“I’m a Heart Doctor—Here Are the 5 Health Metrics I’d Never Trust to My Smart Watch”

As a cardiologist who’s worn virtually every fitness tracker on the market, Jeff Wessler, MD, MPhil, has a message that might surprise you: Your smart watch is probably better at tracking other things than your health. While smart watches show promising accuracy for some cardiac metrics, their reliability varies significantly depending on what they’re measuring and under what conditions, according to a 2023 study published in the European Heart Journal – Digital Health.

Dr. Wessler still wears his Apple Watch daily (and a Withings during workouts), but he’s learned which metrics deserve attention and which ones are more trouble than they’re worth. “I always say the numbers are useful directionally, not diagnostically,” he explains, meaning that the numbers are meaningful for tracking trends but not specific issues. That distinction matters more than most people realize, especially when anxiety-inducing notifications start flooding in.

Here are the five health metrics this heart doctor has stopped using his smart watch for—and what you should know about each one.

1) Sleep tracking

For Dr. Wessler, sleep tracking seemed like a natural extension of health monitoring until it became counterproductive. “I tried the sleep features but found they were more hindrance than help and gave me sleep anxiety because I was overthinking the numbers,” he says. “I sleep much better without them.”

Research backs up his experience. A 2024 study published in Sensors compared three popular devices—the Oura Ring Gen3, Fitbit Sense 2, and Apple Watch Series 8—against polysomnography, the gold standard for sleep assessment. While all three devices showed high sensitivity (equal to or greater than 95%)—a good sign—for detecting sleep versus wake states, their accuracy for identifying specific sleep stages varied dramatically. The Apple Watch struggled most, with sensitivity ranging from just 50.5% to 86.1% for different sleep stages.

A 2023 study published in JMIR mHealth and uHealth found that consumer sleep trackers often misreport the time spent in each sleep cycle and the total number of cycles per night. These are crucial aspects of sleep health that directly impact how rested you feel—not to mention that even if the numbers are mostly accurate, what exactly do you do with them? “Unless there’s a clear course of action, the results are more distracting than beneficial,” Dr. Wessler says.

2) Oxygen monitors

“The O2 monitors are notoriously inaccurate,” Dr. Wessler says. While this feature might seem like a valuable health safeguard, the science reveals significant limitations.

A 2023 study published in PLOS Digital Health compared four commercial smart watches against a clinical-grade pulse oximeter. The Apple Watch Series 7 came closest to the reference standard with a mean absolute error of 2.2%, while the Garmin Venu 2s showed the largest discrepancy at 5.8%. However, a separate 2024 study published in Mayo Clinic Proceedings – Digital Health found that when tested on hospitalized COVID-19 patients, the Apple Watch reported a sensitivity of only 34.8% for detecting clinically significant hypoxia (low oxygen levels).

Environmental factors can make accuracy even worse. Separate research published in Sensors in 2023 showed that cold temperatures cause blood vessels to constrict, reducing blood flow to extremities and making the watch pulse oximeter even less precise—a particular problem at high altitudes where oxygen monitoring matters most. “If you get a warning about low oxygen, check it independently with a store-bought oxygen monitor—they’re more accurate,” Dr. Wessler says. He adds that if you have additional symptoms, along with a low 02 warning, then you should call your doctor.

3) Heart rate variability 

Heart rate variability tracks the variation in time between heartbeats, and it’s become a trendy wellness metric. Dr. Wessler doesn’t bother with it. “[Heart rate variability] has mixed data on how valuable it is,” he explains. “It’s so unique to each person and wildly affected by many different factors day to day.”

The 2023 European Heart Journal study found that while smart watch-measured heart rate variability provides excellent accuracy compared with ECG-based heart rate variability, individual variability remains a challenge. Everything from stress and hydration to sleep quality and even meal timing can dramatically alter these readings, making it difficult to draw meaningful conclusions from the data. Without clear benchmarks or actionable thresholds, heart rate variability numbers often create more questions than answers—exactly the kind of metric Dr. Wessler considers more distracting than helpful.

4) Heart rate accuracy

Here’s where things get particularly concerning for cardiac monitoring. “Heart rate smart watch data is only accurate for middle-range heart rates,” Dr. Wessler explains. “It gets very inaccurate for high or low heart rates. This is important to know because the ECG watch features rely on it, but they’re looking for tachycardia and it’s not good at measuring high heart rates.”

