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Fitbit Sense: Feeling stressed? This fitness tracker can tell

Exclusive: The new Fitbit smartwatch has a sensor that actually checks your skin to see how stressed out you are.

Scott Stein Editor at Large
I started with CNET reviewing laptops in 2009. Now I explore wearable tech, VR/AR, tablets, gaming and future/emerging trends in our changing world. Other obsessions include magic, immersive theater, puzzles, board games, cooking, improv and the New York Jets. My background includes an MFA in theater which I apply to thinking about immersive experiences of the future.
Expertise VR and AR, gaming, metaverse technologies, wearable tech, tablets Credentials
  • Nearly 20 years writing about tech, and over a decade reviewing wearable tech, VR, and AR products and apps
Scott Stein
9 min read
Richard Peterson/CNET

The Fitbit Sense is the company's first major watch update since the Fitbit Versa in 2018, and it adds a variety of new sensors: an ECG similar to the Apple Watch and Samsung Galaxy Watch 3, stress sensing via an electro dermal activity skin response sensor, and temperature sensing similar to the Oura ring.   

I haven't tested the Fitbit Sense yet because the Fitbit Sense I was sent isn't set up to pair with Fitbit's app yet, but I'm holding the $329 (£300, AU$500) watch in my hands. While I can't say how all those sensors will work in everyday use, I can say the watch's build has some improvements over the previous Fitbit Versas. The curved watch shape is a similar thickness to the $199 Versa 2, but has more metal and glass in the finish.  

The side button for starting workouts and calling up Amazon Alexa (or now, Google Assistant) has been replaced with an indented touch area that vibrates when pressed, like the Fitbit Charge 3 and 4 have (will this be better when one is sweaty? I don't know). There's a speaker now, for voice assistant and phone calls. The Sense wrist straps detach a lot more easily than the Versa's ever did (which also means: new custom watch straps). The charger is also new, a magnetic snap-on that's more similar to what the Apple Watch has and is supposed to fully charge in about 15 minutes. It's better than the alligator-clip-charger Fitbit Versa had before.

There are other overdue additions too, like GPS, a redesigned optical heart-rate sensor that promises better accuracy when running and sleeping, and maybe richer data collection for future health research (optical heart rate is already where blood oxygen, heart rate, respiration are drawn from; Samsung's already exploring blood pressure possibilities, too). Fitbit's newest software updates add specific SpO2 blood oxygen readings to Fitbit watches, and a daily "stress readiness" score that will add measurements like respiratory rate to the mix, which are also part of a new step-down model being introduced Tuesday, too, the $230 Fitbit Versa 3. 

But the Sense's ambitious triple-add of new sensors points towards Fitbit's striving for new data to analyze and add to a growing machine-learning health picture, which makes this look like Fitbit's closest attempt to a health-and-wellness super-wearable. I'd welcome the idea of something on my wrist that can help me understand how I'm doing on a daily basis, even beyond how much I'm walking around and exercising.

I've seen ideas around a health super-wearable come and go over years. Will Fitbit Sense be that device? Since I haven't reviewed it yet, the answer for now is a strong maybe. And Fitbit looks to be leaning on its premium subscription health service, launched last year, more than ever to make the most of Sense's data (more details on that below).

Fitbit Sense: An up-close look at the design

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There are a few reasons to wait and see. New sensors on wearables are always a bit of an unknown. Maybe Fitbit is breaking through to a whole new territory here. Or, maybe not. I remember other wearables' promising new sensor tech before (the Jawbone Up 3 had bio-impedance; the Microsoft Band 2 had UV sensing, and then there was the sensor-studded research-focused Samsung SimBand), and some of these never did anything to live up to their concepts. Will EDA make as much of a difference on a watch as Fitbit promises? I'm extremely curious to find out. Temperature sensing is still pretty unique. ECG is becoming a common standard on fitness watches.

