How Do Sleep Trackers Work?
“If you can’t measure it, you can’t improve it.”
This quote, attributed to management consultant Peter Drucker, now applies to so much more than just business performance. In virtually all aspects of life, we are collecting and interpreting more data than ever before.
For decades, sleep data came from subjective surveys or in-depth testing in a sleep clinic. In recent years, though, a whole slate of sleep trackers have become available to provide hard data without having to go to a lab.
The rapid rise of sleep trackers has raised many questions for consumers relating to how they work, how accurate they are, and which are the best.
This guide helps you understand what you need to know. It reviews the types of sleep trackers that are available, what information they collect, how they analyze that data, their strengths and limitations, and how to make the most of them.
What Types of Sleep Trackers Are Available?
Sleep trackers can be organized into four main categories.
Wearable sleep trackers are kept directly on the body and move with you.
The most well-known wearables are general activity trackers such as those made by Fitbit and Jawbone. These devices can collect information about daily steps, exercise, and sleep. Most often they are worn around the wrist.
A much less common type of wearable is sleep-tracking headgear. Because they may not be comfortable and are not designed for all-day use, they are not nearly as popular as other wearables.
These sleep trackers are thin pieces of fabric with sensors embedded in them that are placed underneath your sheet. Because of their proximity to your body, they can collect data about your sleep.
Standalone / Contactless Sensors
These sleep trackers do not have to be worn or be in contact with your body. Normally they are kept on a nightstand within a few feet of your bed, allowing sensors to take in data about your sleep and bedroom environment.
Mobile apps use data collected by your phone to generate sleep metrics. These apps may require you to have the phone very close to your body in order to receive accurate information.
Some other sleep tracking devices, such as wearables or standalone sensors, may couple with smartphone apps to help you review your sleep data.
Understanding Sleep Tracking Technology
The goal of any sleep tracker is to provide accurate, meaningful, and usable information. Though we think about sleep tracking as one thing, it is really made up of two separate and important components: first collecting data and then analyzing that information.
How Do Sleep Trackers Collect Data?
The way that data is collected by a sleep tracker depends on a number of factors including the type of tracker and its built-in technology.
The following sections provide an overview of the most common data collected by sleep trackers and the methods used to acquire it.
For any specific product, you can review its technical specifications to understand how it tracks your sleep.
Most sleep trackers keep tabs on your movement. Because we move so much less during sleep than when we’re awake, movement data can play a big part in analyzing sleep.
An accelerometer is a tool that keeps track of how quickly you move. It can be found in most smartphones and wearable sleep trackers. Actigraphy is the evaluation of data from an accelerometer over a period of time.
Sonar, which uses sound waves, or other bio-motion sensors can be used to track movement and are used in many standalone sleep trackers and some mobile sleep tracking apps.
In addition to major motions, such as getting in and out of bed, some trackers try to detail your smaller movements in the night to help understand your progression through sleep stages or to evaluate the rise and fall of your chest as you breath.
Many wearable sleep trackers are used for exercise tracking and have built-in heart rate monitors. As they are frequently worn on the wrist, these trackers can take a person’s pulse with a high level of accuracy using optical heart rate monitoring.
Optical heart rate monitoring detects information about blood flow based on how a specific light reflects off your skin. Smartphone sleep tracking apps apply this same approach but with a greater distance from the body than a wearable tracker. This may reduce accuracy, but hard data comparing optical heart rate monitoring in wearables and mobile phones is lacking.
Non-wearables such as trackers that go under your sheets can measure heart rate using ballistocardiography (BCG). Every time the heart beats, there are extremely small motions within the body that can be detected and analyzed. BCG tracks these motions to evaluate heart rate.
Noise tracking can be useful to learn both about your sleep and your sleep setting.
Some sleep trackers have noise sensors that can detect, and in some cases record, noise through the night. This information may be useful for determining if you have issues with snoring, including breathing issues indicative of sleep apnea.
This kind of sensor can help make it clear if you are dealing with potential disruptions to sleep from a noisy bedroom. Clear indications of noise problems may point toward the need for earplugs or use of a white noise machine or app.
Temperature is an important part of our sleep environment, and some sleep trackers have a sensor that observes how hot or cold your bedroom is at night.
