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Fitbit Sense review: An ambitious smartwatch that takes on too much

The Sense tracks your stress, temperature, blood oxygen and sleep. But interpreting all this data can be puzzling.

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The Fitbit Sense adds a whole slew of sensors to the Fitbit lineup to track everything from stress to blood oxygen levels, temperature, sleep and even has an FDA-cleared electrocardiogram (ECG or EKG). On top of all that, the $329 (£299, AU$499) Sense also doubles as a regular smartwatch and fitness tracker. And while the Sense still has fitness at its core, it wants to be your daily wellness coach now, too.

Fitbit is trying to bridge the gap between fitness and wellness with the Sense, a zone most wearables were already navigating even before the current COVID-19 health crisis. The Apple Watch has been leaning to wellness and health over the last few years, with a new blood oxygen feature, ECG app and fall detection feature. Samsung's newer Galaxy Watches include these metrics, as well as a stress test of its own, while the Oura ring also collects temperature data like the Sense. The end goal for most of these is that all this data may someday help identify the onset diseases before the user experiences any symptoms. But in the meantime all those charts, numbers and scores from the Sense can feel overwhelming, especially for someone with no medical training.

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After two weeks wearing the Sense, we have mixed feelings about it. In short, if you wanted a health device to monitor your daily stats, and also wanted ECG on a Fitbit, this is your watch. But otherwise, the lower-priced Fitbit Versa 3 (which has the same general features of the Sense, without ECG and stress sensing) would be plenty.

CNET's Lexy Savvides and Scott Stein both wore the Fitbit Sense for this review.

A familiar design if you're upgrading your Fitbit

The Sense has a similar design to the Versa, except with a stainless steel edge around the square watch face instead of aluminum and a host of new sensors inside, which does make it a tiny bit thicker. Along with the touchscreen, you interact with the Sense through an indented haptic side button, which can do everything from launching the Alexa smart assistant to starting a workout. It feels more comfortable than the Versa 2, especially during workouts and at bedtime, thanks to its more rounded finish. Those fiddly toggles used to switch out straps on earlier Fitbits are gone, thank goodness. They've been replaced with quick release buttons to make swapping bands out a lot easier on the Sense. 

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Lexy Savvides/CNET

The Fitbit Sense also has a faster processor than the Versa 2, which makes interactions with the watch feel snappier, but we still noticed some lag when opening apps, raising the watch to wake the screen or swiping up to see daily stats. It also takes about 30 seconds to sync new watch faces like with earlier Fitbits.

What it has improved is its charging station. Instead of the alligator-style clips from earlier Fitbits, the Sense uses a new magnetic charger that easily attaches to the back of the watch. 

The ECG feature is easy to use

The Sense is the first Fitbit to include an onboard ECG app capable of producing a single-lead electrocardiogram read in 30 seconds. Fitbit says that the ECG on the watch will also screen for possible arrhythmias that could indicate atrial fibrillation, or aFib, but it can't detect heart attacks or other cardiac conditions. It recently received FDA clearance in the US and it's now available for use in the US, some European countries and Hong Kong. You can find a full list here.

To take an ECG, you first need to go through a quick Heart Rhythm Assessment briefing in the Fitbit app (go to Discover > Assessments & Reports). Once complete, the ECG app should appear on the Sense. Take a seat and place your thumb and index finger on opposite corners of the watch and you'll see one of three results after your scan is complete, depending on the heart rhythm: normal sinus, signs of atrial fibrillation or inconclusive. You can also review the results in the Fitbit app and share with your doctor.

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Lexy Savvides/CNET

Samsung has received a similar clearance for the feature on its two newer Galaxy Watches, while Apple's ECG app has been active on the Apple Watch since the company launched the Series 4 in 2018. It also notifies you of irregular heart rhythms indicative of aFib, plus high and low heart rate alerts, as does the Sense. The ECG and heart rate notifications work in a similar way between the Sense and Apple Watch, although to take an ECG on the Apple Watch you place one finger on the digital crown rather than the two fingers on the rim of the Sense.

Stress tracking is only helpful if you already know how to manage it

While the Sense isn't the first wearable device to track stress, its method of retrieving this information is unique. Rather than focusing on heart rate like Samsung's Galaxy Watches, the Sense also uses sweat data from its new electrodermal activity, or EDA sensor to determine stress levels. To measure your levels, you place the palm of your opposite hand over the stainless steel rim on the top of the watch. The palm's contact on the watch's metal rim completes a circuit, then uses the EDA sensor to measure possible sweat-triggered stress markers. The entire process takes two minutes.

