For a few decades we have marveled at the ability of our fitness trackers to record and analyze our every move. They cheer us on when we reach our fitness goals and remind us when we need to get moving. Recently these small devices have done even more to improve our health. They now tell us how we are sleeping. When you wake up in the morning, you can now check the app that is tied to your fitness tracker to see how well you slept. So the question is, how do fitness trackers track sleep? Read on to find out.
What Is a Fitness Tracker?
A fitness tracker, such as a Fitbit or Apple Watch, is a device you wear like a wristwatch and that not only tell you the time, but also track your activity, heart rate, and even your sleeping habits. Most fitness trackers collect data on your movement using a 3-axis sensor called accelerometers. We will discuss this science in more detail later in this article, but basically that device you wear on your wrist tracks your movements in any direction, at any speed, to determine your activity levels throughout the day and night.
Most of the more advanced models track your heart rate. These fitness trackers have small LED lights on the back of the face that, when worn, lay against the skin of your wrist. As your heart pumps blood through your body, these LED lights can sense the volume and movement of blood in your capillaries. From this information, your tracker can calculate your heart rate and even classify it in various levels based on previous data it has collected about your age, activity level, and overall health.
You know your fitness tracker is collecting data about your movement and heart rate, but it is also linking up to a plethora of information from other users. This allows the companies behind the fitness trackers to create algorithms. These algorithms are programs used to analyze large collections of data received from all users, and this data can be studied by scientists and doctors to determine patterns in movement, heart rate, and overall body functions.
Fitness trackers link to your smart phone, and from the application on your phone you can access all of your own activity, heart rate, and sleep habits; but you can also see how you compare to people similar in age, body type, and gender. Overall, these fitness trackers give us a great picture of our general health and habits. This includes your sleeping habits. But how do fitness trackers track sleep?
How Do Fitness Trackers Track Sleep?
While the science behind how our fitness trackers keep up with our movement and heart rate make sense, how do fitness trackers track sleep? The simple answer is that they take your data and compare it to not only your own activity and sleep habits but also all the data of other users.
These algorithms look for patterns in activity and heart rate to determine not only if you are asleep, but in some models, how deep of a sleep. To understand this better, let’s take a detailed look at the three main factors behind the question of “how do fitness trackers track sleep:”
Most fitness trackers use the accelerometers to track our actigraphy. This is a continuous measurement of all the movements detected by the fitness tracker. It is a continual log of every move you make. The longer and more consistent you are at wearing your fitness tracker, the more data it collects and the more accurate its determinations about your sleep/wake patterns will be.
Typically you wear your fitness tracker on your non-dominant hand. Throughout the day and night the accelerometers are tracking every movement, big and small. The device then takes all of this data and analyzes it within the fitness tracker app. One of the greatest benefits of using an actigraphy device is that they can continuously track this information in a natural session instead of a clinical test that can only track a finite amount of data within the confines of an unnatural setting.
Heart Rate Monitors
Most of the newer fitness trackers also include a heart rate monitor. As mentioned, these devices use LED lights to calculate the volume and speed of your blood as it passes through the capillaries of your wrists. The addition of the heart rate monitor within these fitness trackers allow them to determine not only when your body is probably asleep but also what your circadian rhythm (sleep cycles) is like.
Each night our sleep pattern typically follows five different stages of sleep. Each stage takes your body deeper into a sleep/rest mode, with the final and deepest sleep being the last. This stage is known as REM sleep (rapid eye movement), and this is the state where you dream. Heart rate monitors within the fitness tracker are able to analyze your heart rate as you sleep to determine the stage of sleep you are in.
As your body enters the first, lighter sleep cycle, your heart rate will slow down. As it moves through light and then deep sleep, the heart rate becomes even slower. Then, as the body enters a REM cycle, your heart rate becomes more erratic. This is possibly because of the dreams you experience during this stage. This is part of the answer to the question “how do fitness trackers track sleep.”
The ability to collect large amounts of data on a person’s activity and sleep patterns is one thing, but being able to analyze that data and compare it to millions of other people is where the real impact is made. Algorithms are basically a step-by-step method of solving a problem. It will take in the data given to it and follow specific pre-described steps to come up with an answer or solution.
For fitness trackers, experts in the field have programmed algorithms that can collect data from millions of fitness tracker users (depending on the brand you purchase) to determine activity and sleep patterns. From this data they have determined sleep patterns for users based on their age, activity level, heart rate, and gender. The analysis of your own personal data coupled with the trends and patterns established by these algorithms have the ability to give each user an overall picture of how well they are sleeping each night.
How Accurately Does a Fitness Tracker Track Sleep?
For decades the gold standard in tracking someone’s sleep cycles has been the use of a polysomnography. This test not only analyzes movement and heart rate, but breathing and brain waves. Because of the complex equipment and monitoring involved in these tests, they are generally only used within a clinical setting.
These tests are typically performed on patients with known or suspected sleep disorders. Experts agree that if you are comparing the accuracy of a fitness tracker to the tests involved in a polysomnography, the fitness trackers fall short.
Compared to the clinical sleep studies, the science behind using actigraphy alone for determining sleep patterns has a high margin of error. A 2011 study comparing an older version of the Fitbit to a polysomnography found that the Fitbit overestimated sleep in adults by an average of 67 minutes per night but underestimated sleep in children by 109 minutes per night. Michael Scullin, a post-doctorate fellow at Emory University concluded that consumers cannot expect reading to be as accurate when the readings don’t include brainwaves.
Should You Use It?
While the consensus among scientists is that the actigraphy data alone is not the most accurate way to determine sleep length and quality, the addition of the heart rate monitor improves the readings exponentially. In addition, as the algorithms gather more and more data, their overall accuracy will improve. A 2013 study found that with the algorithms these fitness trackers add value to individuals tracking their sleep patterns, especially those adults who have an overall good health with no diagnosed sleep disorders.
As the data keeps being collected, and the science behind these devices improves, the accuracy of their sleep studies improve. A study on sleep parameters performed by Terry Blackwell in 2008 concluded that fitness trackers are an accurate method of collecting sleep when you consider the convenience and affordability of the devices. Four of the best sleep trackers on the market are the Fitbit Inspire HR, Fitbit Versa, Apple Watch Series 4, and the Withings Steel HR Sport.
The consensus is that, though they are not as accurate as a clinical test, our fitness trackers do a good job of making us focus on our sleep health. Using the technology to track our movements and our heart rates, these devices can determine if the average healthy person is getting enough shut eye. If you feel fairly rested each day, and if you aren’t noticing excessive snoring, waking up, or unrestful nights, then tracking your sleep with a fitness tracker, especially one that uses heart rate and actigraphy, is a healthy move.