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The accuracy of the Fitbit sleep tracking has been a matter of debate and study for numerous years. Researchers published articles and found that when compared to a gold standard polysomnograph (PSG)device, the accuracy of the sleep detection ranged from 0.81 to 0.93, which should tell us that Fitbit devices reliably detect sleep patterns.
Backed up with all these praises and scientific evidence, we should be confident that Fitbit fitness trackers and smartwatches will reliably detect our sleep phases. Yet, we somehow asked ourselves if the results would be consistent whatever the Fitbit device.
We decided to put the Fitbit Sense and Fitbit Charge 5 to the test and wore both devices while sleeping. The Charge 5 is on our left arm and the Sense on our right. We, of course, made sure to use the proper dominant vs. non-dominant hand and wear them with what is considered the right degree of tightness.
Our protocol was set, the devices were firmly on our wrists, and our bed was comfy. We expected the results to be crystal clear and primarily identical between the two devices. The next day, we were puzzled by the results, and this experiment raised more questions than it gave answers.
Want to know why? Keep on reading. We will tell you everything.
We assume that you already how Fitbit devices detect when you sleep and compute your sleep score. If you do not and need a bit of a refresher, please read the articles below.
Understanding how We Analyzed the Data
Accessing the sleep results can be done in two different ways. The first one is to access the sleep results on the App directly on your phone. It is simple and can be done in just a couple of seconds. However, keep in mind that getting access to more detailed analyses when subscribing to Fitbit Premium will provide more details and even individual scores about the Time Asleep, the Deep and REM sleep episodes, and the overall Restoration.
Fitbit Dashboard offers the possibility to download the sleep data as a CSV file. To do so, log into your dashboard on the Fitbit website. However, we were a bit disappointed by the data provided this way. We expect to obtain the raw data relative to our sleep and then be able to calculate the correlation between the two devices. Unfortunately, it was not the case, and only the time spent on the various sleep patterns was provided.
We then decided to rely on the App and compare the individual parameters.
Did the Fitbit Sense and Fitbit Charge 5 Give Us the Same Sleep Score?
The sleep score is at the center of the Fitbit sleep tracking feature. Using a proprietary algorithm, Fitbit will provide an overall score relative to the quality of your sleep. As a result, most of us will fall between 70 and 80.
Was the score identical between the two Fitbit flagship devices? In short, they were NOT! The Sense gave us an 82 considering our sleep as “good,” but the Charge 5 only graded our sleep 78 for a modest “fair.” We agree that a 4-point difference on a scale ranging from 0 to 100 may not be much but keep in mind that most users will fall between 70 and 80, making the scale more a 60 to 90.
This first difference in the score made us wonder if we could detect more discrepancies. And we did!
What About the Total Sleep Time and The Sleep Stages?
This is where things started to become exciting and different.
The Charge 5 considered that I slept 6h59 when it was 8h05 for the Sense. How could it be possible? I then looked at the individual sleep stages, where things got messed up.
As you can see in the picture below, the Charge 5 systematically under-detected some sleep stages and sleep times. My two arms are firmly attached to my body, and I was genuinely baffled by the discrepancies between how the two Fitbit sleep trackers detected different REM sleep, deep sleep, and light sleep episodes.
Overall when looking at the sleep stages, it seems like the Charge 5 and the Sense mainly were different when considering the light sleep, 61% vs. 55%) and deep sleep stages (12% vs. 16%.)
Somehow when wearing modern sleep trackers from the same manufacturer, we expected the results to be more consistent. After all, the sensors are mostly the same for both devices, and we assume that the algorithms are identical.
Could it be linked to the Fitbit Premium add-on? Our Charge 5 was not using it. So we don’t know, but it may be worth a try.
Our next question was to define if the estimated oxygen variations were detected in the same way with such results.
Is the Oxygen Variation the Same?
They are not! And they are very different. For example, look at the two graphs below. It isn’t easy to consider that the data were collected the same night on the same person…namely me.
It is as if the Fitbit Sense is much more sensitive to detecting the oxygen variation during the night.
The blood oxygen saturation was also overall different. Not massively different, but different somehow. The Fitbit Sense granted me a 94% saturation when it was only 93% with the Charge 5. Not a massive difference but a difference somehow.
To Wrap Up
The Sense and Charge 5 are two of the best wearable devices currently available. Both devices praise their health metrics detection accuracy and reliability.
Our little experiment raises the question of how both devices track sleep and the quality of the data. However, we agree that it was only a one-night study and that there is low scientific value to our homemade experiment.
What is the reason for such differences between the two trackers? We have no idea, but something is going on here and deserves a more rigorous and detailed analysis. Stay tuned as we will continue our analyses with different metrics.
Don’t guess us wrong, though. We love our Fitbits, and we regret not having more arms to put to the test the other devices such as the Fitbit Luxe and Fitbit Versa 3.