Some wearable sleep trackers have thermometers that can provide information about your body temperature while you sleep.
How Do Sleep Trackers Analyze Data?
Sleep trackers generally don’t just dump all of this raw data at your feet. Instead, they use algorithms and complicated analysis to make sense of the data. They then have an interface, such as a desktop or smartphone dashboard, that delivers this information to you in a simple and understandable format.
It is important to remember that these algorithms, though powerful, are not perfect. Often, the more types of data they have to work with, the more they can integrate it to draw conclusions. Even then, general algorithms can have a margin for error when applied to any specific person’s situation.
Every sleep tracker has its own set of metrics that it provides, but some of the most common include:
Total Time Asleep
Normally calculated using motion data, this metric is a simple summary of the sheer quantity of time you spent sleeping.
This metric is based on how soundly it appears that you slept based on the data collected. For example, if you tossed and turned constantly throughout the night, the tracker may determine that your overall sleep quality was lower.
Some sleep trackers provide an estimate of the amount of time spent in each sleep stage.
When we sleep, we progress through different stages of the sleep cycle. The first three stages are part of non-rapid eye movement (NREM) sleep, and the fourth stage is REM sleep. A full sleep cycle usually lasts 90-120 minutes.
Stage 1 is the lightest sleep during which we are starting to doze but can be easily awakened. Stage 2 is deeper and involves slowing of the heart rate and lowering of body temperature. Stage 3 involves slower brain waves and deep, restorative sleep. The REM stage involves higher levels of brain activity that are similar to when we are awake.
Proceeding through each of these stages at a regular rate can be an indication that the mind and body are getting the rest that they need, and some sleep trackers claim to follow your time in each sleep stage based on your movement, heart rate, and other data.
This is a visualization of brain waves during different stages of sleep. Many sleep trackers attempt to track the user's progression through these stages in order to measure their quality of sleep.
A sleep tracker may offer a metric based on the quality of your sleep setting. This may tell you if there are regular disruptions that could upset your sleep.
Other Health Metrics
Wearable sleep trackers commonly provide other data about wellness in addition to sleep. These metrics may be related to exercise, diet, heart rate, or stress. Some of these metrics require you to input data beyond what the tracker collects independently.
How is a Sleep Study Different From Sleep Tracking?
The current gold standard for sleep data is polysomnography. Polysomnography is usually conducted by staying overnight in a sleep clinic. In the clinic, sensors are attached to various parts of the body to gather detailed information.
The powerful and refined equipment used in polysomnography gives more in-depth detail about your sleep. For example, polysomnography usually involves electrocardiography (ECG) to monitor your heart, electroencephalography (EEG) to track your brain waves, and electrooculography (EOG) to follow your eye movement. Your breathing and oxygen levels are monitored, and all of this information can be reviewed by a trained medical professional.
In addition, because it is conducted in a specialized lab, the overall setting is more controlled and the equipment better calibrated, and this allows firmer conclusions to be drawn about the characteristics of your sleep.
Of course, a sleep study is just a snapshot of your sleep on the night or nights that you are in the lab. It is not practical nor cost-effective to try to use all of these tools in the home setting. For this reason, while a sleep study offers more reliable and detailed results, it may not be available to everyone.
Should I Use a Sleep Tracker?
There is no simple yes or no answer to whether or not you should use a sleep tracker. It’s a decision you need to make for yourself based on if you think there would be any potential benefits to tracking your sleep. Often, sleep tracking comes along with tracking other things like movement and exercise.
How Accurate Are Sleep Trackers?
Research in adults and adolescents has pointed to the general accuracy of wearable sleep trackers, especially in measuring basic metrics like total sleep time. However, the accuracy of sleep trackers is still up for considerable debate.
Because many of these products are relatively new to the market, long-term data is often lacking. In addition, companies continue to innovate both in their technology and in their algorithms, which is good for consumers but makes it hard for researchers to stay current and evaluate the accuracy of the latest releases.
The diversity of sleep trackers on the market complicates an assessment of their accuracy. Research supporting or negating the accuracy of any one tracker may not be applicable to a tracker that uses different technology or methods of data analysis.
