How Does a Smartwatch Know You Are Sleeping?
You spend around a third of your life comatose, with the amount and quality of sleep having a huge impact on the balance of your life.
So it's natural to be curious about how long you lot really sleep for, and whether you're getting enough good quality slumber without whatever slumber disorders to address.
Until just a few years ago, your options for assessing your sleep were somewhat limited: you could keep a sleep diary if you had the dedication; ask your partner most your sleep; or record yourself sleeping (an unlikely activity!)
The merely way to accurately assess your sleep would be to spend a night or two in a specialist sleep clinic. But now there's another option to improve your accuracy of cocky-assessment: the personal sleep tracker.
A question of accuracy
How practise consumer sleep trackers piece of work though? How practise they compare to the equipment and expertise in a slumber lab? And can yous actually rely on the data they provide?
These are of import questions to consider, especially if y'all're thinking of spending a fair flake of money on one.
I've been conducting a sleep tracker review for several years, regularly testing new clothing and non-wearable devices. And what I've come to realize is that there's quite a range in terms of what they measure out, and how they practise it.
So I decided to investigate what researchers take to say about the accuracy of both the devices themselves and the engineering science behind them.
Hopefully, this article will requite yous a better understanding of what y'all can expect from your sleep tracker. And how much you tin trust all the information about your slumber they claim to provide.
How slumber specialists mensurate sleep
To sympathize the strengths and weaknesses of personal sleep trackers, information technology's useful to first take a look at how specialist sleep centers mensurate sleep.
If your doctor suspects you lot have a sleep disorder, they might refer you to a sleep dispensary for a sleep study, also known every bit polysomnography.
This typically takes identify over a dark or ii, with different pieces of equipment measuring your sleep stages and cycles. Someone will usually find you while you slumber as well, so a lot of data is gathered about your sleep, including:
- Brain waves.
- Breathing.
- Heart rate.
- Trunk movement.
- Leg movement.
- Eye movement.
- Blood oxygen levels.
- The positions y'all sleep in during the night.
All of this information is so used by sleep specialists to assess your sleep and diagnose whatsoever slumber disorders. This detailed analysis and apply of encephalon wave monitoring is why polysomnography is often referred to as the aureate standard of sleep assessment.
Sleep trackers Vs polysomnogram
Logically, it's hard to imagine how a small sleep tracker worn on the wrist, a strip placed nether your mattress, or fifty-fifty a smartphone App could do the aforementioned every bit all these high-tech measuring devices. A major difference is that consumer slumber trackers don't measure out brain waves, which is a central way a polysomnogram determines slumber stages.
So I think information technology's useful to conduct this in mind when it comes to the expectations you might take of your personal slumber tracker. It likewise leads to an important question: if personal sleep trackers can't measure all of those factors, what exactly can they reliably do?
As you'll come across later, some of them are reasonably good at measuring basic sleep information, such as the total time you spend asleep and how often you wake up in the night. To sympathise what they can actually do though, allow'due south get-go take a look at the technology inside the trackers.
The science backside wearable sleep trackers
One of the cardinal measuring tools of vesture sleep monitors is chosen actigraphy. Actigraphy substantially involves recording movement through a measuring device called an accelerometer.
The idea is that a certain amount of motion corresponds with existence awake, and periods of being yet indicates that yous're asleep.
As Fitbit says on their assistance page:
When you haven't moved for about an hour, your device assumes that you lot're asleep. Additional data—such as the length of fourth dimension your movements are indicative of sleep beliefs (such equally rolling over)—help confirm that y'all're asleep.
Why actigraphy can exist useful
Actigraphy has been used by sleep clinicians for decades to mensurate sleep. Even though polysomnography is the gold standard for sleep measurement, actigraphy also plays an important role, especially as polysomnography isn't without its own issues.
When a patient is hooked upwards to various machines with multiple electrodes stuck to their head, and in a lab rather than their own bed, they understandably might not sleep normally.
And so not only is actigraphy less expensive than polysomnography, but people can clothing a device in their own habitation for a week or two, giving a more than natural insight into their sleep than only i or two nights in a clinic.
Merely how authentic is wrist actigraphy, and what can it accurately tell you about your slumber?
