You spend around a third of your life asleep, with the amount and quality of sleep having a huge impact on the rest of your life.
So it’s natural to be curious about how long you really sleep for, and whether you’re getting enough good quality sleep without any sleep disorders to address.
Until recently your options for assessing your sleep were somewhat limited: you could keep a sleep diary; ask your partner about your sleep; or record yourself sleeping.
The only way to accurately assess your sleep would be to visit a specialist sleep clinic. But now there’s another option to improve your accuracy of self-assessment: the personal sleep tracker.
A question of accuracy
How do they work though, and how do they compare to the equipment available in a sleep lab? And can you actually rely on the information they provide?
These are important questions to consider, especially if you’re thinking of spending a fair bit of money on one.
I’ve been conducting an ongoing sleep tracker review, testing wearable and non-wearable devices. And what I’ve come to realize is that there’s quite a range in terms of what they measure, and how they do it.
So I decided to investigate what researchers have to say about the accuracy of both the devices themselves and the technology behind them.
Hopefully, this article will give you a better understanding of what you can expect from your sleep tracker. And how much you can trust all the information about your sleep they claim to provide.
How sleep specialists measure sleep
To understand the strengths and weaknesses of personal sleep trackers, it’s useful to first take a look at how specialist sleep centers measure sleep.
If your doctor suspects you have a sleep disorder, they might refer you to a sleep clinic for a sleep study, known as polysomnography (PSG).
This typically takes place over a night or two, with different pieces of equipment measuring your sleep stages and cycles. Someone will usually observe you while you sleep as well, and so a lot of information is gathered about your sleep, including:
- Brain waves (EEG).
- Breathing, including how easily or not you breathe, how often you stop breathing and for how long.
- Heart rate and rhythm.
- The flow of air in and out of your lungs.
- Muscle activity.
- The positions you sleep in during the night.
- The movement of your eyes.
- Oxygen levels in your blood.
All of this information is then used by the sleep specialists to assess your sleep and diagnose any sleep disorders. It’s easy to see then why polysomnography is often referred to as the gold standard of sleep monitoring.
Logically then, it’s hard to imagine how a small sleep tracker worn on the wrist could possibly do the same as all these high-tech and expensive machines.
So I think it’s useful to bear this in mind when it comes to the expectations you might have of your personal sleep tracker. It also 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 see later, they can be relied on to measure some basic sleep information, such as the total time you spend asleep and how often you wake up in the night. To understand what they can actually do though, let’s first take a look at the technology inside the trackers and what it’s capable of.
The science behind wearable sleep trackers
One of the key measuring tools of wearable sleep monitors is called actigraphy. Actigraphy essentially involves recording movement through a measuring device called an accelerometer.
The idea is that a certain amount of movement corresponds with being awake, and periods of being still corresponds with being asleep.
It has in fact been used by sleep clinicians for 30 years to measure 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 up to multiple machines with around 20 electrodes placed on their head, and in a lab rather than their own bed, they understandably might not sleep normally.
So not only is actigraphy less expensive than polysomnography, but people can wear a device in their own home for a week or two, giving a more natural insight into their sleep than just one or two nights in a clinic.
The key questions though, especially if personal sleep trackers depend on it, are how accurate is wrist actigraphy, and what sleep information can it accurately measure?
Fortunately, there’s been a fair bit of research into those two questions.
How accurate is wrist actigraphy?
In 2011, Martin and Hakim published some fascinating research into the accuracy and usefulness of wrist actigraphy for sleep assessment. Importantly, they looked at how actigraphy compares to key forms of sleep assessment:
- Clinical interviews and sleep questionnaires.
- Daily sleep diaries.
- Laboratory Polysomnography (PSG).
- Videosomnography in children.
They state that wrist actigraphy is useful for assessing sleep in a natural environment, rather than in a laboratory setting. However, they do advise caution against relying on it solely:
Although actigraphy should not be viewed as a substitute for clinical interviews, sleep diaries, or overnight polysomnography when indicated, it can provide useful information about sleep in the natural sleep environment and/or when extended monitoring is clinically indicated.
In their research, they go on to say that wrist actigraphy can be quite accurate when it comes to estimating information such as total time asleep, sleep percentage, and how long after sleep waking occurs.
However, they also state 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 insomnia, and who tend to lie still in bed awake rather than move about.
This is also one of the key points which consumers complain about with commercial sleep trackers. Looking at online customer reviews, for example, will turn up many complaints that time lying in bed watching television or reading a book was recorded as being asleep.
Comparing two types of assessment
Looking at more recent research, in 2013 researchers in the United States also looked into the accuracy of wrist actigraphy compared to polysomnography. They found very similar results to the 2011 research, concluding:
…we conclude that wrist actigraphy with current algorithms is of value for individual-level estimates of both sleep duration and wakefulness after sleep onset
They also mention the idea that results from actigraphy might help people get an objective measure of their sleep, especially when having treatment for insomnia for example. Considering many people tend to underestimate how long they sleep for, this could help them see that the treatment is in fact helping.
Another research study published 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 find that actigraphy is reasonably accurate for people to use in their homes:
Actigraphy does not replace 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.
So it seems that although wrist actigraphy does have some limitations, it’s definitely very useful. And it does appear that many manufacturers of sleep trackers are aware of the limitations.
To try and deal with the problem, many devices allow you to state when you’re trying to go to sleep rather than relaxing. And some offer options to manually correct any mistakes you think the device has made.
