A recent study concludes that when it comes to resting heart rate (RHR), “normal” does not exist. Although most people’s RHR is relatively stable over time, from person to person, rates vary by as much as 70 beats per minute (bpm).

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RHR is easy to measure, but what does it mean for our health?

Although doctors routinely measure people’s RHR, it is still not clear what constitutes “normal,” and what variations from this norm mean for health.

Over the years, large studies have come to a consensus on what doctors should now consider the normal range.

For instance, the National Health and Nutrition Examination Survey, which analyzed data from almost 50,000 people, found that the average RHR for adults is 72 bpm.

The American Heart Association (AHA) consider an RHR of 60–100 bpm normal for adults.

However, RHRs that fall well within the normal range can still increase a person’s mortality risk.

In general, higher rates are associated with increased cardiovascular risk, but one study found that people with an RHR below 65 bpm are also at risk. In the same study, the authors showed that a significant change in RHR over the 3 year follow-up period also increased cardiovascular risk.

In short, results are mixed. As the authors of the new study explain, “a single measurement of heart rate provides very little useful information about the current health of an individual, unless well out of the expected range of normal.”

Rather than taking RHR measurements at discrete points in time, the researchers behind the new study took a “long view of individual changes in cardiac performance.”

They have now published their findings in the journal PLOS ONE.

Today, wearable sensors mean that it is possible to take accurate heart rate measurements continuously. Alongside monitoring heart rate each second, this technology also records sleep duration and levels of activity.

As the study authors explain, the popularity of these sensors provides a “unique opportunity to better understand how RHR varies over time for and between individuals over the span of days, weeks, years, and, eventually, lifetimes.”

If these all-pervasive wearable sensors can preempt the onset of disease, this would make a significant impact on healthcare at a population level. For this reason, it is vital to understand what information heart rate can provide.

If we can decode the secret messages that our hearts tap out, we could continuously monitor our health and, potentially, receive advanced health warnings.

The researchers had access to data from 92,457 adults across 50 states. Each participant wore a heart rate monitor for at least 2 days each week for at least 35 weeks between March 2016 and February 2018. Each day, they wore it for at least 20 hours.

In total, these data provided 33 million daily RHR values.

Overall, the average RHR was 65.5 bpm plus or minus 7.7. The minimum and maximum RHRs for each individual were 39.7 and 108.6 bpm, respectively. In other words, between individuals, “normal” could vary by around 70 bpm.

When the researchers delved into the data, they found some significant relationships. For instance, there was a considerable difference between men’s and women’s RHRs, as the authors outline:

“We observed that 95% of men had an RHR between 50 and 80 bpm, while the corresponding range for women was between 53 and 82 bpm.”

The scientists also found significant differences in RHR related to age, body mass index (BMI), and sleep duration. On average, RHR increased steadily with age, peaked at 50, then began a slow decline into older age.

When they examined the relationship between heart rate and BMI, they found that individuals with moderate BMIs tended to have a lower RHR than those with low or high BMIs.

After analyzing sleep duration, the scientists found that the lowest RHRs occurred in people who slept for around 7 to 7.5 hours each night.

Interestingly, they also found that a person’s RHR tended to change with the seasons by an average of 2 bpm. They explain:

“The RHR peaked in the first week of January for both men and women, after which the average RHR decreased to the yearly minimum at the end of July. After this minimum, the average RHR steadily increased until the end of the year.”

The scientists also note that some people experienced small but significant changes in their RHR for brief periods, such as a week. These slight changes tended to be around 3 bpm.

Sleep duration, BMI, sex, and age all influenced RHR, overall, the researchers showed that these differences were only “minimally associated with individual characteristics.”

Although a person’s RHR changed with the seasons and for short periods, these fluctuations were “an order of magnitude narrower” than the differences between different people’s RHRs. The study authors write:

“Individuals have a daily RHR that is normal for them but can differ from another individual’s normal by as much as 70 bpm.“

According to the authors, an “estimated 20% of consumers in the [United States] now possess a smartwatch or fitness band capable of passively and unobtrusively measuring continuous [heart rate] over long periods.”

Because access to information about RHR is now so widespread, it makes sense to use it to its fullest potential. The researchers hope that, in the future, health professionals might use RHR data to help diagnose conditions such as cardiovascular and pulmonary diseases and even provide insight into reproductive health.

The authors conclude, “The ability to detect early acute illnesses, such as infections, and early exacerbations of chronic diseases remains a promising avenue to explore.”