Fortunately, there'south been a fair fleck of research into those two questions.
How authentic is wrist actigraphy?
In 2011, Martin and Hakim published some fascinating inquiry into the usefulness of clinical wrist actigraphy. Importantly, they looked at how actigraphy compares to primal forms of sleep assessment:
- Clinical interviews and sleep questionnaires.
- Daily sleep diaries.
- Laboratory Polysomnography.
- Videosomnography in children.
They state that wrist actigraphy is useful for assessing sleep in a natural environment, rather than in a laboratory setting. Still, they do advise caution against relying on it solely:
Although actigraphy should not be viewed equally a substitute for clinical interviews, sleep diaries, or overnight polysomnography when indicated, information technology can provide useful information most sleep in the natural sleep surround and/or when extended monitoring is clinically indicated.
They claim that wrist actigraphy tin be quite accurate when it comes to estimating data such every bit total time comatose, sleep percentage, and how long after sleep waking occurs.
Yet, they besides country that the main limitation is mistaking being awake and motionless for being asleep. This could then make it difficult for those who wake many times, or with severe indisposition, and who tend to prevarication all the same in bed awake rather than move about.
This is also i of the fundamental points which consumers complain most with commercial sleep trackers. Looking at online client reviews, for example, will turn up many complaints that time lying in bed watching boob tube or reading a book was recorded every bit existence asleep.
Comparing two types of assessment
In 2013, researchers in the U.s. also looked into the accuracy of wrist actigraphy compared to polysomnography. They found very similar results to the 2011 enquiry:
…we conclude that wrist actigraphy with current algorithms is of value for individual-level estimates of both sleep duration and wakefulness after slumber onset
They also mention the idea that results from actigraphy might help people become an objective measure of their sleep, peculiarly when having treatment for insomnia, for example. Considering many people tend to underestimate how long they slumber for, this could help them come across that the handling is in fact helping.
The limitations of actigraphy
A indicate I've seen made in many online manufactures where sleep experts have been contacted by the author is that sleep trackers perform improve when you lot sleep reasonably well.
Information technology's when y'all have a disrupted nighttime'southward sleep that errors go more than apparent. And it seems it'south not only the skeptical owners of sleep labs that think this – there'south research to dorsum it up.
Sleep trackers perform better when you sleep reasonably well. It's when you have a disrupted night'due south sleep that errors become more than apparent.
Study 1
A study by Terri Blackwell in 2008 compared actigraphy with polysomnography in older women. She cautions that the more disrupted the sleep, the less accurate the sleep tracking. However, she did detect that actigraphy is reasonably authentic for people to utilise in their homes:
Actigraphy does not supervene upon polysomnography in sleep estimation, but was a convenient, affordable and accurate method of collecting measurements of sleep in a large epidemiologic study of older women…Sleep parameters from actigraphy corresponded reasonably well to PSG in this population…Those with poor sleep quality had the largest measurement error between the ii procedures.
Report ii
A team of Korean researchers in 2018 compared the sleep diaries of 78 people over fourteen nights with several sleep trackers:
- ActiGraph GT9X Link
- SenseWear Mini armband
- Footing Peak Fitbit Charge HR
- Jawbone UP3
- Garmin Vivosmart
Even though some of these are at present sometime models or discontinued, some of their findings are still interesting:
Overall, we found that the SenseWear, Fitbit Charge Hr, Jawbone UP3, and Garmin Vivosmart can be valid measures of TST (Full Slumber Timre) and TIB (Time In Bed) when compared with a sleep diary in a healthy developed population in a complimentary-living setting. However, these trackers cannot be considered valid regarding wake times during a night of sleep…our study likewise plant greater discrepancies between reported variables during nights that appeared to consist of more disrupted sleep (i.e., the sleep diary reported 7 h of slumber and each band would report between two and vii h of sleep for that night).
And then it seems that although wrist actigraphy tin be useful for some measurements, it struggles in certain circumstances. And frustratingly, information technology seems they struggle most when you're not sleeping well – which is in theory when many people would capeesh some accurateness!
Manufacturers aware of the issue
Manufacturers of sleep trackers are aware of these limitations. In response, some devices permit y'all to manually override the automatic sleep tracking – either to say exactly when you're trying to slumber rather than relax in bed, or to right errors in the forenoon.