But how will these all interconnect? For that, I talked with Fitbit's Shelten Yuen, head of research, and lead research scientist Conor Heneghan about what these sensors are aiming to do.


The Fitbit Sense's temperature sensing will happen nightly and show relative changes over time instead of a specific reading.

Richard Peterson/CNET

Temperature: Not like a thermometer, but a nightly relative baseline 

Temperature on wearables started getting a lot of attention after the coronavirus pandemic shut down the world this year. The possibility of detecting symptoms of illness through a watch or ring with temperature scanning, or pulse oxygen readings, began to feel like a promise of wellness forecasting. 

"The origin of this was prepandemic," Fitbit's Yuen tolds me over a Zoom call. "A while ago, we had this hypothesis that with a wearable device, the temperature sensor could be used to detect a fever and even predict a fever was going to occur."

The $300 Oura ring has become the most recognized temperature wearable, but there aren't many others. The Fitbit Sense seems to work in a similar way, collecting skin temperature while sleeping and delivering results the next morning to show relative change.

It doesn't do an instant temperature scan, and it won't give specific standard body temperatures to show if you're 98.6 or 100.3, for instance. But it will show relative temperature changes on a daily basis. That's because skin temperature readings on a wrist are different. I haven't tested the Sense yet, but I wore an Oura ring for months to check my temperature, and it was mostly a graph of gently fluctuating relative scores. (I wasn't sick during that time, so it's hard to judge.) The temperature awareness is more like one more data point in a set of points that Fitbit's using for its daily stress score calculations.

What's really wild is Fitbit acknowledging that existing Versa watches already have a temperature sensor that is used for device battery management. Those sensors helped the  company prepare for what the Sense could do. "We have some data from that to direct where this could go in the future," Heneghan says.

Those other temperature sensors are not as skin-focused as the Sense's temperature sensor, and may not be as accurate. But it could help turn a bunch of Fitbit Versa devices into temperature-aware wearables, adding a lot of devices into the mix fast for possible Fitbit research. It sounds like the Sense's temperature sensor is more fine-tuned for the job.


The Sense's rear sensors and metal rim on the face measure temperature, ECG, and galvanic skin response for stress measurements.


Stress sensing happens in a whole new app

The Sense's electrodermal activity, or EDA, sensor uses the metal outer rim of the Sense to conduct and get a reading when the palm of your hand is pressed against it. Fitbit has a new EDA app that will take a reading over time and sense your overall stress. But this will be a different app than the heart rate-based Relax app that currently calculates heart rate variability and also encourages mindfulness. Fitbit's also launching a new set of mindfulness and meditation features into its Premium subscription service that specifically uses the EDA sensor.

"Your body's sympathetic nervous system is sending little signals to your sweat glands all the time," Heneghan says of the EDA sensor. "That creates a tiny bit of extra moisture to your skin, and that changes the conductivity of your skin."

The two-minute stress scan in the app would require you to hold your hand over the watch the whole time, as would any meditation features using EDA.

"You put your hand over the device, and basically it is a very, very small microcurrent that's going through the palm of your hand. And we can pick up the individual spikes of nerve sympathetic nervous system activity" adds Heneghan. "In the EDA scan app, we're going to count up the number of spikes we see every 30 seconds and report that back to you."

ECG, but no background arrhythmia scanning yet 

The Sense's one-lead electrocardiogram sensor, which Fitbit and Apple call ECG, but is the same as what doctors call EKG, works by finger-pinching the metal bezel on the edges of the watch face to complete a circuit. It takes a reading that scans for possible atrial fibrillation much like the Apple Watch, recent Samsung watches or other devices such as the Withings Move. Like other watch ECG features, the Sense won't detect heart attacks or other conditions that more advanced ECG readings could pick up. Also, the ECG feature is still waiting for FDA clearance, a process that's expected to be finished this year.

And the Fitbit Sense won't continuously scan heart rate for signs of arrhythmia -- at least not yet. The Apple Watch already does this, and Fitbit says it's working on it. The company announced its larger-scale atrial fibrillation study in May. 