It's a little weird at first because you can't see anything on the screen while you're doing the scan, but once it's complete, you'll receive a vibration. You can also log how you feel at the end of the test, see your EDA responses and check if your heart rate went up or down. Fitbit offers guided audio meditation sessions for Fitbit Premium users to pair with the EDA scan, although we didn't find them particularly helpful or relaxing.

The stress results so far have been vague. One of Scott's initial readings showed a few EDA moments, which are those sweat-triggered incidents. But for the most part he seemed to be stress-free, according to the Sense. Considering he wore the Sense throughout a pandemic with two small children at home and during one of the busiest work weeks of the year, this reading did not reflect the reality he experienced. The app also didn't seem to provide any meaningful context as to why he experienced those early incidents or how to improve on his results. Lexy had similar difficulty interpreting the EDA moments, of which she had 17 during one particularly stressful day on deadline. The Fitbit gave no indication as to whether this was a normal amount or cause for concern. 

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After each EDA scan you can see your heart rate, how many EDA responses were measured, then reflect on how you feel, ranging from "very stressed" to "calm."

Screenshot by Scott Stein/CNET

Fitbit does, however, provide a new Stress Management score at the beginning of each day that takes into account sleep, physical activity and stress to give you a "how you're doing" number. It's like the daytime equivalent of the Sleep Score from previous Fitbits. This data could be helpful for recovery: For example, if you have a lower score you might want to focus on getting more restorative sleep rather than pushing yourself on a workout.

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The daily stress score in the Fitbit app.

Screenshot by Scott Stein/CNET

But the Stress Management readings right now aren't particularly user-friendly and it can take a few days for them to show up after you first start wearing the Sense. Lexy's scores varied from 78 to 92, usually averaging in the 80s. A higher Stress Management score means things are generally good. But even on days when her score managed to tip into the 90s, it didn't necessarily correlate with the amount of stress she felt. The Fitbit app doesn't provide personalized feedback to help you interpret or improve your score so figuring out what to do with it can be a bit of a mystery. 

You don't need a Fitbit Premium account to access much of this data, but you'll need one to get the extra meditation features, insights from the stress sensor, extra stats about your sleeping heart rate and minute-by-minute skin temperature variations during sleep. Fitbit is making additional health data, such as heart rate variability and SpO2 trends over time, available to everyone with a compatible device in the coming months. But for that other data you'll need to pay $10 a month for a premium subscription, which is becoming an important part of the Fitbit experience. 

Read more: How the Fitbit Sense tracks stress.

SpO2 tracking is only while you sleep

The Fitbit Sense doesn't take SpO2 or blood oxygen readings on demand like Samsung's Galaxy Watch 3 or the Apple Watch Series 6. Instead, it measures blood oxygen levels while you sleep. The Series 6 also measures SpO2 levels at night.

To do this you have to download and select the specific SpO2 watch face before you go to bed. About 45 minutes to an hour after waking, you'll see the SpO2 score appear on the watch face. You'll also be able to see the graph of your blood oxygen variations in the sleep section. (There are no specific numbers, just an indication if your oxygen variation is high or low.) Remembering to switch to the SpO2 watch face manually every night is annoying and easy to forget before you collapse into bed, but Fitbit is working on ways to bring the SpO2 score to other watch faces. As with the stress score, it's difficult to know what to do with your SpO2 reading unless you're a medical professional. And it's not possible to test the sensor against a pulse oximeter, a device doctors use to measure blood oxygen levels from your fingertip, as the Sense's SpO2 readings are taken at night and averaged out.

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Scott Stein/CNET

It's also worth noting that SpO2 is also available on the Versa 2 and Versa 3 with the same watch face. Sleep tracking is great on the Sense and it works in pretty much the same way as earlier Fitbits with a detailed breakdown on your stages of sleep and gives you a sleep score each morning.

Temperature tracking on the Sense is also only at night

Temperature tracking on the Sense is similar to SpO2 in that it doesn't provide a measurement on demand, but rather shows whether you've deviated from your baseline in a daily graph. Needless to say, it won't replace your thermometer any time soon. You'll need to log about three nights of sleep for the Sense to establish a baseline from which to go by. Like the Oura ring, which we've also been testing for a few months, it's a potentially helpful way to get an idea of your temperature fluctuations over time and indicate possible fevers before you might be aware of them.