Comparing personal sleep trackers to polysomnography
Several research studies have highlighted the lack of research and evaluation of the accuracy of consumer sleep trackers. In fact, there’s very little research into any of them, so you’re left to make decisions based on the reviews and experiences of other people.
The only study I could find was conducted by Dr. Christopher Winter and published in the Huffington Post in 2014.
Dr. Winter wore a Fitbit Flex, Jawbone UP, Basis Chrome and Philips Spectrum Actiwatch whilst undergoing a Polysomnography in a sleep clinic.
It’s interesting to see how the 4 devices correlate with the PSG, with none of them being totally similar, as you’d expect. However, even more interesting is that all 4 of them give different readings.
While hesitant to draw too many conclusions, he does state that the Basis was the most similar to the PSG recording. Unfortunately, the Basis is now discontinued!
It’s unlikely that sleep clinics will be opening their doors to many people who want to do these kind of tests, so it’s interesting to see this rare piece of personal experimentation. The good news is that it does at least show that consumer products have the potential to be accurate.
Beyond actigraphy: the advancing technology in sleep trackers
To make matters even more complicated, in 2015 personal sleep trackers started to appear which promised to be even more advanced when it comes to sleep tracking.
One such device which claimed to lead the way is the Jawbone UP3. Jawbone claim that rather than measuring brainwaves – as happens in Polysomnography – instead they use a range of physical sensors to identify REM, deep and light sleep.
So in addition to actigraphy, they included other sensors such as bioimpedance, which measures heart rate. The idea is to use the measurements together with a complex algorithm to identify the physical characteristics of the different sleep stages.
Unfortunately, Jawbone has now been discontinued…
Fitbit’s new sleep tracking
Fitbit leads the way in the wearable sleep tracking world, though they haven’t been without problems either. Historically, they received a fair bit of criticism over the accuracy of not just the sleep tracking, but the activity tracking too.
In 2017, they announced a big update to their sleep tracking, offering sleep stage tracking with the Charge 2, Blaze, Alta HR, and Ionic.
It seems that Fitbit now believe they have the technology to accurately calculate sleep stages, combining movement sensors with constant heart rate monitoring. And interestingly, they are making use of the colossal amount of data they now have, thanks to millions of users.
So now you don’t just get total sleep time and restless. You get total sleep time, time awake, light sleep, deep sleep and REM sleep. Then you can compare that with what the typical time in those stages is for people of your sex and age.
Could it be then that personal sleep trackers are now able to correctly identify your sleep stages and cycle in addition to just the total time you spend asleep?
For now it doesn’t appear that much research has been done into how accurately they can actually do this.
My current App data looks very realistic, and is measuring up well against my mental note-taking. But my personal opinion would still be to take all the latest claims with a small pinch of salt until they are proved accurate in independent tests.
Non-wearable bedroom sleep monitors
Devices such as the BeautyRest, S+ by ResMed, Emfit QS, Sleepace Reston, Withings and Beddit take a very different approach.
These are dedicated non-wearable sleep trackers which are only used in the bedroom and don’t go where you do.
Although there are a couple of exceptions, the norm is to have a thin sensor which you lie across your bed or under your mattress and sleep on top of.
They each have different claims as to their accuracy, ranging from the Sleepace which says it has medical grade sensors to the Withings and Beddit measuring breathing and snoring.
Again, these devices haven’t undergone rigorous independent testing to my knowledge. However, they do seem to be an interesting alternative for people who don’t want to wear a tracker on the wrist while sleeping.
They make some bold claims as to being the most accurate sleep trackers. For now though, independent reviews by customers, technology review websites and myself have found that, like wearable devices, they have their own unique pros and cons.
I think they can definitely provide some very useful information about your sleep, especially if you sleep alone so your partner won’t lie on top of your sensor by accident. But whether they are a better option for you than a wearable device is very much a personal choice.
You can find out more about some of these devices in my extended review of sleep trackers.
It’s particularly tricky to assess how reliable personal sleep trackers are, and even the question of how they work has more than one answer.
Many of them rely heavily on movement sensors, which has been shown in scientific research to be accurate for measuring factors such as the total time spent asleep.
However, what’s still not clear is how accurate personal sleep trackers are compared to the medical grade devices used in those research studies. There just hasn’t been much independent scientific research to test all of the devices you can buy yourself.
In addition, wearable technology is a rapidly advancing market, with new devices and new promises of accuracy appearing all the time.
Even if researchers decided to put a range to the test, by the time they published the results, a new range will already be in the pipeline.
So for now, my advice would be to try to have realistic expectations of what you can get from your sleep tracker. They are definitely a useful tool for giving you an overview of your sleep. But do be aware that they aren’t perfect yet, and may make mistakes from time to time.
And that doesn’t necessarily mean they aren’t good, rather that it’s still a developing technology. It will be very interesting to see just how accurate they do get in the coming years.
For now though, if you want an insight into your sleep, they will certainly give you that. But above all, if you do suspect you have serious problems with your sleep, do remember to speak to your doctor about it.
At the end of the day, they and the sleep specialists they can refer you to are still the most reliable way to find out about your sleep and diagnose any sleep disorders.
My recent test of the accuracy of different trackers
In 2018, I tested 3 different sleep trackers at the same time, comparing them to my manual sleep diary. It’s still not the most objective test, but the results are interesting nonetheless.
What do you hope to find out from using a sleep tracker? Which device have you tried before, and did you find it to be accurate, or provide you with useful information?