A new study published in The Lancet found that using heart rate variability (HRV), wrist-worn health trackers can provide a range of predictive cardiovascular health metrics.

Researchers gathered data from over eight million users of smartwatches made by Fitbit, the company that funded the study. Over 70 countries were represented in this user group, though nearly half were based in the United States.

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

Heart rate variability (HRV) refers to the variation in time between successive heartbeats and represents a non-invasive index of the autonomic nervous system. Because the autonomic nervous system regulates heart rate during sinus rhythm, HRV summarizes complex non-linear cardiovascular accommodative responses, which are dictated by the parasympathetic and sympathetic nervous systems, to dynamic physiological variations.

Researchers analyzed HRV metrics across the time (measured via the root mean square of successive RR interval differences and SD of the RR interval, frequency (measured by high-frequency and low-frequency power), and graphical (measured by Poincare plots) domains.

Findings show that HRV metrics decrease with age across each type of measurement used in the study, reports MobiHealthNews.

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, reaching peak values in the early morning hours. Several authors have discussed the effect of physical activity on HRV, and studies have shown beneficial results. According to a study by one group, the correlation between the number of steps measured by Fitbit devices compared with steps counted by researchers is variable during walking and jogging, but this variation is within 7%.

“We aimed to characterize HRV metrics to understand variations by age, sex, time of day, and physical activity level, and to provide benchmarks for HRV metrics among users of Fitbit,” the authors wrote.

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

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“Our results might have important implications for the remote monitoring of human health given the widespread availability of wrist-worn trackers,” said researchers. “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.”