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How do Fitness Trackers know when you’re asleep?

One of the main things that Exist does is sleep tracking, they currently have Jawbone UP and Fitbit integration, meaning they’re able to export the sleep data from both of these trackers and analyse it. Exists are also planning on letting users choose whether they want to optimise either their sleeping or activity tracking using Exist, but before we talk more about all that, lets find out just how accurate these devices are at tracking your sleep patterns.

When you set your Jawbone UP or Fitbit to its sleep mode, both devices will use their accelerometers to track your movements, including the speed and direction you’re moving. This is the main way they track your activity during the day and also how they tell when you’re sleeping. So when you sync your device the next morning the software will actually translate these movements into your sleep data, and if you’ve ever done this then you’ll know how this works. Your data during ‘awake’ time is re analysed so it knows then you were actually sleeping, then you’ll see this as your sleep data. The method which is used to track and measure your movements during sleep is called actigraphy, which you can find more about here.


See also: 4 Most promising Wearable Fitness Trackers coming in 2014

When research is done for actigraphy, a purpose built actigraph device will be used, rather than the Jawbone UP or the Fitbit. The main focus of this test has been focused on measuring the accuracy of it when compared to the likes of the standard way of measuring sleep patterns which would be carried out by a Polysomnography (PSG) test. The results of the study have been varied, but for the most part the results have shown that an actigraphy is accurate enough to track sleep in healthy adults with normal sleep patterns.

The main problems arrises when your sleep has been disrupted, for example if you have a sleeping disorder or you have disrupted your sleep on a regular basis. So the more disrupted your sleep, the less accurate the actigraphy will be. This is because an actigraphy only tracks one thing, movement. But sleep researchers know there’s much more to compiling accurate sleep data, such as brain waves and eye movements which can be used to assess sleep phases, something the fitness trackers can’t do.

“Consumers should not expect that these devices will be able to distinguish between sleep stages because these devices rely on movements, whereas sleep stages are defined primarily by brain activity” said Michael Scullin, a post doctoral at Emory University School of Medicines Department of Neurology.

What he basically means is, if you’re using a fitness tracker to measure your sleeping patterns and claims to tell you how long you spend asleep in each sleep stage, then it’s probably not very accurate at all.


See more: Leaked image show that Huawei is also working on wearable fitness tracker

So what does all of this mean, should you be trusting your fitness tracker to know when you’re asleep and how your sleeping patterns are looking? Well from what the sleep researchers are saying and the results of the actigraphy, it seems like its okay to trust it as a general guide to your sleep time and efficiency, but if you want a more in depth and complex look into your sleep patterns, the phases of your sleep or when you’re are deep into REM sleep, then I’d suggest taking a PSG test. As at the moment no doctor is ready to vouch for the accuracy of fitness trackers based on movement alone.