How Do Sleep Trackers Work?

Thanks to a happy confluence of stressful days, sleepless nights, and a growing awareness of the importance of sleep, sleep tracking is becoming increasingly popular.

Sleep trackers tout many benefits, such as helping you ensure you get a sufficient amount of sleep each night, analyzing how much time you spend in each stage of sleep, and even making you aware of potential sleep issues like snoring and sleep apnea.

But have you ever wondered how they do it, exactly? What kind of technology are these sleep trackers relying on that allows them to make such claims?

The accuracy of sleep trackers remains a hot debate among the scientific community. Nevertheless, many agree that while their ability to distinguish among various stages of sleep is dubious, they can tell whether you’re asleep or awake.

Let’s take a look at how the best sleep trackers do it.

To understand how sleep trackers work, it’s helpful to first break down what they track, before getting into how they track it.

Common sleep tracking features

In their most basic form, sleep trackers track whether you’re asleep, and how much time you spend asleep. Typically, however, sleep trackers often measure several aspects of your sleep:

• The amount of time you spend asleep. The sleep tracker will analyze whether you’re asleep or awake, and chart the total amount of time you spend asleep.
• The quality of your sleep. Sleep trackers will analyze any movement during your sleep, to assess whether your sleep is restful or restless.
• How much time you spend in each stage of sleep. Some sleep trackers claim to track how much time you spend in each stage of sleep. Using this information, they’ll schedule your alarm to go off at a time when you’re likely to be in light sleep, making it easier for you to wake up.
• The quality of your sleep environment. Sleep trackers may analyze other factors that can impact the quality of your sleep, such as the light or temperature in your bedroom.
• Sleep-related health metrics. Much of what you do during the day can actually impact your sleep, such as your diet and exercise. Throughout the day, some sleep trackers may track your physical fitness, heart rate, food consumption, and stress levels to analyze how your daily life may be affecting your sleep.

How do sleep trackers know when you’re asleep?

The main way sleep trackers know when you’re asleep is through your body movement. While we switch positions during sleep and may twitch some during our dreams, we’re generally a whole lot more still when we’re asleep than when we’re awake. This makes body movement a useful indicator for measuring your sleep vs. awake status and your overall time asleep.

Depending on the type of sleep tracker you have, it will measure your body movement through either actigraphy or accelerometry. Most wearable sleep trackers leverage actigraphy, where an actigraph sensor is worn around your wrist. Many smartphone sleep tracking apps, on the other hand, rely on the phone’s accelerometer to measure your body movement and assess whether you’re asleep or awake.

In addition to your body movement, sleep trackers may also measure the following personal and environmental factors to determine when you’re asleep or awake:

Noise

Sleep trackers may include a microphone to capture noise from the room or your body. The lack of or presence of noise can help the device determine whether you’re asleep, and provide insight into the quality of your sleep.

The microphone may measure your respiration, in order to detect any snoring. Snoring can interfere with the quality of your sleep. Some sleep trackers will even record your snoring so you can assess whether it was snoring, or whether you’re making gasps or choking sounds that may be indicative of a more serious sleep disorder like sleep apnea.

The microphone may also capture ambient noise, to help you assess the suitability of your bedroom for sleep. If the room is very noisy from atmospheric elements like a TV, sleep trackers may suggest to you that the noise may be interfering with your sleep. (A quiet, dark room is the ideal sleep environment.)

Your heart rate

Sleep trackers with a heart rate tracking feature can measure your variations in heart rate (or lack thereof) to assess when you’re asleep or awake, and make a more informed assessment of when you’re in REM sleep.

When you fall asleep, your heart rate slows down and blood pressure both lower. Your heart rate stays fairly steady through the duration of your sleep—with the exception of REM sleep. In REM sleep, as when you are awake, you can experience an increase in heart rate and overall more variations in your heart rate.

Wearable sleep trackers use personal heart rate monitor (HRM) technology to measure your heart rate, while mattress sleep trackers use ballistocardiography (BCG). Ballistocardiography is an alternative way of measuring your heart rate by observing the repetitive motions of your body, as opposed to being attached to your skin through a HRM sensor. The mattress strip in mattress sleep trackers may include a BCG sensor to measure how your body movement affects the movement of the strip.

Temperature

Sleep trackers with a thermostat feature may monitor your bedroom temperature to assess its effect on your overall sleep quality.

The ideal temperature for sleep is in the low to mid 60 degrees Fahrenheit for adults, and a bit warmer for children. If your sleep tracker observes a temperature outside of that range, it may suggest you adjust the thermostat for better sleep.

Motion sensors

Some sleep trackers do not use actigraphy at all, as they’re not worn on your person or in your mattress. These no-contact sleep tracking devices still find a way to observe movement, though.

No-contact sleep trackers typically sit on a nightstand in your bedroom and use environmental factors to determine whether you’re asleep or awake. For instance, they’ll have light and temperature sensors to detect when bedtime approaches. Then, they’ll rely on motion sensors to observe when you enter the room, and when movement stops in the room—indicating you’ve fallen asleep.

One of the main differences between personal sleep trackers and a professional sleep study performed in a lab is the wealth of measurement used. Personal sleep trackers rely primarily on actigraphy or accelerometry. They may also measure noise and heart rate to track your sleep, but the core measurement is focused on your body movement.

When you undergo a sleep study in a sleep clinic, on the other hand, the sleep technicians monitor a host of elements related to your sleep, including your body and eye movement, your brain waves, your heart rate, your blood oxygen levels, and your respiration. In addition to actigraphy, they also employ the following modes of measurement:

• Electrooculography (EOG), which measures your eye movement
• Electrocardiography (ECG), which measures your heart rate
• Electroencephalography (EEG), which monitors your brain waves

This last one, regarding your brain waves, is the biggest distinction between the quality of sleep tracking provided by sleep trackers versus polysomnograms performed in a sleep lab. To fully and accurately assess the quality of your sleep, your brain waves must be monitored.

