When Apple announced its new lineup of smartwatches—Apple Watch 12 and Apple Watch Ultra 4—in September, one of the biggest highlights was that the devices are now capable of keeping tabs on your pulse roughly every five seconds around the clock, with even greater accuracy than before. This is a 60 times the improvement over the previous models. To do so, Apple had to create larger, brighter green LEDs on the back side of the watch. Those same LEDs also allow the device to check in on your heart rate variability (HRV), an indicator of stress and recovery, 24 times as often, throughout the day.
The biggest question we had following the announcement: How does this not nuke the battery on a device that’s already criticized for needing daily fill-ups?
We spoke with the team at Apple that was tasked with not only hardware modifications but also algorithmic advancements that allow the Watch to take your HR more often while being more power efficient.
Apple also claims that the new Health Sensing System results in the most accurate heart rate tracking of any wearable—and went so far as to distribute the findings of its own study on the matter.
So, does it work? And how well? Let’s dive in.
More Light + Larger Sensors = Better Reliability
In a nutshell, all wrist-based optical heart rate monitors work in roughly the same fashion: LEDs shine light into your skin, your blood absorbs some of that light, and photodiodes read the amount of light that gets reflected back. From there, the watch can determine your heart rate and HRV.
There are some complications to overcome, however. The physical real estate on the backside of a watch is limited. And there’s a ton of movement that happens at your wrist, which makes accurate readings challenging—anything that requires a strong grip, like strength training, creates significant interference.
For the new watches, Apple looked to maximize the use of that sensor space. If you look at the backside of the older models of Apple Watch, you’ll see eight small dots in a circle at the center of the unit. Four of them shine a combination of green, red, and infrared light. The other four are sensors to capture the reflected light. Those are tiny in comparison to what’s been rolled out on AW12 and AWU4.
On the Series 12, Apple separated the LEDs—four exclusively emit green light, because that works best in challenging daytime conditions and workout scenarios. The other four combine red and infrared, and activate when you’re largely motionless or sleeping—times when those are adequate. They’ll be less bothersome while you sleep, and the red, in particular, is needed to measure blood oxygen levels.
The sensor on the back of the Apple Watch Ultra 4 (and Series 12) feature a redesigned sensor array.
Those larger and brighter LEDs, though, take up the space where the old photodiode receptors were set, so Apple replaced them with a ring of eight trapezoid-shaped photodiodes arranged in a ring surrounding the LEDs.
A New Back Crystal
The larger sensors improve the readings, but only if the bright light shining out of the LEDs can be properly isolated. To prevent any light leakage, Apple had to re-engineer the crystal covering the sensors. That casing is not there just to keep out dirt and water, but it’s set up so light can be directed at your skin, yet shielded from the sensors.
“What's really different here, we took red and white zirconia and did a couple processes with a lot of heat and pressure,” said Eugene Kim, vice president of hardware engineering at Apple. “It's a very unique singular piece of zirconia that has specific opaque and clear areas—black and clear—which really helps us isolate the light source from the receiving source, because you only want to see the light off your blood, not just random stray light that might be emitting from the LEDs.”
Apple used a new process to create those black and clear sections of glass. Previously, the housing was made up of multiple pieces of crystal that were essentially glued together. Now, it’s created using a “double-shot” process where the red zirconia is molded first for the areas that will end up being dark. Then a second injection mold of the white ultimately becomes the clear sections. After it goes through the heat and pressure processes, it’s polished to give the single, shiny glass cover you see on the back of the Watch.
But What About Battery Life?
One knock on Apple Watch versus sport-specific GPS devices is its relatively short battery life. Compared to many other smartwatches, Apple Watches have historically needed to be charged every day. Devices from Garmin and Coros, on the other hand, can last days or weeks—you charge them so infrequently you might forget where you placed the charging cable. The battery life of an Apple Watch Ultra, say, has been extended, but you’re still putting it on the magnetic puck every other day at least. So, Apple knew it couldn’t reduce those run times, even though it wanted to improve the HR sensing capabilities.
“The name of the game there is just efficiency,” said Kim. “We really wanted to improve our performance by improving the efficiency of the system. And I mean that in very broad terms. It's physical packaging, it's energy usage, it's computation.”
Apple didn’t plan to just pack in a bigger battery, but to ensure that everything required less power to operate.
“We actually took our work on the AirPods heart rate,” Kim continued, “where that is also a very power-constrained product, and we took the [machine learning] approaches there and actually brought that to the Watch to increase efficiency of the algorithms, to not only improve performance but also use less power.”
Steve Waydo, Director of Health Sensing, explained that the biggest impact is the improved efficiency. “For workouts, for example, we expect similar heart rate performance for that five-second stream, but we're achieving that performance with a lot less power. So that can translate into longer workout durations or just a smaller overall impact on your battery life.”
The efficiency improvements also mean bigger changes to the rest of the day. Previously, HR features were based on infrared light largely because they required less power. But, with the more efficient LEDs and sensor, and the ability to provide the data with a lower power cost, Apple is able to fire up the green lights all day without overly taxing the battery.
I have been fairly impressed with the battery on the AWU4. One of my first runs with it was a 10-miler that included five fast intervals each one mile long. After 75 minutes of running, I only drained 6 percent of the battery. In the first mile, I realized that I messed up the transfer of my cellular data plan from my previous Apple Watch, so I couldn’t fire up music to power me through that workout—which would certainly have delivered a bigger drain on the battery. (I got it all sorted later that day.)
