A woman sleeping while wearing a smartwatch.
Olena Hromova/Shutterstock.com

According to analyst house Gartner, consumers are expected to buy 86 million smartwatches in 2020. It’s a market worth $23 billion globally. Beyond fitness and sports, a major factor prompting people to buy these stylish wrist-worn computers is the ability to monitor sleep.

This is an almost universal feature, found on even the cheapest of devices. But why would you want it? And, more importantly: Does sleep tracking actually work?

How Sleep Trackers Work

Hidden within your fancy Apple Watch or Samsung Galaxy Watch is an array of sophisticated motion trackers, which can detect even the slightest of movements with a decent amount of precision.

These sensors (most notably the accelerometer) form the basis of your wearable’s sleep tracking system, with your body’s movement serving as an indicator for whether you’re asleep or not. This approach has a complicated scientific name—actigraphy—but it’s more straightforward than it sounds.

When you fall asleep, your body becomes prone, making far less movement than it otherwise would while awake. This provides the wearable the primary indicator that you’ve gone to bed.

As you progress through the several stages of the sleep cycle, your body behaves in different ways, allowing your wearable device to track whether you’re in deep or light sleep.

The first two stages of the sleep cycle are the lightest. During these periods, your body might involuntarily twitch. These movements are called “hypnic jerks” and are a type of movement called myoclonus. According to a study published in the October 2016 article of the Sleep Medicine journal, these movements occur randomly and are prevalent in between 60 and 70 percent of the population.

As you enter the deeper stages of sleep, your body’s movements reduce, giving your wearable a decent barometer of where you are in the sleep cycle.

Finally, there’s REM (or rapid-eye movement) sleep, which is the part where you typically experience dreams and nightmares. It’s at this part where your body is at its most motionless.

Your Heart Is the Twitchiest Muscle of All

A woman sleeping while a smartwatch shows her heartbeat.

It’s worth noting that many wearable devices contain heart-rate monitors, which can improve the accuracy of sleep tracking. This is because your heart rate varies wildly between the different sleep stages.

During light sleep, your heart rate slows slightly, only to normalize as you enter deep sleep. Heart activity increases further as you start dreaming, with breathing also becoming irregular.

While standard actigraphy-based sleep trackers can tell when you’re asleep, devices with heart-rate monitors are more capable of providing a qualitative analysis of your sleep, thereby telling you how restful your shuteye actually is.

Are Sleep Trackers Accurate?

When you invest in a smartwatch, remember that the sleep-tracking technology isn’t perfectly accurate. These are lifestyle products, after all, and not precise medical appliances.

One study, published in the June 2019 edition of the JMIR mHealth and uHealth journal, looked at the accuracy of the Fitbit Charge 2, particularly in comparison with a medical-grade tracker. It found that the FitBit had a propensity to overestimate how long an individual was in a deep-sleep state, while underestimating when the wearer would transition to another stage.

In general, medical literature tends to be quite critical of consumer-grade sleep trackers, particularly when it comes to the methodology used to derive their results.

A 2015 review of the technology, published by researchers at the Johns Hopkins University School of Medicine, looked at several of the leading trackers at the time. Of the six devices reviewed, only one provided information on the accuracy of their sensors, while two provided information on how they interpreted that data to produce results.

What Will You Do With That Data?

A woman using a smartphone in bed at night.

Another criticism of sleep-tracking devices is they often fail to guide people on how to improve the quality of their sleep—save for obvious advice on basic sleep hygiene. For example, a sleep tracker might recommend increasing the amount of exercise performed during the daytime, or avoiding using electronic devices before going to bed.

One paper, published by academics from the University of Tokyo and Queensland University of Technology, argues that consumer sleep tracking technology fails to provide any long-term benefit. The studio also argued that, to be useful, sleep trackers should use more sophisticated lifestyle and health data in their metrics.

RELATED: How to Get Better Sleep

So, Is It Worth It?

Ultimately, the results produced by your smartwatch’s sleep tracking technology should be regarded as purely advisory. They are not diagnostic tools.

If you believe you have a sleep disorder, you should speak to your physician, who will be able to arrange for you to undertake a sleep study under rigorous, controlled conditions, using the most sophisticated of medical gear.

If you do decide to invest in a smartwatch for the purposes of tracking your sleep quality, it’s important you get one with a built-in heart-rate monitor, like a FitBit Versa 2 or an Apple Watch, which has third-party apps that can track your sleep. The analysis published by the Johns Hopkins was somewhat damning about the accuracy of devices that use actigraphy as their sole method of monitoring.

RELATED: 12 Items That Will Help You Have a Better Night's Sleep

Profile Photo for Matthew Hughes Matthew Hughes
Matthew Hughes is a reporter for The Register, where he covers mobile hardware and other consumer technology. He has also written for The Next Web, The Daily Beast, Gizmodo UK, The Daily Dot, and more.
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