Unfortunately (or fortunately), we haven't been sick in the period where we've been tracking our temperatures and haven't experienced any significant variances from the baseline to report on. Fitbit is promising that the Sense will also reflect temperature variations due to menstrual cycles, and it did seem to show a dip in Lexy's skin temperature before the start of a cycle. Basal body temperature is often used by women to predict fertility, as ovulation often causes a slight increase in temperature, or a dip in temperature before a period. Like other Fitbits, the Sense has cycle tracking which you can log in the app, or see where you are in your cycle on the watch.

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Screenshot by Lexy Savvides/CNET

Fitness tracking: Not much has changed

One of the downsides of Fitbits that launched after the Ionic in 2017 was lack of onboard GPS. Starting with the Charge 4 earlier this year, Fitbit has finally brought back GPS so you don't have to take your phone with you on an outdoor run or ride to track your route. It takes around 10 seconds to acquire a lock when you start an outdoor activity, with or without your phone nearby.

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The Sense can show you what heart rate zone you're in and encourage you to push harder (or back off).

Lexy Savvides/CNET

If you've used any other Fitbit in recent years, the rest of the fitness tracking features on the Sense will seem familiar. You'll still be able to track your steps, start a goal-based workout, see your heart rate zone and keep an eye on calories burned. What is new is the addition of Active Zone Minutes, which we first saw in the Charge 4. This uses your age and resting heart rate to show you how hard you worked out during an activity. You'll also receive real-time alerts when you've changed zones, which can help you take action during your workout, whether that's pushing yourself a bit more or easing off depending on your goals. For Lexy, it was most helpful during an outdoor run so she knew when to go a bit faster (usually, that's all the time).

Despite the Sense having the same general fitness features, it has its advantages in a workout. The screen is brighter than the Versa and earlier Fitbit trackers, so it's relatively easy to see in sunlight as long as you ensure the brightness is set to maximum. It's also comfortable to wear, and thanks to its flat profile it doesn't get in the way even when you're working up a sweat.

We compared the Sense against a chest strap to test heart rate tracking accuracy. While it matched up fairly consistently to the strap for resting heart rate, it took at least 5 to 10 seconds to catch up to the strap during a workout when heart rate spiked (like when going from a gentle jog to a full sprint).

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Lexy Savvides/CNET

Battery life is good, but not as strong as other Fitbits

With the always-on display active, two 30-minute workouts, a few stress measurements and a full night of sleep tracking, the Sense met the two-day battery life claim. Turning off that always-on display and just using raise to wake helped boost the battery to around 4.5 days between charges. That said, outdoor workouts seem to be a pain point for the battery. Lexy noticed after a particularly strenuous 2.5 hour outdoor bike ride, the battery dipped almost 50%.

More smartwatch features coming soon (we hope)

The Sense works with Android and iOS and the experience is consistent across ecosystems, with the exception of not being able to respond to text messages from the watch when you're on iOS. There's a microphone and speaker onboard so you'll be able to take quick calls from your wrist with your phone nearby. If you have an Android phone, you'll also be able to use dictation or voice-to-text to respond to messages. Google Assistant support is also on the way, hopefully by the end of the year, which may improve on the slow and limited Alexa functionality.

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Lexy Savvides/CNET

You can store songs for offline listening from Pandora or Deezer if you have a premium subscription, but the Sense will only serve as a remote control for Spotify. 

You can't store your own music on the Sense, however, which is a big downside if you like to listen to songs during a workout and don't want to take your phone with you. 

A good, but not great smartwatch for the price

The Fitbit Sense hits the mark in many ways: It has strong battery management, excellent sleep tracking and an array of new sensors that could be helpful to some people. But it feels like it's trying to do a little too much at once, with features like stress management seeming more confusing than helpful. That said, Fitbit has a track record of adding new features, like unlocking temperature tracking in earlier versions, or adding a color always-on display to the Versa 2 a few months after launch. So there's more potential in the Sense and how it interprets all this data for you further down the line. Even so, you might be better off with a more basic and capable Fitbit Versa 3. That way you'll save some dollars and wait for Fitbit to work out its new sensor tech for a later generation of Sense.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.