Only let's face it – if you have to continue doing this, your religion in your device is going to collapse.
Across actigraphy: heart charge per unit and respiration in consumer sleep trackers
When I first started reviewing sleep trackers, actigraphy was all there was. But now, some include other measures, such as middle charge per unit, heart rate variability, and respiration to help improve their estimations of your sleep.
To make matters more complicated, sleep trackers seem to have branched off into 2 major types, within which there are subtypes (as I call them):
- Wearables: most are worn on your wrist, and also role as activity trackers and/or smartwatches. Some recent devices are worn on the caput.
- Non-wearables: Measuring strips that sit under your mattress and work by ballistocardiography, tracking small movements made as your heart pumps claret and y'all breathe. Or a device that sits on your nightstand and measures points similar animate, movement, light and racket. One or ii that clip onto a pillow. Some smartphone Apps likewise endeavor to track sleep.
Going dorsum to the Fitbit help page, which also explains how they now track sleep stages, they summarize how they 'judge' them.
And this is a fundamental point: sleep trackers don't exactly measure out your sleep, but they estimate it based on data they collect and algorithms the manufacturer creates:
While you're sleeping, your device tracks the beat-to-beat out changes in your center rate, known as heart rate variability (HRV), which fluctuate as you transition between low-cal sleep, deep sleep, and REM sleep stages. When you sync your device in the morning, nosotros utilise your movement and middle rate patterns to estimate your sleep cycles from the previous night.
Sleep stage tracking: the unmeasurable elephant in the room
Estimating your sleep based on a combination of actigraphy, center charge per unit or breathing is neat (when it works), and my feeling is that it will get better in time as engineering science improves.
Even so, along with the inclusion of new measuring tools arrived a giant elephant in the room: sleep stages.
Whether slumber tracker manufacturers decided effectually the same time that the engineering was skillful enough to offer slumber stage data to users, or they were worried about non selling if they don't offer what competitors exercise isn't for me to say.
But here'south the problem: unless you take access to a sleep lab and brain moving ridge measuring, you have absolutely no way of knowing if your slumber stage data is accurate.
So we have a situation where some consumers don't even worry about it and trust the way their expensive new toy works, while others have suspicions but no way to confirm them.
And neither of those is ideal.
Sleep researchers compare sleep trackers with polysomnograms
I suspect many people would like to know how big the elephant in the room is, simply we merely don't own elephant measuring tools. And there's no dominance that regulates sleep trackers to help united states out.
However, in that location have been a handful of studies in recent years where slumber researchers have compared sleep trackers with polysomnograms, EEG or sleep diaries.
The problem, as I encounter it, is that manufacturers regularly release new models or update the firmware. So almost available published enquiry involves slumber trackers that are either older models or unabridged brands that have since discontinued (such every bit Jawbone).
Still, they are interesting to consider as at that place are some common findings that crop upwardly more than once.
one. Portable EEG + Fitbit Versa (older model)
A study in 2019 particularly interested me as it involved one of my favorite wearables – the Fitbit Versa (even though the Versa 2 is now out).
20 people used the Versa for 14 nights, as well equally a portable EEG. They institute the Versa was similar to the EEG for full sleep time, time in bed and calculated sleep efficiency. However, information technology differed when it came to sleep stages, time to fall asleep and time of first waking. They concluded:
The consumer sleep tracker could be a useful tool for measuring sleep duration in longitudinal epidemiologic naturalistic studies albeit with some limitations in specificity.
2. Polysomnogram + Jawbone UP3 (at present discontinued)
Even though the Jawbone UP3 is now discontinued, this 2017 study is interesting because 17 patients wore the UP3 while undergoing a polysomnogram in a sleep lab.
The results weren't the best for Jawbone though. They establish it significantly overestimated total slumber time and sleep efficiency. And it was found to overestimate REM latency, total REM sleep fourth dimension and deep sleep time. And it underestimated the time to fall comatose and fourth dimension of start waking.
The grim decision was:
The JB3 inaccurately estimated sleep duration and efficiency relative to PSG. Further investigation to determine JB3 specificity, sensitivity, and accuracy for sleep staging, as well equally the evaluation of the JB3 to measure sleep during naps, is warranted.
