Smartwatches Keep Tabs on Your Overall Health While Still Tracking Your Fitness

Electrocardiograms measure the timing and strength of the electrical pulses that keep your heart pumping. On the Apple Watch, the ECG kicks into action when you launch the app and hold your finger against the digital crown, the home button on the upper right side of the watch, for 30 seconds. Electrodes are built into the back crystal and digital crown of the watch.

If you receive a “sinus rhythm” result, it means the watch has detected a normal heartbeat, not A-fib. Regardless of the result, you’re urged to seek medical attention if you experience any symptoms. Apple makes it clear that its watch does not check for signs of a heart attack.

Fitbit’s irregular heart rhythm notifications about possible A-fib work across several models and are powered by Fitbit’s photoplethysmography (PPG) algorithm, which passively assesses your heart rhythm in the background while you’re still or sleeping.

2. Skin sensor. Fitbit added an electrodermal (EDA) sensor to its Fitbit Sense watch and its successor, Fitbit Sense2. When you place your palm over the face of the device, sensors can detect subtle electrical changes in the sweat level of your skin. Factored with your heart rate, sleep and activity data, Fitbit calculates a stress management score of 1 to 100, with a higher number translating to fewer physical signs of stress and a lower number meaning more signs.

Similar sensors on Pixel Watch 2 can also detect stress.

Fitbit may recommend breathing exercises and other mindfulness tools to help you manage stress. But after a free trial period, you’ll have to sign up for a premium subscription service for a deeper dive into the metrics. The subscription costs $9.99 monthly or $79.99 annually.

Another sensor on the Fitbit Sense can detect variations in skin temperature. Apple added wrist temperature sensors to the Series 8 and later watches, including the more rugged $799 Apple Watch Ultra. For its part, Samsung added an infrared temperature sensor on Galaxy Watch5 and later smartwatches.

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3. Sleep tracking. Some devices, including high-end Fitbits and Galaxy smartwatches, track the quality of your slumber time right down to various sleep stages and the number of times you wake up in the middle of the night. Apple’s watch is more about setting up the conditions so you get a good night’s sleep, starting with a wind-down routine before bedtime. But it, too, has the ability to detect sleep stages.

Of course, if you’re planning to sleep with any of the smartwatches, make sure they are charged before you go to bed, and if needed, give them some extra juice when you wake up.

4. Blood oxygen. Fitbit/Google, Garmin, Mobvoi and Samsung all have models that can measure blood oxygen levels. The Apple Watch Series 6, Series 7 and Series 8 obtain a measurement from a quartet of clusters of green, red and infrared LEDs on its rear and four photodiodes spaced and isolated between them to determine the color of your blood.

Measurements are automatically collected throughout the day or when you’re asleep. You also can launch an app to take a manual reading by steadying your wrist on a table, with the watch display facing upward, tapping Start, and then waiting patiently while a timer counts down for 15 seconds.

But Apple removed the blood oxygen feature on its Series 9 and Ultra devices following a patent infringement dispute with an Irvine, California, company called Masimo. The watches are now for sale in the U.S. without the feature.

Masimo’s own $499 FDA-cleared W1 Medical watch includes real-time monitoring of a users’ oxygen saturation and pulse rate and is expected to be out this year.

While most healthy adults report blood oxygen levels between 95 percent and 100 percent, what does a lower reading signify? Blood oxygen is an indicator of early signs of circulatory, heart or lung function issues, such as anemia, neurological problems or sleep disorders, says Leslie Saxon, M.D., a professor of medicine and executive director at the Center for Body Computing at the University of Southern California.

Keep results in perspective

Don’t freak out if any result is out of kilter. See how it compares with your baseline, yet don’t ignore warning signs.

“The most important thing is how one is feeling and the symptoms,” Isakadze says. “If someone is not feeling well, I would say no matter what the numbers show on the oxygen levels or the ECG, it would still be very important to seek care.”

These are still early days for what smartwatches can track. Neither the Apple Watch nor the Fitbit can monitor blood glucose levels or high blood pressure without the use of other devices right now, which would provide a huge benefit to patients with diabetes or hypertension.

In fact, the FDA issued a warning in February that noninvasive ways of measuring your blood sugar — meaning without a prick of a needle as with a continuous glucose monitor — pose a life-threatening risk to diabetes patients because of inaccuracies.

Splurge on cell service to keep connected

If you don’t want to carry your smartphone with you when you’re out for a run or doing other exercise, get a smartwatch that has a cellular connection. This will give you much of a phone’s functionality on your wrist. Smartwatches without cellular connectivity are more like an extension of your phone and will work well only when near that device.

Meanwhile, data culled from the wrist is becoming fertile ground for researchers.

In 2020, Apple announced a study with Anthem and the University of California, Irvine, to examine how its watch might help individuals manage asthma. Another Apple study, with the Ted Rogers Centre for Heart Research at the Peter Munk Cardiac Centre in Toronto, investigated how its watch might lead to better outcomes for patients with heart failure.

“Looking at data from wearables such as Apple Watch and partnering to understand the patient experience, coupled with the information from sensor technology, is a way for us to develop the knowledge that will allow us to take better care of patients. This is the future,” says Heather Ross, M.D., at the University of Toronto. 

Behavioral health experts at the University of California, Los Angeles have been collaborating with Apple on a digital health study to measure how factors like sleep, physical activity, heart rate and daily routines relate to symptoms of depression and anxiety. Data comes from the Apple Watch and iPhone.

While the results of research studies typically take time, you can learn a lot by looking at your wrist right now.

This story, originally published Sept. 28, 2020, was updated to reflect newer models and industry developments.