All of this is to say that it is best to be realistic about the information being provided by a sleep tracker. For most people, trackers can offer interesting data and an opportunity to reflect on improving sleep hygiene. At the same time, remember that they are not infallible because there can be errors in the data or in data interpretation.
What is the Best Type of Sleep Tracker?
There is no universal type of sleep tracker that is best for everyone. Each type has benefits and downsides.
|Wearable||Studies point to overall accuracy; often track heart rate and other health metrics; extensive range of brands and models||Have to be kept on the body; more advanced models can be expensive|
|Under Sheet||Easy to set up; normally collects data with ballistocardiography||Data collection may be inaccurate for couples; may affect feel of bed|
|Standalone||Requires no contact with body; sensors can add data about sleep environment||Must be kept close to the bed; may be inaccurate for people who share a bed|
|Smartphone App||Low cost; requires no extra equipment||More limited data collection potential; have to keep phone close to bed|
For specific recommendations and our top picks, check out our guide, the Best Sleep Trackers.
What Are the Limitations of Sleep Trackers?
Sleep trackers can deliver intriguing information about your sleep, and it’s natural to want to take advantage of this information. However, there are some limitations to be aware of with sleep tracking.
- Sharing a bed may compromise data: for all the sleep trackers that aren’t worn directly on the body, sensors may not be able to distinguish between individuals if you share the bed with a partner, pets, or your children.
- Proximity is important: most sleep trackers require being very close to the body to function. Obviously, wearables have to be kept attached, and standalone trackers and mobile apps normally need to be within a few feet to collect good data.
- Algorithms are not perfect: trackers use software to draw conclusions from the data that is collected. These algorithms are carefully designed, but for any given individual, they may not perfectly capture the dynamics of their sleep.
How Can I Make the Most of a Sleep Tracker?
If you are about to buy a sleep tracker or if you are already using one, there are some tips to help you get the most out of your device.
Standardize How You Use It
Data collection is a scientific process, and part of that process is eliminating variables that can confuse the results. If you are constantly adjusting your sleep tracker - for example: how you wear it or how far it is from your bed - it may create noise in the data.
In addition, some people may only want to use a sleep tracker when sleep problems arise. Doing this, though, means that you don’t have a baseline for comparison. Use your sleep tracker consistently so that you can make meaningful comparisons.
Consider The Big Picture
Treat the information from a sleep tracker as one data point. Also consider your own assessment of your sleep. If you are waking up refreshed but have a sleep tracker telling you that you have terrible sleep quality, don’t just assume the sleep tracker is right.
Another part of considering the big picture is not overreacting to the results from any short period of time. You’ll be able to know a lot more about your sleep if you monitor results over a period of weeks or months, especially if you’re making changes to your sleep routine or setting.
Alongside your sleep tracker, it may be helpful to keep a sleep journal in which you write down what time you went to bed each night and what time you woke up, plus any notes about the perceived quality of your sleep or sleep disruptions. This provides a second point of data that you can use to compare with the information from your sleep tracker.
In general, look to the sleep tracker for feedback about trends in your sleep rather than for hard-and-fast, infallible metrics.
Talk to Your Doctor
If you have ongoing sleep problems, your best bet is to talk with a doctor. A sleep tracker may give you general data, but your doctor can help get to the bottom of your sleep issues and can refer you to a sleep clinic for in-depth polysomnography if necessary.
How Does a Smart Alarm Work?
One feature of some sleep trackers is a smart alarm. The goal of this alarm is to wake you up at the optimal time during your sleep cycle.
That optimal time is generally during the first phase of sleep when you are most easily awoken. A tracker may evaluate your heart rate, temperature, or movement to assess your sleep stage and wake you up based on that stage, within a preset time window, rather than at a specific hour of the morning.
Should I Do a Sleep Study?
Because a sleep study is an involved process that is usually done overnight in a special clinic, it typically requires a referral from a doctor. The doctor makes that referral based on whether they think that a sleep study will offer useful information in diagnosing the cause of your sleep problems.
If you’re worried enough about your sleep to be thinking about a sleep study, then it makes sense to talk with your doctor who can help you to understand and diagnose your sleep situation.