Written by David Zar, CTO, Nuream Advisory Board Member

Sleep is essential for our physical and mental well-being, and many of us are turning to technology to help us understand and improve our sleep. Consumer sleep trackers, like fitness watches and bedside monitors, have become increasingly popular, promising to provide insights into our sleep patterns. But how accurate are these devices, really? And can they truly measure the quality of our sleep?   

While sleep trackers can be helpful tools, it's important to understand their limitations, especially when compared to polysomnography (PSG), the gold standard for sleep study. PSG is a comprehensive test used to diagnose sleep disorders in sleep labs. It involves monitoring various physiological signals, including brain waves, eye movements, heart rate, and breathing patterns, to provide a detailed picture of your sleep architecture. This comprehensive approach allows sleep specialists to accurately identify different sleep stages and diagnose conditions like sleep apnea, insomnia, and other sleep disorders. [1][2]

A Convenient but Limited View

Sleep trackers include wearables (like smartwatches, smart rings, and fitness bands), nearables (bedside devices or those placed under the mattress), and smartphone apps. They typically rely on accelerometers and heart rate monitors to estimate sleep stages. By measuring movement and heart rate variability, they determine when you're asleep and make educated guesses about your sleep phases. While this can be useful for tracking sleep duration and identifying potential sleep disruptions, it doesn't provide the same level of accuracy as PSG.   

One major limitation of sleep trackers is their inability to measure brain waves. Brain wave activity is crucial in determining sleep stages and assessing sleep quality. PSG directly measures brain waves using EEG, allowing for accurate identification of sleep stages. This detailed analysis is essential for diagnosing sleep disorders and understanding the nuances of your sleep performance.

Another limitation is that sleep trackers often overestimate total sleep time and underestimate wakefulness; They primarily rely on movement to determine sleep and wake states. As a result, periods of stillness, such as lying in bed while watching TV, may be misclassified as sleep.   

The biggest problem with consumer sleep tracking devices is their struggle to accurately differentiate between sleep stages, especially light and deep sleep. Because they don’t look at EEG, they often overestimate the time spent in deep sleep and underestimate the time spent in other stages. This can lead to an inaccurate picture of your sleep quality.   

The Benefits of Sleep Trackers

Despite their limitations, sleep trackers can offer some benefits:

• Increased awareness: They can help you become more aware of your sleep patterns and identify potential sleep disruptions. If you consistently wake up feeling tired despite seemingly getting enough sleep, a tracker might reveal frequent awakenings or a lack of deep sleep.   

• Motivation: They can motivate you to prioritize sleep and make changes to improve your sleep habits. Seeing your sleep data might encourage you to establish a regular sleep schedule, create a relaxing bedtime routine, or change your sleep environment.   

• Convenience: They provide a convenient and relatively inexpensive way to track your sleep at home.

Conclusion: Sleep Trackers as a Starting Point

Consumer sleep tracking devices can be a helpful starting point for understanding your sleep patterns. They can provide valuable insights into your sleep duration, identify potential disruptions, and motivate you to change your sleep habits positively. However, it's crucial to remember that they are not a substitute for PSG. If you have concerns about your sleep, it's always best to consult a sleep specialist for a comprehensive evaluation.

The field of sleep tracking technology is constantly evolving, with new devices and algorithms emerging regularly. While current consumer devices are limited when measuring sleep stages and quality, they can still be valuable tools for increasing awareness and promoting healthy sleep habits. As technology advances, we expect more accurate and sophisticated sleep-tracking devices.

In the meantime, it's vital to approach sleep tracking with a balanced perspective. Use sleep to focus on long-term trends rather than daily fluctuations, and don't hesitate to seek professional guidance if you have concerns about your sleep. Combining technology with lifestyle changes and expert advice can pave the way for better sleep and improved overall well-being.

[1] Evan D Chinoy, Joseph A Cuellar, Kirbie E Huwa, Jason T Jameson, Catherine H Watson, Sara C Bessman, Dale A Hirsch, Adam D Cooper, Sean P A Drummond, Rachel R Markwald, Performance of seven consumer sleep-tracking devices compared with polysomnography, Sleep, Volume 44, Issue 5, May 2021, zsaa291, https://doi.org/10.1093/sleep/zsaa291

[2] Robbins, R.; Weaver, M.D.; Sullivan, J.P.; Quan, S.F.; Gilmore, K.; Shaw, S.; Benz, A.; Qadri, S.; Barger, L.K.; Czeisler, C.A.; et al. Accuracy of Three Commercial Wearable Devices for Sleep Tracking in Healthy Adults. Sensors 2024, 24, 6532, https://doi.org/10.3390/s24206532