How Daytime HR Monitoring May Help You
As a competitive distance runner who follows a structured training plan, I use my watch to keep tabs on my sleep, too. I want to know not only how well I slept but also how well I’ve recovered from the workouts of the previous day or two. Tracking overnight HRV stats can give me insight into that, but can also be factored into a Readiness score (1 to 10) that Apple surfaces in the Health app. The score is an easy way for me to know whether I’m recovered enough to tackle another speed session on the track, or if I should pull back and go for an easy jog—or take a rest day completely. (To be fair, most other watchmakers offer a similar metric.)
The new “readiness” score gives you a holistic way of looking at exercise and sleep tracking to help you adjust your fitness plans on the fly.
But even non-marathoners and athletes can benefit from the overnight data as well as the new all-day HRV readings. Dr. Asha Chesnutt, a member of Apple’s clinical team, explained why that matters.
“As a clinician, obviously, continuous heart rate monitoring is part of the hospital, the ICU where I come from,” she said. “For some users, they really just like to be able to connect the dots—like not drinking enough water corresponds to increasing your heart rate, or drinking that extra cup of coffee and making you jittery might increase your heart rate. For me as a clinician, I think that that's pretty obvious, but to a lot of people it's not. Even before you physically have a fever and get sick, you might see some subtle changes in your heart rate and HRV. I think it helps people become more in tune with their bodies.”
Is It Really the Most Accurate Wearable?
As I noted above, Apple shared its own findings that suggest the new Watch Series 12 and Watch Ultra 4, using the new Health Sensing System and the S11 chip, have the most accurate heart rate measurements of any wearable. It compared the new watches to the Garmin Forerunner 970, Google Pixel Watch 4, Huawei Watch 5, Samsung Galaxy Watch 8, Whoop 5, and Oura Ring 5.
I didn’t simply want to take the brand’s word for it, though. I’ve used the Watch Ultra 4 to track more than 200 miles of runs over the past month, and have been looking at how the data recorded compares to a Garmin Fenix 9 Pro Solar and Forerunner 970, as well as heart rate captured with a Coros armband monitor and Garmin HRM600 chest strap.
If I want the most accurate heart rate data, say for a session of fast intervals where my HR rises and falls quickly, I still reach for the chest strap. Nothing I’ve seen from any optical sensor comes close to its accuracy. AFF of athletes get chafing or irritation from a chest strap—and you obviously won’t wear one casually all day.
The Apple Watch Ultra 4 (red line) didn’t report any HR for nearly a minute at the start of a running interval, but then tracked remarkably close to the Garmin HRM600 chest strap (blue).
By taking a closer look at graphs of my heart rate from the various devices during workouts, I’ve seen the Apple Watch Ultra 4 mirrors the strap remarkably closely—with one quirk.
While Apple says that the Watch measures heart rate every five seconds, it’s not truly every five seconds exactly. In my data files, I see gaps—sometimes a minute or longer—where there is no recorded value. I asked Apple’s team why that happens, when other devices display measurements up to every second.
“We took a strong stance, really from the early days of the first Watch, that if we're going to claim to be doing a measurement—if we're claiming to measure heart rate—we need to have the data coming from the sensor to back that up,” said Waydo. “Any optical measurement at the wrist—a hardware device interacting with physiology—the signals are not perfect all the time. If we don't have real data to support the measurement that we're going to present, we'll choose to withhold that measurement rather than present something that we aren't really sure is the right answer.”
When I asked what goes into that “confidence check” to determine whether or not to present a reading, he explained the following:
“The algorithms are a large machine learning–driven neural network that takes in the optical signals from the sensor as well as the motion data from the accelerometer. And inside the system, it outputs two things: It outputs, ‘here's what I think the heart rate is,’ and ‘here's how sure I am about that.’ And we train the, ‘here's how sure I am’ component based on data from thousands of people, where we're looking at the ability of the system to accurately estimate the heart rate on their signals.”
When that’s combined, the algorithm might have low confidence even if the reading is correct—so it leaves a blank gap. What that means is the data that is actually reported has a stronger likelihood of being the true measurement, compared to devices that just give you a number regardless.
The times I have found the watch to struggle the most were at the beginning of runs, or at the start and end of speed intervals. In the case of the beginning of runs, I’d see a flat line on my heart rate graph, but then the Watch’s data would snap up to match what my chest strap was reading nearly dead-on.
In the case of intervals, I think what happens is that I mark the beginning and end of my efforts by pushing the Action button on the side of the Watch to record a segment. In doing so, I may be jostling the Watch just enough to create a gap between the sensor and my skin, destroying the algorithm’s confidence in what the sensor is seeing.
A Step Toward a Chest Strap-Free World
In the end, the changes that Apple rolled out with these devices do move the needle to make wrist-based heart rate monitoring more useful and reliable. The accuracy, even if not as spot-on as a chest strap, proves to be good enough that most of my running is done without a chest strap—and I’m confident that I’m pushing at the right effort level.
And the efficiency improvements give you better access to your heart rate all day. The usefulness of that insight right now might not be apparent, but that data will continue to be rolled into features and apps for even more meaningful actions. It’s currently being used within the Heart Rate app, where you can see a graph of your last two minutes as well as your resting HR and even your walking HR average.
Best of all, Apple managed to make these improvements without any hit to the Watches’ battery life.
Jeff is Runner-in-Chief for Runner’s World and the director of product testing. He has tested and reviewed running shoes, GPS watches, headphones, apparel, and more for nearly two decades. He regularly tests more than 100 pairs of shoes each year, and once had a 257-day streak running in different models. Jeff can usually be found on the roads, racing anything from the mile to a marathon, but he also enjoys racing up mountains and on snowshoes. When he’s not running, you’ll probably find him hanging from a ladder making repairs and renovations to his house (he’s also director of product testing for Popular Mechanics).