How accurate are sleep trackers?

How accurate are sleep trackers?

the The NHS recommends sleep six to nine hours a night for most adults, which can often be difficult in our busy modern lives. However, even if we hit the hay at a reasonable hour, many of us struggle to sleep, waking up restless. According to research by Mental Health Foundationmore than a third of adults said poor sleep made them more anxious.

Studies also show that women tend to sleep longer than men, but have more nocturnal awakenings and are more likely to develop insomnia. It’s no wonder, then, that many of us turn to sleep tracking devices in an attempt to figure out what’s wrong.

Home sleep tracking

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

Devices typically track sleep duration, sleep quality (i.e. how long you’ve spent in certain stages of sleep) and some even record your weird little nighttime noises (shout out to all sleep talkers ).

“Our sleep is made up of a series of sleep cycles in which the brain goes through different phases of sleep,” says Dr. Verena Senn, neurobiologist and head of sleep research at Emma Sleep.

“We typically cycle through the sleep stages an average of four to six times per night, with a cycle of around 90 minutes. More generally, the stages of sleep can be divided into two different types of sleep: non-REM sleep and REM sleep, named after their eye characteristics of non-rapid eye movement and rapid eye movement. »

Basically, the stages are drowsiness, light sleep, deep sleep, and REM. Research suggests that it’s best to wake up at the end of a cycle (after REM) in order to feel well rested and avoid that drowsy feeling so familiar to many of us.


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

You don’t have a laptop? Place your phone on the mattress and an app like Sleep Cycle will use the device’s accelerometer to detect movement, train you while 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 through your phone’s microphone and speaker to detect your breathing and movement, detecting how long you’ve spent in each sleep phase . The phone doesn’t have to be on the mattress – it can be next to you on the bedside table.


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

“Clinical sleep studies (polysomnographies) look in detail at your movements while you sleep as well as your brain waves, heart rate, breathing, and blood oxygen levels,” says Senn.

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

A study 2018 comparison of 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 benchmark trials, however, it was limited in detecting sleep deep. Studies point out that most commercial technologies are accurate at detecting when you are asleep or awake, but are quite limited otherwise.


“Trackers become problematic because they are inaccurate in several respects,” says the adviser Ruth Micallef.

“Trackers often trick us into overriding our intuition, for example, our tracker says we slept badly but woke up rested. Over-reliance on this type of technology almost gets on our nerves: we stop listening to our own body’s needs and wants and can become unhealthily obsessed with data, causing anxiety.

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

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 Frances Nicolson, another tracker.

“I find it useful to remember what time I actually fell asleep as well as how long I slept. The app also gives a note on how consistent my sleep schedule is, which is useful because I have trouble noticing long-term trends in my habits.

“Sometimes after seeing the data and seeing that I slept well, I feel better, but more often than not I feel like crap, then look and see my sleep data is all over the place, with hours of sleep/ irregular awakenings and only six to seven hours of sleep. I think to myself, ‘Ah, that explains that!’

“The tracker also tells me how much deep sleep I’ve had, but that definitely doesn’t match how I feel. It keeps telling me I haven’t slept deeply enough, yet I’m While the data isn’t entirely accurate, the app has helped me try to get more regularity with my sleep schedule, which can only be a good thing.

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

“If you suffer from sleep issues, collecting data about your sleep can be a great first step in finding a solution because it provides you with information that can help you spot trends in your sleep,” advises Senn.

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