The simple answer to this is comprised of two components: Heart rate monitoring and an accelerometer.
Heart rate monitoring is a vital feature that fitness trackers have and it tracks your heart rate throughout the day and during the various kinds of activities you do.
They can, with statistics, tell which of your heart rates indicate sleep, which indicate being still, which indicate mild activity, and which indicate vigorous activity. Using this information, a sleep tracker can tell when you’re asleep or not.
Some trackers even come with automatic sports recognition which can easily tell when you’re doing different activities like walking, running, cycling, dancing, playing sports, or swimming.
An accelerometer is a sensor that measures the acceleration of your moment throughout the day, to indicate different activity levels. They can also indicate the number of steps you took and even the steps you’ve climbed if the tracker has an altimeter.
Both of heart rate monitor and accelerometer combined together work to give you sleep monitoring that is as accurate as possible.
How does your fitness tracker know when you’re asleep?
A question that asks itself, however, is whether sleep tracking feature in those fitness trackers is reliable and how accurate the information is.
Well, for a regular everyday consumer, the answer would be that they give you a good idea and a general background on the quality of your sleep. Although it is not a hundred percent accurate.
However, if you’re a person with a medical condition or a sleep disorder, these fitness trackers may not be the most accurate devices to use. This applies especially to the tracking of sleep stages which is something some of the fitness trackers’ manufacturers claim that their trackers do.
To monitor the stages of your sleep, experts and professionals use brain waves and eye movements to give an accurate estimation of how much time you spend in each stage of sleep, broken down into light, deep, and REM sleep.
This is the most accurate way and anything else wouldn’t give you reliable information that you can use for treatment or improvement with your sleeping disorder.
Compared with the polysomnography –a scientific evaluation of a person’s sleep-, the fitness trackers were easy to trick due to their great dependence on the accelerometer.
How Does Sleep Tracking On Fitness Trackers Work? | Bottom Line
So the bottom line is, if you’re using the tracker to give you a general idea on the quality of your sleep, then there are no worries and you’ll get a good background on how well you sleep.