And considering the lack of studies like this done for any sleep trackers, it didn't inspire much confidence in general!
3. Polysomnogram + Jawbone
Just to exist fair to Jawbone, especially since we're all hoping for a resurgence in the future, at that place was an interesting study in 2015 of the Jawbone Up before slumber stages came along. The results were more positive than the previous report, every bit the authors say:
Jawbone Upward shows good understanding with polysomnography in measures of total slumber time and wake later on sleep onset in adolescent boys and girls.
iv. Polysomnogram + Bodymedia SenseWear Pro Armband (medical actigraphy) + Withings pulse 02 + Jawbone Up
This study compared 2 commercial trackers – the Withings Pulse 02 and Jawbone Upwards with PSG with medical lab sleep tests, specifically with 36 obstructive slumber apnea patients.
They again institute mixed results with the only positives existence that they correctly assessed time in bed and light sleep. But no other measures were accounted to be accurate.
v. Polysomnogram + Actiwatch-64 (medical actigraphy) + Fitbit
This study, done in 2012, is at present pretty old and Fitbit has made significant changes to its sleep tracking. However, since highlighting Jawbone'south issues, information technology seems just fair to bring this i upwardly.
Interestingly, despite being caught out by the polysomnogram, it was at to the lowest degree no worse than the medical form actigraphy, as noted by the authors:
Fitbit has the same specificity limitations as actigraphy; both devices consistently misidentify wake every bit sleep and thus overestimate both sleep fourth dimension and quality
six. EEG + Fitbit Accuse two (older model)
If heart rate changes are used when estimating slumber, surely that's an easier one to check – especially during exercise.
The Charge 3 is now the latest version, but notwithstanding, a written report in 2018 found some worrying results while recording the centre rate of xv participants riding a stationary bike.
Compared to an electrocardiograph, they found that while it wasn't too far off in some cases, at that place were incidences where the fault was large:
Whilst there is only moderate bias on average, precision is poor for individual measurements, which could plausibly be underestimated by equally much as 30 bpm.
To exist fair, the authors do notation that the mode the device is worn on the wrist can lead to errors. And this, presumably, tin as well happen during sleep:
…instability and improper positioning of the device may potentially explain different results and poor-quality HR information.
7. My own tests in 2018
In 2018, Itested 3 different sleep trackers at the same time, comparison them to my manual sleep diary. I didn't compare them with an objective polysomnogram, but I thought the results were interesting nonetheless.
One standout result was that the slumber phase reporting of all three was wildly different. And so that ruled out the absolutely small chance that they were all accurate!
And with other points I could mensurate myself, like estimated time of falling asleep and time of waking, I found the Versa and Beautyrest were 'reasonably' accurate, while the Emfit struggled.
And that raises another point: where do we draw the line where accurateness is concerned? One person's definition of 'reasonable' could exist another'southward motive to log into eBay.
Determination
The results of the studies higher up aren't exactly reassuring – to put it mildly.
As I said though, engineering advances apace, and to my knowledge, in that location are no published contained studies of the latest devices made by the companies which have been put to the sword. And there are a few brave new companies entering this lucrative, withal vulnerable to self-devastation, market.
So what tin can you take from all of this?
Well, my personal take remains the aforementioned as information technology did when I outset wrote about sleep trackers six years ago: take the results with a pinch of salt, and don't get too caught up in the finer details of the sleep tracking data – especially where sleep stages are concerned.
I think it's skilful to have realistic expectations of what you can go from your sleep tracker. They are potentially a useful tool for giving y'all an overview of your sleep. Just be aware that they aren't perfect yet, and will brand mistakes from fourth dimension to time.
It'south notwithstanding a developing engineering, and it will be very interesting to see just how authentic they become in the coming years.
For at present though, if you take sleep bug, recall to speak to your doctor/physician about information technology.
At the cease of the day, they and the slumber specialists they can refer you to are still the most reliable way to find out about your sleep and diagnose any sleep disorders.
Your views
What do you hope to discover out from using a sleep tracker? Which device have you tried before, and did yous discover it to be accurate, or provide you with useful information?
Source: https://www.nosleeplessnights.com/how-do-sleep-trackers-work/
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