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Your heart rate can vary depending on the time of day or even the season. Getty Images
  • Researchers found that daily resting heart rates differed between individuals by as much as 70 beats per minute (bpm).
  • Most men had a daily resting rate between 50 and 80 bpm, while most women had a daily resting rate between 53 and 82 bpm.
  • Compared to men, women of childbearing age showed greater variability in their individual resting rates.

Doctors and nurses routinely measure patients’ heart rates during clinical exams.

If their heart rate falls outside of what’s considered “normal” at a population level, it may be taken as a cause for concern.

However, relying on population-level averages has its limits.

That’s because resting heart rates vary significantly between different people: What’s normal and healthy for one individual may not be normal or healthy for another.

But now thanks to the advent of wearable technologies such as smart watches and other fitness trackers, experts may be able to gain a better sense of what’s normal for individuals.

When researchers from Scripps Research Translational Institute evaluated wearable tracker data collected from nearly 92,500 people across the United States, they found that daily resting heart rates differed between individuals by as much as 70 beats per minute (bpm).

The research team found significant differences in average resting heart rates across age, sex, body mass index (BMI), and sleep habits. But when taken together, those factors accounted for less than 10 percent of the variation observed between individuals.

“Day-to-day changes in resting heart rate could be the first true individualized digital vital sign, which is only now possible to measure thanks to wearable sensor technologies,” the authors of the new study said in a press release.

“These variations in resting heart rate may allow for the identification of early unexpected changes in an individuals’ health,” they added.

The new study was published this week in the open-access journal PLOS ONE.

All participants in the study wore a Fitbit heart rate tracker for at least 20 hours per day, on at least 2 days per week, over 35 weeks or more between March 2016 and February 2018.

The research team found that across all participants, the daily resting heart rate ranged from 40 to 109 bpm.

Most men had a daily resting rate between 50 and 80 bpm, while most women had a daily resting rate between 53 and 82 bpm.

The average daily resting heart rate for men and women increased with age until participants were about 50 years old. Then, the average rate began to decline.

Men with a BMI of 23 and women with a BMI of 21 tended to have the lowest resting heart rates, while people with very low or very high BMIs tended to have higher rates.

The investigators also found a small change across seasons. The average daily resting heart rate in both men and women peaked in early January, before declining to a yearly low at the end of July.

“The findings are not surprising,” Dr. Satjit Bhusri, a cardiologist at Lenox Hill Hospital in New York City who wasn’t involved in the study, told Healthline.

“There is no one standard heart rate. The heart beat changes its rate due to normal physiologic factors,” he continued.

Although most participants had fairly stable daily resting heart rates, 20 percent of individuals in the study went through at least one week when their resting heart rate fluctuated by 10 bpm or more.

Compared to men, women of childbearing age showed greater variability in their individual resting rates. This difference disappeared by the time men and women reached age 50.

Compared to younger adults, individuals over age 60 showed less variability in their individual resting heart rates.

When someone’s daily resting heart rate changes over weeks or months, it might indicate changes in their cardiovascular fitness or the development of a chronic medical condition.

“For example, a thyroid problem called hyperthyroidism, or overactive thyroid, can cause rapid heart rates, whereas hypothyroidism can cause slower heart rates,” Dr. Michael Goyfman, director of clinical cardiology at Northwell Health’s Long Island Jewish Forest Hills in Queens, New York, told Healthline.

On the other hand, changes over the course of days might be a sign of an infection, menstrual cycle effects, or other acute triggers.

“Conditions such as infections can cause higher resting heart rates, as can stress, anxiety, pain, or other unknown or nonspecific conditions,” Goyfman said.

If someone’s heart is beating in a regular rhythm at a rate that’s higher or lower than the population-level average, that’s not usually a cause for concern on its own, Goyfman told Healthline.

What matters more is how much their heart rate deviates from their normal rate, or individual baseline, he said.

“For example, an athlete may have a resting heart rate of 40, and be perfectly healthy or indeed healthier than most,” Goyfman explained.

“On the other hand, someone whose heart rate is usually 85 but now is 50, may be having hypothyroidism, or medication overdose, or some other medical issue,” he continued.

If someone’s heart rate changes significantly from their baseline, it might be a sign of a medical emergency that requires prompt treatment.

Commercial wearable tracking devices provide a novel tool for monitoring individual changes in people’s resting heart rates.

In turn, this might help experts learn more about individuals’ cardiovascular health, menstrual cycle phases, and more. It might also help them detect infections, chronic disease flares, or other acute illnesses.

But according to Goyfman, more research is needed to learn how data from commercial tracking devices may be translated into clinical practice.

The new study relied on a convenience sample of people who can afford and choose to wear a Fitbit device. As such, the findings may not be representative of the general population.

Commercial tracking devices are also less accurate then medical-grade monitors. They may not be able to detect the difference between a slow but healthy heart rhythm, on the one hand, and an abnormal heart rhythm that’s cause for concern, on the other hand.

“Overall, with wearable devices such as Fitbits and Apple watches,” Goyfman said, “we are able to collect and aggregate more data than we ever had in history.”

“However, the question remains of how useful that data is,” he continued. “As technology evolves, it will be interesting to see if we can actually detect some patterns in the data that offer meaningful diagnostic or prognostic use.”