A 2023 study published in Bioengineering examined four popular wrist-worn devices during intense exercise. The research found that as exercise intensity increased, accuracy decreased significantly. Another 2024 comprehensive review in Quality in Sport found that even the best-performing smart watches were inaccurate 40% of the time during vigorous physical activity.

This accuracy gap creates a serious problem: When you’re experiencing a rapid heartbeat—precisely when you’d want reliable monitoring—your smart watch is the least trustworthy. But if you receive a high heart rate notification and have other symptoms like chest pain or shortness of breath, Dr. Wessler does recommend calling your doctor immediately or going to the emergency room.

5) Calorie tracking

“I don’t like the diet features because they take a lot of work and aren’t accurate, especially calories burned,” Dr. Wessler says. Multiple studies support his skepticism about energy expenditure estimates.

Researchers tested three smart watches during outdoor walking and running and found that the watches were off between 10% to 32% depending on the device and activity, according to the 2022 study published in Frontiers in Physiology. “The problem is it really can’t know certain things about your body,” explains one researcher quoted in the study. “For example, it’s guessing about your relative proportions of muscle and fat, and those things have a lot to do with your resting energy expenditure.”

Dr. Wessler’s advice applies broadly here: Unless you can take clear action based on the data, it’s probably not worth the mental energy. Calorie tracking via your watch often falls into that category, requiring significant effort for questionable returns, and there are much better apps for that.

Smart watch features worth considering

Despite his skepticism about certain metrics, Dr. Wessler hasn’t abandoned his smart watch entirely. He uses his Apple Watch to log workouts, count steps, and track trends. “For wellness features, I do track resting heart rate and VO2 max [maximum oxygen consumption], but only to see trends,” he says.

But the one he is most interested in is the aforementioned ECG function—which he says has improved significantly over the past few years. “The tech is awesome and has gotten much better at identifying arrhythmias,” Dr. Wessler says. However, this advancement comes with an important caveat: “There are a lot of false positives. I’d say only about one in 100 of these will actually be legitimate; the rest are clogging up offices and just scaring people. We call it Apple Watch Syndrome.”

Research published in Cardiology and Cardiovascular Medicine in 2023 confirmed this phenomenon. Researchers found that 67% of participants who received any alert got a false positive. The Apple Heart Study showed similar results: Of 2,064 participants with irregular pulse notifications, the positive predictive value for atrial fibrillation was 84%—meaning roughly one in six alerts was incorrect.

Still, atrial fibrillation detection has value when used appropriately, he says. Atrial fibrillation is the most common type of arrhythmia and can indicate a blood clot, which may lead to tachycardia and heart failure. And, Dr. Wessler shares that he likes it just for the entertainment value, saying he once matched his own heart palpitations with extra heartbeats detected on his watch. “It was exciting to actually see it!” he says, though he adds that palpitations are common and typically not concerning without other symptoms.

Should you monitor blood pressure on a smart watch?

According to the American Heart Association, maintaining healthy blood pressure is one of the most important factors in preventing heart attacks, strokes, and other cardiovascular events. Dr. Wessler agrees, saying that blood pressure is “the holy grail” of cardiac metrics. “If we could increase tracking of blood pressure, then we could really help more people,” he says.

But Dr. Wessler says current smart watch technology hasn’t quite cracked reliable blood pressure monitoring—despite various attempts. Unlike heart rate, which can be estimated through optical sensors detecting blood flow, blood pressure requires measuring the force of blood against artery walls—a fundamentally different challenge.

Until more consistent technology arrives, Dr. Wessler emphasizes the importance of calibrating whatever smart watch features you do use. “You have to make sure to calibrate your watch to your personal data—age, gender, stride length, height, weight—to get more accuracy,” he says.

The bottom line

As someone who runs a virtual cardiology practice using remote monitoring gear (sometimes including smart watch data), Dr. Wessler has extensive experience in this area—and has learned to separate useful information from digital noise.

The key is understanding what your smart watch can and cannot do. Use it for trends in fitness metrics like step counts and workout tracking. Be cautious about sleep tracking if it increases your anxiety. Verify any concerning oxygen or heart rate alerts with more accurate, independent devices before panicking. Skip the calorie features unless you enjoy them for motivation rather than precision. And remember: Your smart watch is a helpful tool for directional health insights, not a medical diagnostic device.

Most importantly, always discuss any health concerns with your healthcare provider rather than relying solely on wearable technology. A notification on your wrist should never replace professional medical evaluation, especially when it comes to your heart.

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