Fitbit Sense ongoing review: Time will tell how useful stress tracking can be - CNET

The Fitbit Sense isn't the first Fitbit to track SpO2, or blood oxygen saturation, but its new sensors have opened up more ways for the company to connect the dots on all the data that's being collected from your wrist. The $329 (£299, AU$499) Sense can also track temperature, respiration, heart-rate variability, take an electrocardiogram (also known as an EKG or ECG) and even measure stress. We're all for learning about what's going on inside our bodies, but all these wellness reports have started to feel like arcane car performance charts.

Getting a morning oxygen reading from the Fitbit Sense feels a lot like waiting for bread to bake. If you're lucky, it arrives about an hour or so after waking up, but other days it might not show up at all. Just like a homemade loaf, the results can be inconsistent.

With the Sense, Fitbit's trying to bridge the gap between fitness and wellness, 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. Samsung's newer Galaxy Watches also include these metrics, as well as a stress test of its own, and the Oura ring collects similar data that might someday help identify the onset diseases before the user experiences any symptoms. And while the Sense still has fitness at its core, it wants to be your daily wellness coach now, too.

After a week of 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, minus ECG and stress sensing) would be plenty.

CNET's Scott Stein and Lexy Savvides have been wearing the Sense for the past few days. We'll be updating this review once we've finished testing battery life, the ECG and heart-rate tracking. 

A familiar design if you're upgrading your Fitbit

The Sense shares 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. Along with the touchscreen, you interact with the Sense through the indented haptic side button, which can do everything from launching Alexa to starting a workout. It feels more comfortable than the Versa 2, especially when working out and going to bed, thanks to its flatter profile. Those fiddly toggles used to switch out straps on earlier Fitbits are gone (thank goodness) and replaced with quick release buttons to swap bands in and out.

The Sense also has a faster processor than the Versa 2, which helps make interactions with the watch feel snappier, but we still noticed some lag when opening apps or swiping up to see daily stats on the Today menu. And like earlier Fitbits, it can take up to 30 seconds to sync new watch faces or apps to the Sense.

Stress tracking and ECG still to be determined

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 can't detect heart attacks or other cardiac conditions. It recently received FDA clearance in the US, but if you buy a Sense today, you won't have access to it until October. We haven't been able to test it out yet. 

Initially the ECG app will roll out in the US, Canada and Hong Kong. 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 and it also notifies the user of irregular heart rhythms indicative of aFib, plus high and low heart rate alerts like the Sense.

Stress sensing is a totally different proposition. The Sense has an electrodermal activity, or EDA, sensor that uses sweat to determine stress levels. It uses the metal rim on the top of the watch to complete a circuit and measure possible sweat-triggered stress markers over a two-minute reading.

To take a test, you first have to open the EDA Scan app on the watch, then place the palm of your opposite hand over the watch face and hold it there for two minutes, making sure your skin comes into contact with the four edges of the metal frame. It's a little weird at first because you can't see anything on the screen while you're doing the scan, but a vibration lets you know when you can release your palm. Once you're done, you can log how you feel, see your EDA responses and check if your heart rate went up or down. Fitbit also has guided audio meditation sessions to pair with the EDA scan.

The stress reading 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 in 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 and what how to improve on his results. 

Fitbit does, however, provide a new Stress Management score at the beginning of each day that takes into account sleep, physical activity and stress into a "how you're doing" number. It's like the daytime equivalent of the Sleep Score if you're already familiar with this from previous Fitbits.  

The Stress Management readings can be counterintuitive and you have to wear the Sense constantly to get a reading in the first place. Even taking it off for an hour to charge may deprive you of your daily score. After a few days, Scott got a high stress management score of 94, which apparently means things were good, but even then it was confusing. Does that mean he isn't stressed? No. Does it mean he's ready to face the day? Maybe. Knowing what to do with daily scores is a bit of a mystery.

You don't need a Fitbit Premium account to access this data, but subscription is encouraged as this is how you'll get the extra meditation features and insights from the stress sensor. Fitbit's $10 monthly Premium subscription service is fast becoming an essential part of the Fitbit device experience. While you used to need a Premium subscription to view additional health data such as heart rate variability and SpO2 trends over time, Fitbit is making this available to everyone with a compatible device in the coming months. 

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SpO2 tracking is only available for sleep

The Fitbit Sense doesn't take SpO2 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. In the morning, about 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. On a few nights, however, it never appeared and we're not sure why.

Temperature tracking on the Sense is also only at night

Don't expect the Sense to replace your thermometer anytime soon. Temperature tracking 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. 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 potentially helpful to give you an idea of your temperature fluctuations over time and indicate possible fevers before you might be aware of them.

Unfortunately (or fortunately), neither of us have 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, but that hasn't been apparent yet either. Basal body temperature is often used by women to predict fertility, as ovulation often causes a slight increase in temperature. In the same way as other Fitbits, the Sense has cycle tracking which you can log in the app, or see where you are in your cycle on the wrist.

Fitness tracking: Not a whole lot different to earlier Fitbits

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.

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 calorie burn. What is new is the addition of Active Zone Minutes (which we first saw in the Charge 4) which 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 having the same general fitness features, working out with the Sense has its advantages. 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 max. It's comfortable to wear, and doesn't get in the way thanks to its flat profile even when you're working up a sweat.

Still to come: how the Sense compares against a chest strap to see how accurate heart rate tracking is (and how fast it updates) during a cardio workout.

Battery life

• With the always-on display active, two workouts, a few stress measurements and a full night of sleep tracking, we met the two-day battery life claim. 
• Outdoor workouts don't seem to drain the battery too noticeably, as long as you only raise to wake the watch face. After one 40-minute workout with GPS, the battery dropped by 8%. With the always on display active, battery life takes a more significant dip. We'll update with more battery test details soon.
• The new magnetic charging puck is much better than the old Versa alligator clip. And the Sense charges fast, giving you a full day of battery life in 12 minutes. It charges from flat to full in around two hours.

Other thoughts

• The Fitbit app used to be one of the best apps to break down and interpret your fitness and health data. It's still good, but there's now a lot more complexity and finding certain metrics like heart rate variability is harder than it should be. 
• Overall, it feels like souped-up Versa that feels a bit nicer to wear, with a few more sensors. 
• 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 probably more potential in the Sense down the line.