Large resting heart rate variability between individuals, but small variability within individuals
Inter- and intraindividual variability in daily resting heart rate and its associations with age, sex, sleep, BMI, and time of year: Retrospective, longitudinal cohort study of 92,457 adults
Introduction and methods
Several studies have found that changes in individual heart rate (HR) measurements over a prolonged period of time are associated with CV outcomes [1-4]. Longitudinal, individual HR data that are obtained daily in normal living conditions over long periods may therefore provide more information than a single HR measurement. Commercial wearable devices often contain sensors that can measure HR continuously. The accuracy of these sensors is similar to that of standard ECG monitoring, especially at rest [5-7]. The aim of this study was to evaluate the inter- and intraindividual variability in resting HR (RHR) using data from wearable devices.
This retrospective observational cohort study collected data from 92,457 individuals that used a wearable device for a minimum of 35 weeks, for at least 2 days per week, and at least 20 hours per day. The average age was 45.8 ± 14.4 years, 63% were women, average BMI was 28.4 ± 5.8 kg/m². The RHR measured by the wearable device is expected to be representative of the true value of RHR that would be measured in a supine position immediately after waking and before getting out of bed. The association of individual mean RHR, as well as RHR variability, with age, BMI, and average time sleep (measured by the device) was analyzed. Furthermore, RHR variability between individuals and long- and short-term changes in an individual’s daily RHR were studied.
- Mean RHR for all individuals was 65.5 ± 7.7 bpm. The average RHR over time varies widely between individuals, ranging between 39.7 and 108.6 bpm.
- Average RHR increased with age until approximately 50 years of age and decreased after that age in both men and women.
- Women had significantly higher RHR in all age groups. RHR ranged between 50 and 80 bpm in 95% of men. 95% of women had an RHR between 53 and 82 bpm.
- RHR and BMI had a U-shaped relationship, with the lowest RHR at a BMI of 21 for women and 23 for men.
- Average sleep duration was associated with RHR, with the minimum RHR at an average sleep duration of 7-7.5 hours per night in both men and women.
- The average relative variability, expressed as standard deviation, in individuals’ daily RHR was 3.10 bpm for women and 2.90 bpm for men. The maximum weekly fluctuation in RHR for approximately 80% of individuals was <10 bmp.
- A seasonal trend was observed with RHR of individuals changing by an average of 2 bmp, ranging from a minimum at the end of July and a maximum at the beginning of January.
- Intraindividual variability in RHR was associated with sex, age and BMI. Women had a greater variability than men and women of childbearing age had a significantly greater variability than men at the same age or older women. RHR variability was lowest in individuals with a BMI between 20 and 25 kg/m², while greater variability was found in both underweight and overweight participants.
This retrospective observational study investigated the inter- and intraindividual variability in daily RHR and found that the average RHR varies widely between individuals (as much as 70 bpm). Within individuals, RHR was more consistent over time. A prospective study will be necessary to investigate whether information about daily RHR can be of value for the early detection of important physiological changes in individuals.