Resting heart rate can vary by up to seventy beats per minute from one person to the next, a study shows.
Data from almost 100,000 people showed the speed at which the heart beats while sitting still is vastly different between individuals.
For two years, participants wore wristbands that tracked their heart rate, which gets faster or slower depending on activity levels.
Some people’s ‘normal’ resting heart rate was as little as 40 beats per minute, while others was as high as 109 beats per minute.
This was partly be due to age, gender and weight – all play a role in how the ticker works. Other factors include smoking and physical activity levels.
Within individuals, heart rate was pretty consistent all the time. But slight changes may indicate something more sinister, the team said.
Wearable technology is fast becoming an exciting way to study how heart rate varies because it is a long-term measure.
A single time-point measurement with an ECG, for example, doesn’t provide much information on either an individual or population scale.
This study, by researchers at the Scripps Research Translational Institute in La Jolla, California, is the largest of its kind.
It took data from more than 92,000 people across 50 US states who used a wearable daily. Their average age was 46 years old.
Overall, nearly 33million days’ worth of heart rate data were collected. The findings were published in the open-access journal PLOS ONE today.
The researchers used the data to examine variations in resting heart rate (RHR) for individuals over time, as well as between individuals.
The ‘normal’ RHR for adults has long been considered to range between 60 to 100 beats per minute (bpm).
In this study, it ranged between 40 and 109 bmp, with an average of 65.5 bpm. Men had a RHR between 50 and 80 bmp, while woman ranged from 53 to 82 bpm.
Typically, a RHR over 80-100 may be a warning sign of ill health, because a high RHR indicates a person may be physically unfit or stressed. Age can also quicken RHR.
The researchers wanted to see the extent to which these factors explained RHR differences.
Taken together, age, sex, BMI, and average daily sleep duration accounted for less than 10 per cent of the observed variation between participants’ bpm.
The other 90 per cent was not explained, and more research is required.
On an individual scale, the findings show each person’s RHR did not fluctuate too much from one day to the next.
The authors observed also a small seasonal trend in the resting heart rate, with slightly higher values observed in January and slightly lower values in July.
Women had higher variability than men, which could be linked to hormonal changes over each month.
One in five people have one-week periods when their resting heart rate differs by 10 or more bpm from their ‘normal’ range.
The study did not look into why this might be. However previous research has found that brief periods of increased bpm precedes a diagnosed infection or worsened asthma.
Giorgio Quer and colleagues wrote: ‘It is worth considering that a rising RHR may serve as an early warning sign of a physiologic change.’
An increase in RHR over time may be a signal heart trouble – those with higher than average RHR are considered more at risk of heart disease, for example.
Changes in RHR over 10 years, for example, has been easier to measure than minor fluctuations over a week.
The researchers said wearables are making this kind of research possible.
The paper authors said: ‘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.
‘We analyzed the extent of inter- and intra-individual changes in resting heart rate over a prolonged period of time, showing distinct patterns of variation according to age and sex, time of the year, average sleep duration and body mass index.
‘These variations in resting heart rate may allow for the identification of early unexpected changes in an individuals’ health.’