How Does a Smartwatch Measure Sleep? (Is It Accurate?)

Getting the right amount of sleep improves your health a thousandfold, whether you want to believe it or not.

If you still find yourself constantly anxious, irritable, and unwell despite eating a healthy diet and exercising regularly, you might want to check your sleep habits. Choosing to track sleep will improve your sleep quality and overall health for the better.

But what’s a good way to measure sleep accurately? In the age of advancing machinery, the obvious answer is wearable technology, i.e., smartwatches.

So, how does a smartwatch measure sleep? There are several different values that your smartwatch calculates in order to present accurate sleep data. Let’s look at how the technology behind it works and whether the results are reliable.

1. Sleep Tracking Using Smartwatches

Functionalities of tracking devices aren’t just confined to measuring steps walked and calories burned. Such devices have shown effective measurement of sleep patterns.

Fitbit is arguably one of the best sleep trackers out there, so we will be using it as a reference in most of the sections below. Keep reading to find out how it actually knows when you are asleep.

2. How Does a Smartwatch Detect Sleep?

The “sleep” feature on your smartwatch has been designed to perceive sleep through a combination of the following:

2.1 Body Movements

Try going to bed while wearing your Fitbit on your wrist. When the device detects no body movement for about one hour, it assumes that you are asleep. The sleep feature gets activated and starts tracking accordingly.

Your smartwatch can also detect certain body movements, such as when you roll to your side. Small motion detectors in the device can also understand that it means you are sleeping.

2.2 Heart Rate

At rest, a healthy adult’s average heart rate is anywhere within the range of 60 to 100 beats per minute. When sleeping, it usually drops under 60, and the range can start from 40 and end at 100.

So, when the Fitbit sees that you have minimal movement and a steady, low heart rate, it assumes that you have gone to your sleep stage.

2.3 Environmental Factors

Newer versions of sleep tracker devices can also take environmental factors into consideration when measuring sleep duration. For example, the tracking devices can consider the temperature of the room, whether the light is on or off, if you drank coffee, etc.

As we all have experienced, sleep quality greatly depends on a comfortable surrounding and an absence of caffeine.

3. How a Smartwatch Measures Different Sleep Stages

In a sleep lab, there are various equipment and machines that are used to analyze the different sleep stages. Brain activity alongside heartbeat and blood oxygen levels are obtained and analyzed in such facilities.

But as wearable sleep trackers obviously can’t accommodate so many features, they can still collect enough data to be able to detect the different stages of sleep.

3.1 Light Sleep

As you just start to fall asleep, your heartbeat will slowly go down. Your breathing intervals will also become slower. Such actions indicate that the body is in light sleep mode, and the Fitbit thus tracks this stage.

3.2 Deep Sleep

Your brain enters deep sleep mode after a couple of hours when you are no longer aware of your surroundings. During this time, the rate of your heart gradually goes back to normal. So, the change in the pattern of the heart rate implies that the deep sleep stage is on the go.

3.3 REM Sleep

If you have ever watched someone sleeping and noticed that their eyelids are moving in all directions, it means that the person is in the Rapid Eye Movement (REM) stage of sleep. This is the final of the sleep stages where the brain activity is highest. As the name itself suggests, this is the phase where eye movements are fast and intense. 

Dreams occur during this sleep stage, and the heartbeat becomes faster, and breathing becomes unsteady. So, by noticing these differences in breathing and heart rate throughout several hours, the sleep trackers in smartwatches detect the three stages of sleep accordingly.

4. How Is Wake Time Measured?

Apart from the sleep duration, your device tracks other key sleep stats such as how long you stay awake and also restless sleep.

Sleep tracking devices rely on how much movement you make while sleeping. The accelerometers measure the motion of your body and thus detect that you are awake.

So, as it tracks movement this way, it calculates how long you stay awake versus the time you spend asleep.

5. How Is Sleep Score Measured?

So, you have had a good night’s sleep, but how would you know whether you did well or not? In most sleep trackers, including Fitbit, there is a “Sleep Score” option that shows a result of your sleep quality from the previous night. 

After you fall asleep, the Fitbit on your wrist will collect data from your heart rate, time spent awake, restless sleep, and sleep phases. For Fitbit devices, the total score is an aggregate of three separate items:

5.1 Duration

This is your calculated sleep time against the time you were awake. The score will be higher based on how long you have slept overall.

5.2 Quality

Quality of sleep is measured by how long you were in the deep sleep and REM phases. A greater score for this item means longer periods in these two stages of sleep.

5.3 Restoration

The restoration is tracked according to your restless or disrupted sleep period and whether your heart rate is over 50-60 beats per minute. If you were restless for too long and your heart rate was higher than the baseline value, the sleep score decreases.

An excellent sleep score is anywhere from 90 to 100. A score less than 60 is troublesome and if this continues, seeking medical assistance is of critical importance. Generally, healthy people get a sleep score within the range of 70 to 85.

6. How to View All Sleep Logs

After your sleep tracker smartwatch did its job of measuring sleep, it’s time for you to look at the results. When syncing your device, all the logs, including details, trends, and summaries of the sleep information, can be accessed from your smartphone app.

Even if you don’t have a specific device or smartwatch, there are a lot of good smartphone apps you can install to track sleep. The general functionality is the same, where it tracks movement with the small motion detectors in your phone.

As phones are nonwearable sleep trackers, you will need to keep them close to your body while you sleep so that they can measure movement properly.

7. Accuracy in Detecting Sleep Apnea

As implied earlier, proper sleep time is essential for a healthy lifestyle. Even if it is to not remain healthy, getting enough sleep is needed for you to properly function in your daily life.

How much you sleep can help diagnose serious sleep disorders you may have and help catch them early. Most sleep tracking devices are generally accurate in terms of exploring sleep cycles and wake times.

However, to find symptoms of sleep apnea and other sleep disorders, it is necessary to visit a sleep clinic to get proper tests done. Even though there are so many sleep trackers on the market, they still can’t read brain waves.

If you are concerned about any serious sleep disorders you might have, it is advised that you check with a specialist. The smartwatch might give you a reading for sleep, but not for the treatment of sleep apnea or other serious illnesses. 

8. So, Is It Accurate?

As mentioned earlier, brain waves play a huge role in understanding sleep cycle health. And this is something your smartwatch cannot provide at the moment. You will no doubt get a more accurate reading through various tests in a sleep clinic.

A lot of sleep studies have been done, but it is still safe to say that sleep researchers are okay with you relying on your fitness tracker to measure sleep. Estimating sleep time well is the first step to getting enough sleep, which leads to good health and well-being.

Even if smartwatches will not show whether you have a sleep disorder, it still helps to estimate sleep time and sleep quantity, which is still a huge step. Once you get into the habit of tracking your sleep cycle regularly, adjusting your schedules to get a good night’s sleep will become easier, and you will stay healthy and happy.