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How Does a Fitbit Track Sleep: Understand How a Sleep Tracker Works

Activity trackers have become surprisingly popular in the previous few years, and questions like, “How does a Fitbit track sleep?” are not uncommon.

Just a few years ago these devices were nothing more than glorified step counting devices. But they are progressing at an amazing rate. Now, they can do everything from measuring our heart rate to create customized plans to help us achieve our goals.

Even though activity trackers are fairly simple compared to most smartwatches, the increasing number of different sensors are turning them into powerful computers. The latest fitness bands are now being advertised as having ten different types of sensors.

It’s no surprise that our expectations are also higher than ever. Let’s go over the sensors found in these devices. Understanding how a Fitbit works will only help us understand how it tracks our sleep.


An accelerometer is the most basic type of sensor. You will find an accelerometer on every single smartphone, handheld device or fitness band which measures our rate of accelerating based on our movements.

It can be used for a number of things including counting steps, the force of acceleration, general orientation and whether the device is vertical or horizontal. It is the sensor that helps to device understand whether we’re moving or not.

Optical Heart Rate Monitors

We usually think of EKGs that doctors use to measure our heart-rate. An optical heart-rate monitor is quite different from an EKG. It uses light to measure our heart-rate. The sensor shines light through the skin and examines the light that bounces back.

Since blood is conducive to light absorption, changes in light can be used to examine the heart-rate. This process is called photoplethysmography. Yes, quite a mouthful.

An optical heart-rate monitor is quite close to an EKG machine in terms of accuracy but only when used on the fingertip or the chest. But even the wrist it is accurate enough for us to get the information we need.


GPS, although decades old technology, has only recently made its debut in fitness bands. The addition of GPS really enhances the value these devices offer because they can allow cyclists and runnings to map out and analyze the entire workout session.

GPS is still a very power hungry device, and most activity trackers with GPS will have a smaller battery life. However, the benefits of having a GPS far outweigh its costs, and for most runners and cyclists it is an absolute must.


Thermometers constantly measure our body temperature whether we are sleep or out hiking. It is an excellent way to study our body’s response to activity and resting cycles.

It also plays a key role in monitoring our sleep as our body temperature lower as we sleep and enter different stages of sleep.

UV Sensors

Most new activity trackers also have UV sensors that allows the device to know if we are absorbing too much harmful UV radiation. Most outdoor exercises are under the sum, and the sensor can remind us to get some shade and cool off.

Galvanic Skin Response Sensor

Galvanic skin response sensor is one of those sensors that you don’t hear much about. We suspect it has got something to do with its name. We could be wrong, of course. It measures the electrical conductivity of our skin.

The sensor analyzes our sweating patterns whether from exercising or something else. When this sensor is combined with the accelerometer, it can generate data that can use to understand how our body is responding to different exercises and help us do more of what works.

Ambient Light Sensors

The ambient light sensor is what tells a phone to automatically brighten its light when out in the sun and dim it when indoors. A fitness band uses it for the exact same purpose. It makes using these devices seamlessly and offers a fluid experience.

Bioimpedance Sensors

Bioimpedance sensor is an advanced piece of technology. It measures heart-rate, galvanic skin response and respiration rate in order to understand better the physiological changes our body undergoes while we’re working out or sleeping.

The sensor can detect the quality of our respiration and hydration. It does this by simply looking at the oxygen levels in our blood by measuring slight impedance changes within the volume of blood.

Since this is a new addition to activity tracking bands, it offers much greater potential in understanding our body and the quality of our sleep.

How Does a Fitbit Track Sleep?

Now that we understand what makes up a Fitbit fitness band, we are better positioned to understand how does a Fitbit track sleep. All of these sensors with an exception or two are used to track our physical changes in our body as we sleep to understand our sleeping patterns and help us get valuable insights into the quality of our sleep.

Just collecting data from all of the sensors crammed into the device is not enough. The most important thing is machine learning.

Machine learning refers to using incomprehensibly large amounts of data to find recurring patterns and understand them. This is how Google or Amazon’s recommendations work, which are fairly accurate.

Fitbit has collected anonymized data worth billions of night’s worth sleep. It uses that data to better understand how the human body functions as it enters its resting cycle and try to figure out ways to better understand the physical state our body is in during sleep.

It uses this information to determine if there are any irregularities, which once identified can be rectified through different means. For example, Fitbit is currently working to figure out a way to diagnose sleep disorders such as sleep apnea. It definitely has the data and the computing power to achieve such a gargantuan task.

Whether the current hardware is up to the task is still uncertain. However, at the rate hardware technology is progressing it is difficult to make accurate predictions. But it quite exciting. All of these things, hardware, software, computing power and big data all come together and work in conjunction to track our sleep.

There are of course a lot of technical information we didn’t cover. And of course, we did go on a few tangents, but it would only help you understand how your Fitbit is tracking your sleep and how quickly it is improving.

For all we know, Fitbit might even evolve from a fitness tracking and sleep tracking device to a sleep diagnosis device.


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