Fitbit study finds heart rate variability fluctuates by age, gender, time of day and activity level

The researchers used data from more than 8 million Fitbit users over the course of a randomly selected 24-hour period.
By Mallory Hackett
11:56 am
Person observes Fitbit watch on wrist

Using heart rate variability (HRV) data, wrist-worn health trackers can provide a range of predictive cardiovascular health metrics, according to a new study published in The Lancet.

The study, which was funded by Fitbit and used its smartwatches, looked at how devices continuously gathering data from the sympathetic and the parasympathetic nervous system could derive and describe diverse measures of cardiac autonomic function among users.


Findings from the study show that HRV metrics decrease with age across each type of measurement used in the study. The root mean square of successive RR interval differences (RMSSD) and high-frequency power declined faster over time than the standard deviation of the RR interval (SDRR) and low-frequency power.

This finding suggests a more rapid decline of parasympathetic function with increasing age than of sympathetic activity, the study said.

HRV metrics varied throughout the day, with measurements peaking early in the day and subsiding later in the evening. Researchers found the variation to be “substantial” and suggest that people interpret HRV measurements at the same time each day.

The study also found a correlation between physical activity and HRV, such that increased activity could optimize HRV metrics.

There was, however, a stronger correlation for young people, especially when looking at high-frequency power – a metric of the parasympathetic system. Results suggest that people aged 20-24 years could increase their high-frequency power by 1 ms2 with roughly 30 additional steps. In comparison, people aged 50-54 need about 200 additional steps for females and 300 additional steps for males.

“As a result, older individuals can improve their low-frequency power (both sympathetic and parasympathetic activity) more than high-frequency power, with physical activity,” the study said. “These findings imply that parasympathetic function might be more challenging to restore with physical activity versus sympathetic function. We emphasize that the connection between physical activity and increased HRV is merely evidence of correlation, not causation.”


The researchers used data from more than 8 million Fitbit users over the course of a randomly selected 24-hour period on September 1, 2018.

A total of 74 countries were represented. The most common countries represented in the dataset were the United States (48%), the United Kingdom (11%) and Canada (5%), with all other countries contributing less than 5% each.

The mean age for females was about 46 years old and about 49 years old for males.

Data were collected from several Fitbit device models, but primarily from the Charge 2, Alta HR, Blaze, Versa and Ionic.

Fitbit provided financial support towards the collection and analysis of the data, but had no role in study design, data interpretation or writing of the report.


Fitbit’s ability to passively collect data from millions of users has set it up to conduct research across numerous areas of health.

In another study focused on heart health, Fitbit monitors users’ heart rates to search for possible atrial fibrillation, which is then followed up with a free telehealth visit with a live doctor and, if needed, an ECG patch delivered through the mail to confirm the irregularity.

Since the launch of that study, Fitbit unveiled a new line of smartwatches that include the Fitbit Sense, the first smartwatch from the company that has ECG functions.

The company has also been studying the capabilities of its devices in predicting COVID-19 in patients before the onset of symptoms. Findings suggest that Fitbit wearables can detect nearly 50% of COVID-19 cases one day before participants report the onset of symptoms with 70% specificity.


“Although HRV metrics have been previously correlated with cardiovascular health and mortality, our technical advance in the analysis of wearable data at large scale and descriptions of the data now permit its potential use for health promotion through tens of millions of currently available wrist-worn commercial trackers,” the study said.

“Future longitudinal studies can demonstrate how change in health behaviors such as physical activity, diet, sleep, stress, and weight management can affect HRV. Prospective randomized controlled trials are necessary to demonstrate effective ways to use HRV metrics to improve health.”


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(Image: "1 US Bank Note"/geralt via Pixabay, licensed under Creative Commons Zero)

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(Photo by Kwanchai Lerttanpunyapoorn/Getty Images) 



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