How Does Your Smartwatch Know You’re Asleep? | Sleep Clinic Services

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How Does Your Smartwatch Know You’re Asleep?

smartwatch for sleeping

Have you ever wondered how your fitness tracker measures your sleep? Or more importantly, if the data it records is reliable? Whether you own a Fitbit, smartwatch, Garmin or one of the countless other fitness devices on the market, they all work in much the same way.

If you have a sleep disorder or if your sleep is disrupted on a regular basis, wearing a fitness tracker is not going to be a reliable indicator of your sleep health. The best option is to have an in-home sleep study to get a reliable and accurate reading.

So, is it still worth wearing your fitness tracker to bed at night?

sleep tracker smartwatchMost smartwatches and fitness trackers have what’s called a tri-axis accelerometer in them. That’s a techy way of saying there’s an accelerator in your wearable.

It’s a small device made up of axis-based motion sensing and it tracks movement in every direction. Some even come with a gyroscope to measure orientation and rotation.

Using a process called actigraphy, your tracker translates your wrist movements into sleep patterns. 

As you can imagine it’s not always accurate and there’s some guesswork involved in that process. That’s because actigraphy just tracks one thing – movement. And there’s so much more to monitoring sleep than simply tossing and turning.

Brainwaves, eye movements, and breathing are also required to determine the difference between deep REM sleep and light sleep.

Psychologist Kelly Glazer Baron, PhD, MPH from the Rush University Medical Center, published a report in the Journal of Clinical Sleep Medicine on the impact of fitness trackers on sleep.

Baron states that part of the problem lies in the technology itself. Citing multiple studies, Baron’s report states that these increasingly popular devices "are unable to accurately discriminate stages of sleep." As Baron explains, "They are not able to differentiate between light and deep sleep." Furthermore, "they might call it sleep when you're reading in bed."

While reliability is questionable, fitness trackers can be a useful guide.

Countless studies have varied in their results of fitness tracker reliability but for the most part, the consensus is that actigraphy is generally accurate enough to track sleep in healthy adults with normal sleep patterns. It’s just not as accurate as an in-home sleep study using a polysomnogram to monitor brain activity.

Well, it all comes down to psychology. Maybe you give yourself a pat on the back after your fitness tracker tells you you’ve had 8 hours sleep (regardless of how accurate it is).

And that’s a good thing. Even before wearable technology was invented, it was well known that tracking anything related to your health made you more aware of your choices. When you count steps, you tend to do more steps. When you write down your calories, you tend to be more conscious of what you’re eating.

smartwatch trackingTracking can also help you stick to your health goals or resolutions.

Rather than only focussing on a big health goal such as losing 20 kilos, tracking gives you small daily goals towards improved health.

You feel more successful when goals are broken down into smaller bite-size chunks.

On the other hand, if you have a tendency toward perfectionism, not achieving the 8 hours sleep dictated by your tracker could produce more anxiety which in turn affects your sleep.

People pursuing the "quantified self," says Baron, referring to data-driven obsessions, rely on the daily acquisition of data to enhance their mental and physical life. But, she adds, "some people do take it too far, and that can be stressful."

The bottom line is if you feel your fitness tracker is improving your quality of life then keep wearing it. But if you think you may suffer from a sleep disorder such as sleep apnoea or snoring, don’t take the data to heart. An in-home sleep study will provide the accurate results your health deserves.



  • Rush University Medical Center. "Sleep trackers can prompt sleep problems: Journal article sees potential for unintended effects in their use." ScienceDaily. ScienceDaily, 28 February 2017.
  • de Zambotti M, Baker FC, Colrain IM. Validation of sleep-tracking technology compared with polysomnography in adolescents, Sleep, 2015, vol. 38 (pg. 1461-8)
  • Dalva Poyares, Camila Hirotsu, Sergio Tufik; Fitness Tracker to Assess Sleep: Beyond the Market, Sleep, Volume 38, Issue 9, 1 September 2015, Pages 1351–1352

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