Are sleep trackers accurate

Sleep tips: should you use your fitness watch to track sleep, and how accurate are they?

Posted by for Wellbeing

It’s common knowledge that a good night’s sleep is imperative for a good day’s living, so are sleep trackers the answer to our prayers? Sleep-deprived writer and runner Lisa Bowman explores sleep tech and all its benefits… and drawbacks. 

The NHS recommends getting six to nine hours of sleep a night for most adults, which can often be a struggle in our busy, modern lives. However, even if we hit the hay at a reasonable hour, many of us struggle to sleep, waking up feeling unrested. According to research from the Mental Health Foundation, more than a third of adults said that sleeping poorly had made them feel more anxious. 

Studies also show that women tend to sleep for longer than men, but have more nighttime awakenings, and are more likely to develop insomnia. So, it’s no wonder that many of us are turning to sleep tracking devices in an attempt to figure out what’s going wrong.

Tracking sleep at home

At home, sleep can be tracked via a wearable (like a Fitbit or Apple Watch) or with a smartphone app (like SleepScore or Sleep Cycle).

Devices commonly track length of time slept, quality of sleep (ie how long you spent in certain sleep stages) and some even record your strange little night-time noises (shout out to all the sleep talkers).

“Our sleep is composed of a series of sleep cycles during which the brain moves through different sleep stages,” explains Dr Verena Senn, neurobiologist and head of sleep research at Emma Sleep.

“We usually cycle through the sleep stages on average four to six times per night, with one cycle lasting approximately 90 minutes. More generally, the sleep stages can be divided into two different types of sleep: non-REM and REM sleep, named after their ocular features of non-rapid eye movement and rapid eye movement.” 

Essentially, the stages are dozing off, light sleep, deep sleep and REM. Research suggests it’s best to wake up at the end of a cycle (after REM) in order to feel properly rested and avoid that groggy feeling many of us know so well. 


Fitbit estimates your sleep stages using a combination of your movement and heart-rate patterns – if you haven’t moved for an hour, it will assume you’re asleep. Once you’re asleep, the device will compare heart-rate changes to estimate which sleep stage you’re in.

Don’t own a wearable? Pop your phone on the mattress and an app like Sleep Cycle will use the device’s accelerometer to detect movement, working out when you’re awake and in light/deep sleep. It also uses your phone’s microphone to pick up sounds.

SleepScore uses sonar technology to assess your sleep, sending and receiving silent signals via your phone’s microphone and speaker to sense your breathing and movement, detecting how long you spent in each sleep stage. It doesn’t require the phone to be on the mattress – it can be next to you on the bedside table.  


How does this all compare to sleep tracking in a clinical setting? For starters, if you share a bed with a pet or human, your home data will be way off. Data is also limited when movement is used to detect sleep, as you can be awake but lying still. Plus, apps that use accelerometer technology may be affected by the softness of your mattress.

“Clinical sleep studies (polysomnographies) take a comprehensive look at your movements while you sleep as well as brain waves, heart rate, breathing and the oxygen level in your blood,” explains Senn.

“In contrast, commercial tracking apps can only ever make an educated guess about your sleep stages and won’t give you a full understanding of your sleep.”  

A 2018 study comparing the Fitbit Charge 2 to polysomnography technology found it showed promise in detecting when users were asleep, awake and in REM sleep, but compared to gold-standard trials, however, it was limited in detecting deep sleep. Studies highlight that most commercial technology is accurate in detecting when you’re asleep or awake, but is fairly limited otherwise. 


“Trackers become problematic because they are imprecise in a number of ways,” says counsellor Ruth Micallef.

“Trackers often push us to override our intuition, eg our tracker says we slept poorly but we woke up refreshed. Relying too heavily on this type of technology almost gaslights us: we stop listening to the needs and wants of our own bodies and can become unhealthily fixated on data, causing anxiety.”

I started using Sleep Cycle years ago when I discovered it had a smart alarm that wakes you after a sleep cycle to help you feel more refreshed. However, I soon realised I was using the app as an excuse to go to bed later, thinking I could function on less sleep as long as I woke up after an REM stage.

But not everyone uses their tracker as a scapegoat. “I have a Withings watch and use their HealthMate app, which I’ve been using for about a year,” says fellow tracker Frances Nicolson.

“I find it helpful to remember what time I actually fell asleep as well as how long I slept for. The app also gives a score for how regular my sleep schedule is, which is useful as I’m terrible at noticing long-term trends in my habits.

“Sometimes after I see the data and see that I’ve had a good sleep, I’ll feel better, but more often than not, I’ll feel rubbish, then look and see my sleep data is all over the place, with irregular sleep/wake times and only six to seven hours sleep. I’m like, ‘Ah, that explains it!’

“The tracker also tells me how much deep sleep I’ve had, but this definitely doesn’t correlate to how I feel. It keeps telling me that I haven’t had enough deep sleep, yet I feel great. While the data may not be entirely accurate, the app has helped me try to have more regularity with my sleep schedule, which can only be a good thing.” 

It’s important to remember that tracking apps should be combined with good sleep hygiene, eg sleeping and waking at a regular time (yes, even on the weekends) and limiting screen use leading up to bedtime.

“If you’re someone suffering from sleep issues, collecting data about your sleeping can be a great first step in finding a solution as they provide you with information that can help you spot trends in your sleeping,” advises Senn.

“Your best bet is to listen to your body and be conscious of how your habits and daily routine impact your sleep, and raise any concerns with your GP.” 

For more sleep advice, check out the Strong Women Training Club.

Images: Getty

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