How Do Sleep Trackers Work And Are They Reliable?

image of ethan green wearing 2 sleep trackers

I regularly wear more than one sleep tracker at a time to test them

You spend around a third of your life asleep, with the amount and quality of sleep having a huge impact on the other two thirds of your life.

So it’s very 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 deal with.

Until recently your options for assessing your sleep were somewhat limited: you could reflect on your sleep; 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.

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, personally testing the most popular 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. And there, depending on your circumstances, they might decide to do 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 your 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 being then 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.

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

photo of me wearing 2 sleep trackersTo 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, my personal experience has been that the Jawbone sleep tracking isn’t as reliable as I’d like it to be. They haven’t received glowing reviews online either, and it seems likely there will be some effort made to improve them at some point in the future.

Fitbit’s new sleep tracking

Fitbit lead 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 and new 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.

Although my current App data looks very realistic, and is measuring up well against my mental note taking, 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

image of the beddit sleep monitor sensor on a bedDevices such as the S+ by Resmed, Emfit QS, Sleepace Reston, Withings Aura Smart Sleep System, Sense and Beddit Smart Sleep Monitor 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 which even measures external factors such as temperature and noise, and the 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 these three monitors in my article looking at the best sleep trackers.

Conclusion

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 of Fitbits and Jawbones 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.

Your views

I’d like to hear your thoughts on personal sleep tracking and any devices you’ve tried yourself. 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?

Feel free to share your experience, thoughts and ideas in the comments below.

References

Martin, J. L., & Hakim, A. D. (2011). Wrist Actigraphy. Chest, 139(6), 1514–1527.

Marino M; Li Y; Rueschman MN; Winkelman JW; Ellenbogen JM; Solet JM; Dulin H; Berkman LF; Buxton OM. Measuring sleep: accuracy, sensitivity, and specificity of wrist actigraphy compared to polysomnography. SLEEP 2013;36(11):1747-1755.

Terri Blackwell, for the Study of Osteoporotic Fractures Research Group. ‘Comparison Of Sleep Parameters From Actigraphy And Polysomnography In Older Women: The SOF Study’. Sleep 31.2 (2008): 283. Web. 11 Dec. 2015.

24 CommentsLeave a comment

  • Thank you. This is a much needed article. Sleep Apps for wearables are in danger of over promising. In my experience they don’t clearly set out how they work and their limitations.

    I use an Apple Watch App called Pillow, have you reviewed this?

    • Hi Dan
      Thanks for your comment, and I’m glad you liked the article. Yes, it’s a shame really that they aren’t so reliable! I have plans in the pipeline to do a huge test of lots of different ones all at the same time. I’d love to take the test into a sleep lab to compare them with their equipment, though it might not be possible. If I do manage to get round to this kind of test, I’ll be sure to write about it!
      And no, I haven’t used that App – can you tell me a bit about it?
      Regards
      Ethan

  • Hi, thanks very much for your comprehensive and sensible explanation of how sleep trackers work and comparing different devides. I’m 65, female, and over the past couple of years my sleep has been getting less restful. So a few months ago I bought a Fitbit One. Though the accuracy is questionable, comparing its results over many weeks gives me a general sense of how I sleep each night and is actually quite reassuring. It does confuse sleep with awake-still, but the latter state can be restorative too. I’m looking for a device now that checks more factors than just activity and will give a more defined readout. Your articles have been very helpful in choosing which one to purchase. Thank you.

    • Hi Sharon
      Thanks for your comment. I’m glad you found the article helpful – thanks for saying so! The Fitbit One is definitely at the lowed end of the market in terms of what you can get from it, and you’re right in that the sleep readings can be questionable, but over time give a general idea. Hopefully you’ll get some more interesting data with your next choice of sleep tracker.
      Regards
      Ethan

  • I am just wondering if it is also meaningful to have a sleep tracker for baby? Is it meaningful to consider to improve the baby’s sleeping quality – as it sounds babies are always sleeping no matter what environment they are in. But if there is a sleep tracker for them, maybe their emotion can be understood better for “day time” as they cannot “speak” out what they need. If there is some ways to improve their sleeping quality, will it be helped their brain development?
    I believe it is not necessary for the tracker to wrapped around the wrist but anywhere in the body, just for the wrist, the movement is the “most obvious”.

