lab-based study called polysomnography (PSG), a comprehensive test that measures brain waves, eye movement, and muscle activity, major brands like Fitbit, Apple Watch, and Oura perform reasonably well. They are generally good at telling sleep from wake but vary in their ability to accurately identify specific sleep stages (Lujan et al., 2021).

How Accurate Is A Fitbit In Measuring Sleep?

Fitbit has been in the sleep tracking game for a long time, and its technology has improved considerably. Early models that only used accelerometers were found to have very poor specificity, meaning they were terrible at detecting wakefulness (Montgomery-Downs et al., 2012).

However, once Fitbit added PPG heart rate sensors, performance improved. A study on the Fitbit Charge 2 found it had 96% sensitivity (good at detecting sleep) and a much-improved 61% specificity (better at detecting wake) when compared to PSG (de Zambotti et al., 2018).

When it came to sleep stages, the device was most accurate at identifying "light" sleep but struggled more with deep and REM sleep. A more recent study on the Fitbit Alta HR found it was 95% sensitive and 54% specific, with accuracies for classifying light, deep, and REM sleep at 72%, 86%, and 89%, respectively (Chinoy et al., 2020).

How Accurate Is The Apple Watch At Measuring Sleep?

The Apple Watch contains high-quality accelerometer and PPG sensors, making it a capable sleep-tracking device. Although Apple only added a native sleep-staging app in 2020, studies using third-party apps or raw data have shown strong results.

One study that extracted the raw sensor data and applied a machine-learning model found the Apple Watch could achieve 93% sensitivity and 60% specificity, performing on par with or better than many medical-grade actigraphs (Roberts et al., 2020).

Another analysis found that combining motion, heart rate, and an estimate of the time of day based on your body's internal clock allowed the Apple Watch to classify sleep stages with about 65% accuracy for NREM and REM sleep (Walch et al., 2019).

How Accurate Is The Oura Ring At Measuring Sleep?

The Oura Ring is a popular tracker that packs its sensors into a small ring worn on the finger. This placement may offer an advantage, as some research suggests that the arteries in the finger can provide a cleaner PPG signal than the wrist (Longmore et al., 2019).

Validation studies have shown the Oura Ring performs okay. One study found it had an impressive 96% sensitivity but a lower specificity of 48%, meaning it was excellent at detecting sleep but still prone to misidentifying wakefulness as sleep (de Zambotti et al., 2019). In that study, its agreement with PSG for classifying sleep stages was 65% for light sleep, 51% for deep sleep, and 61% for REM sleep. Another analysis found similar results, confirming the device's high sensitivity but more moderate specificity (Roberts et al., 2020).

With all this new technology, are there limitations to be aware of?

What Are the Limitations of Consumer Sleep Trackers?

Consumer sleep trackers are not perfect and have several important limitations, especially for people with sleep disorders or certain physical characteristics. While they are powerful tools for monitoring trends, it's important to understand where they fall short.

Why do trackers struggle with insomnia?

The "quiet wakefulness" problem that affects traditional actigraphy still impacts modern trackers, particularly for people with insomnia. Insomnia is often characterized by a state of mental and physical hyperarousal, but this doesn't always involve tossing and turning. If you are lying awake but calm and still, even a multisensory tracker may incorrectly log that time as light sleep (Lujan et al., 2021). Studies have consistently shown that trackers overestimate sleep time and efficiency in individuals with insomnia compared to good sleepers (Kang et al., 2017).

How reliable is the sleep stage data from a smart watch or smart ring sleep tracker?

The sleep stage data from your tracker should be viewed as an estimate, not a fact. Sleep stages are technically defined by patterns of brain wave activity, which can only be measured with an electroencephalography (EEG), a test that records electrical activity in the brain. Since a watch or ring can't read your brain waves, it uses signals like heart rate and movement as substitutes.

While the algorithms are getting smarter, they are not a substitute for a real EEG. Research has shown that trackers are much more accurate when classifying sleep into three broad categories (Wake, NREM, and REM) than when trying to break it down into five stages (Wake, N1, N2, N3, REM). One analysis showed an accuracy of 78% for a three-stage model, which dropped to 65% for a five-stage model (Zhai et al., 2020).

Can my tracker detect sleep apnea?

You should not rely on a consumer wearable to diagnose sleep apnea unless it has an FDA clearance for diagnosis. Sleep apnea is a medical condition where breathing repeatedly stops and starts during sleep, causing brief arousals and drops in blood oxygen. While some newer devices include a pulse oximeter, a sensor that typically shines a red light through your skin, to estimate blood oxygen levels, they are not yet accurate or reliable enough for clinical use. Standard actigraphy fails to identify those with sleep apnea because of its poor specificity for wake episodes (Middelkoop et al., 1995). Even with newer sensors, one study found that while a Fitbit could help confirm a diagnosis in many cases, its accuracy was still insufficient for clinical screening (Moreno-Pino et al., 2019).

Can skin tone or tattoos affect accuracy?

The PPG technology used by most trackers can sometimes be less accurate on darker skin tones or skin with tattoos. The melanin pigment in darker skin absorbs more of the green light from the sensor, which can potentially make it harder to get a clear reading of the blood flow underneath (Colvonen et al., 2020). Similarly, the ink from tattoos, especially dark or dense ink, can block the light and interfere with the sensor's ability to get an accurate reading (Lujan et al., 2021). While device manufacturers are working to improve their hardware and algorithms to account for this, it remains a potential source of error.

So, where does this technology go from here?

What's Next for Sleep Tracking Technology?

