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How Does Fitbit Know You Are Sleeping?

How does Fitbit know you are sleeping? A question that lingers in the minds of many. The thing is, we too were wondering the same thing and thought it would serve as an excellent linchpin for an article. In the process of writing the article, we would not only satiate our own curiosity but might help you understand as well.

Despite their deceptively small sizes and simple designs, these fitness trackers are powerful devices packed with different types of sensors. After gathering data through the sensors, these devices transfer the data onto their apps, which have a number of algorithms that understand our physiological patterns.

These algorithms can go through the raw data and is able to tell when we’re awake or asleep. When we’re talking or running. The computer does this through pattern recognition, as it already knows what to look for in the data.

The thing is, our bodies are like machines as well. The heart is like a fuel pump. It pumps blood throughout the body and automatically adjusts the flow of blood based on what we’re doing. When we’re sitting on a computer working, our heart is working in a certain way that is identifiable.

If we are out running, our heart rate would naturally accelerate to compensate for the increased physical exertion. And when we’re asleep, our bodies naturally require less energy input and thus, our heart slows down along with other organs in the body.

Our entire bodies work like an interconnected machine compensating for different parts of the body and adjusting to our physical needs.

All of these physiological changes and patterns are identifiable. In fact, researchers and scientist have a fairly decent understanding of how our bodies work.

And all you would need is a mathematician and computer engineer to create a software capable of identifying these physiological patterns in the body and be able to tell when we’re asleep or when we’re hiking.

So that’s the gist of it. But it does really answer the question of how does Fitbit know you are sleeping, right? Let’s get into the specifics.

How Does Fitbit Know You Are Sleeping?

So we got a bit of our nerd on and went through some really boring research papers on how these tiny devices work so well. Here is what we’re learned.


All activity trackers including Fitbit use a sensor called accelerometers to track our physical movements. These tiny sensors can track the direction and speed of our motion.

This is basically how they track all of our physical activity during the day and this is also how they are able to tell when we’re asleep.

Accelerometers are motion sensors that measure acceleration. Acceleration is the change in sleep or velocity in terms of time or divided by time.

For example, if a stationary car was to start accelerating from 0 MPH to 60 MPH in six seconds, it would mean the car was accelerating at 10 MPH per second.


Actigraphy is a term used to describe the analysis of physical movements during sleep. It is done is sleep studies and it utilizes a device called, an actigraph, which is also a device worn on the wrist. A Fitbit might not be an actigraph but it is capable of performing actigraphy.

When you go to sleep with your Fitbit on your wrist, it is still recording data on your physical movements. Since you’re not really moving around as you would be moving if you were awake.

There is a considerable difference between the data your device is constantly recording. You can open the graphs in the app and take a look.

You will, without a doubt, see a visibly recognizable difference in the data being recorded on the graph. Without having any understanding of what you’re looking at, you will be able to tell the difference at a glance.

If you can see the difference with your bare eyes, computers will be able to tell the difference without any real effort.

How Accurate is Actigraphy?

Now that we understand how activity trackers know we’re sleeping, we can look at some of the limitations of actigraphy. Since a Fitbit is only using our physical movements to understand our physical state, there are some limitations to take into consideration.

If you are lying in bed, maybe watching a tv-show or a movie, there is a real possibility that your activity tracker might mistakenly think that you’re asleep. This is especially true if you’re not moving around much and intently watching something.

This is the biggest downside of actigraphy. Periods of no movements will almost always be recorded as sleep time. Due to his, actigraphy generally has a really high margin or error when it comes to sleep quality and sleep time.

Activity trackers will almost always overestimate or underestimate the time you’ve slept sleeping. Some Fitbit models have two sleep modes, normal and sensitive sleep modes.

While in regular sleep mode, the Fitbit will always overestimate your sleeping time and underestimate it while in sensitive-mode.

There is also the possibility of user-based error. If you don’t turn the sleep mode at the right time will affect the accuracy of the data.

Forgetting to wear the device will obviously skew the results as well. Also, the sleeping position can also affect the quality of the data as well.


None of the people we know bought activity trackers to undertake scientific endeavors. Chances are you too are like us. You were curious. You wanted some identifiable figures and metrics to push yourself and stay motivated.

Yes, these devices are not as accurate as a sterile laboratory might be. But that’s not why we bought these devices right? We just wanted some benchmarks to challenge ourselves.

Some metrics to strive for. We wanted to do just a little bit more than we did yesterday. And these devices are an excellent way to motivate us to do more.

We’re not scientists or researchers. We just want to better ourselves. Now that we understand how these devices work and how they are able to tell when we’re asleep or not, we can focus on what truly matters and that’s self-improvement.

And hopefully, we will be a better version of ourselves when we look back to reflect on the time we’ve already lived.