    • Hi Linda
      Thanks for your comment. To be honest, I’ve never heard of anyone using a sleep tracker for a baby. I think the way they are calibrated, you’re likely to get lots of warnings about sleeping badly because of waking up too often and then sleeping for too many hours during the day, so I can’t see that it would be helpful.
      Having said that, there are loads of Apps for babies which take into account things like sleep and sleep patterns. If you have a look online, you’ll find plenty of reviews of them that you could read through and see if any do what you’d like it to.
      Regards
      Ethan

  • Hello Ethan, I have RLS (Restless Leg Syndrome) and take drugs to help the symptoms but I never feel like a have a restful sleep, I wake up exhausted and tense. I don’t care about daytime activity monitoring, just my night time … dare I say “sleep” and what is happening during the night. Can you recommend a type of monitor that would be useful for this type of issue. Thinking to maybe use it to help adjust my medications with my doctor. Oh, also don’t want to spend tons of money either ;-).

    • Hi Alden
      Thanks for your comment. Even though you’re not interested in activity tracking, you might find a wearable device helpful because they work mainly based on movement. So if you move around a lot in the night, they might pick up on that and record it as restless sleep or being awake. But then again, most of the bed based ones will also tell you if you moved around a lot in the night! So if money is also a factor, I’d go with one of the cheaper ones first and see if it gives you the results you’re interested in. What you’re asking for isn’t anything particularly unusual for sleep trackers, so you’ll find most of them will try to record how much you’re restless.
      Regards
      Ethan

  • I’m looking for a sleep monitor that will record my heart rate continuously. I have sleep apnea (I use a CPAP) and I want to see my HR on a graph because I know apneas cause a slower HR and then a higher HR afterward. My DreamMapper app doesn’t give me a graph just the number of apneas per night. Sleep stages would be nice, too

    • Hi Alice
      Thanks for your comment. Many wearable sleep trackers give you your heart rate readings these days. But are you looking for one that will track apneas and correlate it with your heart rate? If so, I’m not aware of one that does that.
      Regards
      Ethan

  • Hello, I took a sleep test and was diagnosed with moderate sleep apnea. After trying different solutions I settled on a customized mouthpiece built by my dentist to increase air throughput. I then went back to take the sleep test with my mouthpiece and the result was I don’t have sleep apnea when wearing the mouthpiece. Although it is night and day compared when I wear the mouthpiece vs not wearing it, I also notice some days are even better than others when wearing the mouthpiece. Which device would you recommend that provides more accuracy for REM and deep sleep per night?

    • Hi Dion
      Thanks for your comment. It’s great that you had such good results with the mouthpiece! To be honest, I don’t think any of them are 100% reliable when it comes to breaking down the sleep stages. However, I’d probably recommend one of the stand alone devices over a wearable one. Have a look at the different ones and see which you like the sound of based on the other features it offers too.
      Regards
      Ethan

  • I am 72, in good shape, still working, doing sports and other activities. At about 10pm I am tired and wish to sleep. As soon as in bed, my tiredness gone and I waste time in bed untill 2am and sometimes even longer. When asleep. I sleep rather well until about 8am. Could you recommend devise which would improve my sleeping habits?

    • Hi Dov
      Thanks for your comment. Many devices will encourage you to stick to sleep goals that you set yourself. So you can say what time you’d like to go to sleep and wake up, and they will give some kind of positive or negative feedback if you get within a target range of that. Fitbits have that option for example, but others do too.
      To be honest though, the best way to improve your sleep habits is by planning them yourself. It may be that in your case, you’re feeling tired but not ready to sleep. I’d have a look at my article about fixing your sleep schedule where I explain more about that.
      Regards
      Ethan

  • My husband was given a Fitbit Flex & we both tried it for sleep monitoring. It didn’t work well (and was very inaccurate at measuring steps & activity, too, but that’s a separate problem).. I especially remember that the Flex said I woke up 46 times in one night… what actually happened that night is that I slept little, and only in 2 or 3 spells. I wish something was a decent monitor, although what I really want is just to sleep better over all. Nothing works, argh.