The future of sleep tracking will likely involve integrating even more sensors to create a more complete and accurate picture of your sleep health. Researchers are exploring a number of new signals that could be measured by a wearable device to improve sleep/wake detection and staging (Lujan et al., 2021).

Future devices may incorporate sensors to continuously monitor blood oxygen saturation, which could help in screening for sleep-disordered breathing. Others might track changes in skin temperature, as the temperature of your hands and feet is closely linked to sleep onset. Some researchers are even developing wearable sensors that can measure biomarkers of stress and inflammation, such as the hormone cortisol, directly from your sweat (Jagannath et al., 2020). By combining all these data streams, the next generation of trackers could provide even more personalized and actionable insights.

How Should I Use My Sleep Tracker Data?

You should use the data from your sleep tracker to understand broad patterns and trends in your sleep, rather than getting fixated on the exact numbers from a single night. These devices are most powerful when used to monitor your habits over time.

Here are a few practical tips:

• Focus on consistency. Use your tracker to see if you are maintaining a regular bedtime and (especially important) regular wake time, even on weekends. Your total sleep time and the amount of time you spend awake after first falling asleep (often called WASO) are likely the most reliable metrics for this.

• Look for trends. Pay attention to how your sleep metrics change in response to your daily activities. Is your sleep more disturbed after alcohol? Does your resting heart rate go up after a stressful day? Use this information to connect your lifestyle choices to your sleep quality.

• Don't obsess over sleep stages. Remember that sleep stage data is an estimation. It can be interesting to look at, but don't worry if your deep or REM sleep numbers seem low on any given night. Focus more on how you feel during the day (deep sleep (N3) is typically only a small percentage of your night anyhow).

• It's a tool, not a doctor. Your sleep tracker is a wellness device, not a medical one. If you consistently feel unrefreshed or sleepy despite spending enough time in bed, or if you suspect you have a sleep disorder like insomnia or sleep apnea, it is important to speak with a healthcare professional.

You can also talk to a NZ sleep clinic like The Better Sleep Clinic for sleep help. Whether it’s an Auckland sleep clinic, Wellington sleep clinic, Christchurch sleep clinic, Hamilton sleep clinic or anywhere in NZ, we can help.

Ask for a free chat below or book an assessment (no referral required) and get started addressing your sleep problems today.

Frequently Asked Questions About Sleep Tracker Accuracy

Q1: How accurate are sleep trackers like Fitbit, Oura, and Apple Watch?

A1: Modern sleep trackers from top brands are generally accurate at telling the difference between being asleep and being awake. Studies show they have high "sensitivity," meaning they are good at correctly identifying when you are asleep (de Zambotti et al., 2019). However, their "specificity," or ability to detect when you are awake but lying still, is lower, which can cause them to overestimate total sleep time (Lujan et al., 2021).

Q2: Can my sleep tracker really tell my sleep stages (light, deep, REM)?

A2: Your sleep tracker provides an educated estimate of your sleep stages, not a precise measurement. Because consumer devices cannot measure brain waves directly, the only way to truly define sleep stages, they use secondary signals like heart rate, heart rate variability (HRV), and movement to create an algorithm-based guess (Lujan et al., 2021). While these estimates are useful for tracking trends, their agreement with a clinical sleep study can be moderate, with one study on the Oura Ring showing 65% agreement for light sleep and 51% for deep sleep (de Zambotti et al., 2019).

Q3: Why does my sleep tracker measure my heart rate?

A3: Your sleep tracker measures your heart rate and heart rate variability (HRV) to get a more detailed and accurate picture of your sleep. Your heart rate naturally slows as you enter deep sleep and becomes more variable during REM sleep (Snyder et al., 1964). By monitoring these predictable patterns, the device can make a much better guess about which sleep stage you are in and more accurately detect awakenings during the night, which are often accompanied by a spike in heart rate (Lujan et al., 2021).

Q4: Can a sleep tracker diagnose a sleep disorder like sleep apnea or insomnia?

A4: No, you should not use a consumer sleep tracker to diagnose a medical condition like sleep apnea or insomnia. These devices often struggle with accuracy in people with sleep disorders, frequently overestimating sleep time for those with insomnia (Kang et al., 2017). While some trackers can estimate blood oxygen levels, studies have found their overall accuracy is still insufficient for diagnosing sleep apnea in a clinical setting (Moreno-Pino et al., 2019). If you suspect you have a sleep disorder, it is essential to consult a healthcare professional.

Q5: Does skin tone or having tattoos affect my sleep tracker's accuracy?

A5: Yes, in some cases, darker skin tones or tattoos can affect the accuracy of a sleep tracker. Most trackers use green LED lights to measure heart rate (a technology called PPG). The melanin in darker skin or the ink in tattoos can absorb some of this light, potentially making it more difficult for the sensor to get a clear and consistent signal from the blood flow in your wrist (Colvonen et al., 2020; Lujan et al., 2021).

Q6: What is the most accurate sleep tracker?

A6: Validation studies show that major brands like Fitbit, Apple Watch, and the Oura Ring have comparable performance when measured against the clinical gold standard (Lujan et al., 2021). They all tend to be very good at detecting sleep (high sensitivity) but more moderate at detecting wakefulness (moderate specificity) (Roberts et al., 2020; de Zambotti et al., 2019). No single consumer device has been proven to be definitively more accurate than the others across all measures, as performance can vary based on the individual user and the specific metric being measured.

References

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Written By The Better Sleep Clinic

Reviewed By Dan Ford, Sleep Psychologist