The redesigned optical heart rate sensors on the back measure light through different areas at once.

Scott Stein/CNET

More accurate heart-rate readings could open other measurements

It's always hard to cast forward and expect too much of sensor platforms, because it's unclear where results will land. For instance: Fitbit's long touted sleep apnea detection still hasn't arrived, despite promises for years. "We are kind of engaging with the regulatory process, and the real challenge is just getting enough volume of data to prove out what we already have," Fitbit's Heneghan says to me about progress on detecting sleep apnea.

Meanwhile, research initiatives with the Fitbit Sense's new sensors could explore wellness forecasting through temperature and SpO2, but no specific plans are in place yet with any studies. "In the future, we hope to develop a regulated SpO2 feature that could help with illness detection and may be an indicator of potential chronic respiratory conditions, like COPD (chronic obstructive pulmonary disease), or acute illness like COVID-19," Fitbit CEO James Park tells me in email answer to my questions.

Fitbit's current COVID-19 symptom detection research has been focused on heart-rate variability, resting heart rate and breathing rate as metrics, which CTO Eric Friedman says "can detect 50% of COVID-19 cases a day before onset of symptoms with 70% specificity." Fitbit will be adding sleep data, temperature and SpO2 blood oxygen measurements into further studies.

The heart-rate reading improvements at the moment seem to be about helping accuracy while running and sleeping, to prevent gaps in data. But it could still open up other channels too: Samsung and Valencell are trying to crack blood-pressure readings through PPG. Fitbit isn't looking at that yet, but Park says, "When it comes to heart conditions, we continue to explore additional metrics like pulse arrival time – a metric that might signal that your blood vessels are getting stiffer, which could be an indication of heart disease or high blood pressure." 

But when it comes to other readings pulled from heart rate, Yuen adds: "We're hoping to explore this data set and then maybe unlock its potential in the future. I don't want to overpromise. But are we interested in things like this? Absolutely."

Fitbit is really leaning on you paying for Premium

A lot of the daily metrics and tools Fitbit discussed with me rely on Fitbit's $9.99-a-month Premium subscription service, which launched last year and now has over 500,000 subscribers. Some measurements like a specific SpO2 measurement will be available through a free watch face and in more limited sets of readings on the free app, but longer history and analysis comes with the premium service. 

That Premium service is an extra cost, and also an extra lock-in. Fitbit's data doesn't flow into Google Fit or Apple Health like some other wearables do, and it means staying in a channel that can feel limiting, especially since most people have an increasing flow of health data from other sources.

Fitbit includes a six-month free trial of Premium with the Sense (and a year with its lower-cost Inspire 2), but sooner or later that's going to be an extra fee. Google and Apple don't have any similar subscription health/fitness service yet, but there are plenty of coaching and fitness apps that do.

Premium doesn't seem necessary, but for a lot of what the Sense offers, it may be the way you'd want to get the most out of the device.


The Fitbit Sense looks ambitious. How will it dovetail with future Google wearable plans?

Richard Peterson/CNET

What does this all mean with Google?

Fitbit's pending acquisition by Google has been underway since November, and Fitbit's subscriber base and data archive could become a launchpad for a new generation of Google health wearables. But the Fitbit Sense has no added hook-ins to Google's ecosystem, other than added Google Assistant voice support in addition to support for Amazon's Alexa voice assistant. Park wouldn't comment on Google. "As you know, the merger is subject to customary closing conditions, including regulatory approvals," he says. "Until the transaction closes, we continue to operate independently and are focused on continuing to create exciting, innovative devices and services for our users."

The Fitbit Sense might be the last new Fitbit under the Fitbit brand. Or, maybe it's the blueprint for a new generation of future devices. Clearly, these new sensors are where Fitbit wants to head next. But what we don't know is how much Fitbit's future will look like its present once Google enters the picture.