    • Hi Julii
      Thanks for your comment. I had a Fitbit Flex some time ago, and remember being less than impressed by the general accuracy. Having said that, I’ve found other Fitbits more accurate, namely the Charge 2 and Surge.
      But yes, it’s unfortunately the case that sleep trackers get confused between awake and still or asleep. So if you’re awake and still a lot, trying to sleep, but then inevitably toss and turn frequently, it might think you were asleep and keep waking up. That’s why I think sometimes when you have a particularly bad night, it’s good to correct the information manually the next day on the App. Though of course, in an ideal world you’d never need to do that because it would be totally accurate! Have you tried any of the other bed-based trackers, like the Withings Aura, Beddit or Sleepace, or perhaps even the S+ by Resmed? It might be worth trying one of those to see if it’s more accurate for you.
      As you say, the main thing though is to sleep better overall. And for that, there’s plenty of advice around this website. If you haven’t already taken a look, I highly recommend having a look at the sleep hygiene section, where I’ve tried to compile as many good ideas for better sleep as I could.
      Regards
      Ethan

  • In a very short test period, I have noticed that my wrist monitor only seems to tell me I have been in Deep Sleep at times when my wife is not in the bed alongside me.

    Does having a partner in bed with you affect the results if they are restless during the night??

    • Hi Alan
      Thanks for your comment. It depends which sleep monitor you have, but it’s definitely possible. Which one are you using? I think more so with those that have measuring strips on the bed which your partner can touch with their body. Wearable trackers on the wrist I think are harder for a partner to disrupt. But just thinking off the top of my head, I wonder if a partner is very restless, perhaps it could make you more restless if you’re a light sleeper? I have no evidence for that idea though – I’m literally thinking out loud here.
      Regards
      Ethan

  • I am searching for a device to monitor my sleep. Like susan I have narcolepsy but I’m not sure that’s relevant. I feel like there’s a lot of moment where I am awake in the middle of the night and I would like to monitor it in order to give like a sleep diary to my slepcialist. I think the Jawbone UP3 could do that. I could use only at night (seems that the bracelet is not lasting long…). I found on other device the ability to add hashtag to the day to see what factors could improve or damage sleep quality. I’d like to have that but I’m not sure the UP3 does that. Anyhow, it happens also often that I fell asleep while I didn’t plan to do so and start my night too early, often I wake up to put my night cloths as others people would do to pee but I have no idea how long I’ve been sleeping, what does it do to my sleep cycles, etc. I could do a basic diary with a pencil and everything but I know I woul forget at some point to do it.

    • Hi Sabine
      Thanks for your comment. I think the main thing for you then would be one which records sleep automatically, rather than having to tell it yourself that you’re going to bed now. The UP3 will do that. And then one which is particularly good at recording nighttime wakings. Again the UP3 will do that, but then so will many others. The good thing about the UP3 is that it’s very inexpensive compared to many others. So if you want to try one, but don’t want to spend too much, it’s a good choice. However, if you want to get a really detailed view and aren’t interested in activity monitoring during the day, you could perhaps try one of the bed based ones such as the withings aura or beddit.
      Regards
      Ethan

  • My intention is to measure ‘how much’ I sleep rather than ‘the quality’ of my sleep. Meaning assessing depression not insomnia. I currently use the leaf and love it except it only tracks sleep for a defined period, not 24 hrs: 9PM – appears to be 10 AM. To credit the Leaf you have the ability to add naps but I would prefer to not do so everyday.
    So I’m looking at google apps that would allow me to turn it on and off, on and off with nap, on and off with nap, on sleep….. I’m sure there’s something but I feel the market is more geared to insomnia. I’ve a sleep test but it wont definitively track time.

    • Hi Amy
      Thanks for your comment. Many, if not most, sleep trackers allow you to manually input the time you went to sleep and woke up, if that’s what you mean? But if you want one which is constantly on and automatically monitors your sleep 24 hours a day, you can get that also.
      Regards
      Ethan

  • I have been using the Fitbit Flex to monitor sleep . I have narcolepsy and and am on Adderal for daytime sleepiness . Have had a sleep study to confirm . I usually wake at 9 00 and go to bed at 11 30 . My age 68 yrs . I sleep about 5 to 6 hrs , according to Fitbit .. I feel exhausted after waking up until 12 00 or so and then have about 7 hrs of fairly productive time . I guess Fitbit must be right. I wish I did not have to spend so much time in bed to get so few hrs of sleep.

    • Hi Suzanne
      Thanks for your comment. It’s interesting that the Fitbit is reflecting what you think is the case with your sleep – that’s a good thing! Yes, it can be frustrating to be in bed and not sleeping. Have you spoken to a sleep specialist about your sleep schedule? Perhaps they can suggest a routine for you that isn’t so frustrating. It’s in fact very normal to sleep for less hours as we get older. Have a look at this article about how much sleep we need. Sometimes it can be helpful to know that actually we’re getting enough sleep, as often the worry about not sleeping enough makes us feel worse.
      Regards
      Ethan

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