To understand why that is, let’s quickly review how sleep works. As you sleep, your brain waves change as you cycle through the different stages of sleep.

• During stage 1 of non-REM (NREM) sleep, your eyes move more slowly and your muscle tone relaxes. Your brain waves also begin to slow.
• As you enter stage 2 of light sleep, your eyes stop rolling. Your brain waves continue slowing down, but they also experience distinct bursts of activity called sleep spindles, and they display sleep structures called K complexes.
• During stage 3 of sleep, known as deep sleep or slow-wave sleep, your brain waves slow further to delta waves.
• Finally, in REM sleep, your brain waves become active again, mimicking activity similar to when you are awake. REM is short for rapid eye movement. During this stage, your eyes move rapidly from side to side.

You’ll cycle through all four of these stages several times a night, spending more time in REM sleep as the night goes on.

As you can see, your brain waves experience distinct changes during the stages of sleep. Your eye movement also changes during sleep. Meanwhile, your body stays relatively still. However, your brain waves and eye movement can only be accurately measured by EEG and EOG, respectively, neither of which are includes in personal sleep tracking devices.

To measure these in a sleep study, along with your heart rate, muscle movement, and respiration, sleep technicians attach sensors and electrodes to your head and person. That’s a lot more involved than a simple wearable on your wrist or a fabric strip on your mattress.

When it comes to sleep quantity, most commercial sleep trackers can measure the amount of time you spent asleep fairly accurately. They use actigraphy to track your sleep, just like polysomnograms do. However, actigraphy doesn’t provide the full picture. It’s just one of many methods polysomnograms use to measure your sleep, along with electroencephalogy, electrooculography, and more. To fully track sleep quantity and quality, your brain waves must be included in the measurement.

Comparing sleep trackers vs. polysomnograms: what do they measure?

Some sleep tracker devices are beginning to include more methods of measurement, such as the Philips SmartSleep Deep Sleep headband. Worn over your head while you sleep, the headband includes sensors to detect when you’re in deep sleep. It also includes headphones which emit noise to help improve your sleep. However, this technology is designed to be used by people with insomnia who are consistently sleeping fewer than 7 hours a night, not for the average person simply looking to track their sleep.

While many people think of a smartwatch or fitness wearable when they think of a sleep tracker, sleep tracking devices actually come in a few different forms. Depending on the type of sleep tracker you use, it may measure different facets of your sleep.

The biggest distinction between sleep tracking devices is the wearable/non-wearable divide.

• Wearable devices are worn on your person, typically as a watch or band around your wrist or as pendant you wear around your neck. These include Fitbits, Jawbone, and Apple Watch.
• Non-wearable devices, unsurprisingly, are not attached to your person. These are typically a thin fabric strip that lies under your mattress or a cover that lies above it. Alternately, it could be a no-contact device that sits on your nightstand table, or an app on your smartphone.

How your particular sleep tracker works depends on the technology involved. Typically, however, a wearable sleep tracker simply requires you to keep wearing it when you go to bed. The device will then rely on actigraphy to know when you’re asleep, and the heart rate sensor (if included) will monitor your heart rate.

As for no-contact sleep trackers, you can get straight into bed. If you’re using a mattress sleep tracker, the sensors in the strip will begin working once it senses your body weight. If you’re using an app on your smartphone, the instructions will vary by app, but you’ll typically just need to keep it in the bed with you and turn on the app.

Many sleep trackers include a “smart alarm” feature, where the device rouses you through vibration during your lightest stage of sleep. You program in a window for your ideal wakeup time, such as between 7 and 7:30am. Then, the device will wake you up sometime during that time frame, at a point when you’re most likely to be in light sleep.

As we’ve mentioned already, the assumption that these devices can reliably “know” whether or not you’re in light sleep is still debatable, since they’re unable to measure your brainwaves.

However, they can make a qualified assumption. When we sleep, we cycle through all four stages of sleep four to five times a night, with REM sleep taking up an increasing amount of each subsequent cycle. We spend about two hours in the first sleep cycle, and ninety minutes in each of the remaining cycles. Based on the time your sleep tracker assumes you fell asleep, in addition to input from your heart rate, it can make a guess about when you might be in a light stage of sleep during your last sleep cycle.

If you’re the kind of person who benefits from knowing you’re being monitored to reach your goals (or if you’re simply interested in seeing the metrics), a sleep tracker may be worth it. For the most part, these devices reliably know when you’re asleep and how much time you spend asleep.

The recommended amount of sleep is 7 to 7.5 hours for adults per night. However, studies show people often overestimate how much sleep they’re getting. This is one area where a sleep tracker comes in handy, as you can use it to find out if you’re getting enough sleep per night. Also, if your sleep tracker includes a mic, you can use yours to find out if you’re snoring at night.

On the whole, sleep trackers can help you do a better job monitoring when you go to sleep, when you wake up, and how much sleep you get overall. However, you may want to take their graphs about your various sleep stages with a grain of salt.

Also, if you’re the kind of person who gets stressed out from monitoring your health so closely, a sleep tracker may do you more harm than good. Some people can even become obsessed with monitoring their sleep, a condition known as orthorexia.

Want to learn more about sleep trackers? Check out the articles below.

• Best Sleep Trackers – 2019 Reviews and Buyer’s Guide
• Guide to Sleep Tracking
• Top Sleep Apps
• What Happens During a Sleep Study?
• Is Sleep Tracking Bad for Your Health?