Heart rate variability coefficient of variation during sleep as a digital biomarker that reflects behavior and varies by age and sex

The heart rate variability coefficient of variation (HRV-CV) is an index of day-to-day cardiac autonomic fluctuation that may serve as a scalable digital biomarker for behavioral monitoring and health risk stratification. We investigated how many nights of sleep-derived HRV were needed to reliably estimate 7-day HRV-CV, and examined associations with behavioral and demographic characteristics linked to health. We analyzed ∼2 million nocturnal HRV readings from >21,000 wearable device users, stratified by age and sex. Seven-day HRV-CV was calculated, and a simulation determined the minimum nights required for reliable estimates. Associations with alcohol, physical activity, sleep, and variability of these behaviors were evaluated. Additional models examined associations between HRV-CV with age, biological sex, and body mass index (BMI). At least five of seven nights were required to achieve acceptable agreement with full-week HRV-CV values (intraclass correlation coefficient ≥ 0.80). Higher HRV-CV was associated with greater alcohol consumption, lower physical activity, shorter and less consistent sleep, and greater behavioral variability (<i>P</i>s < 0.001), with stronger associations for alcohol and sleep compared with HRV. HRV-CV increased with age in males after ∼40 yr and showed a U-shaped pattern in females, declining through midlife and rising after ∼50 yr. HRV-CV increased with BMI in both sexes (<i>P</i>s < 0.01). HRV-CV measured during nocturnal sleep can be reliably estimated from at least five nights of data, with higher values associating with less favorable behavioral profiles, older age, and higher BMI. These findings support the use of HRV-CV as a scalable, behavior-sensitive digital biomarker with potential applications in personalized health monitoring and risk stratification.<b>NEW & NOTEWORTHY</b> We identify HRV-CV measured during sleep as a scalable measure of day-to-day HRV fluctuation. Analyzing nearly 2 million HRV readings from >21,000 individuals, we document that five nights are sufficient for reliable 7-day estimates. Higher HRV-CV is associated with greater alcohol consumption, lower activity, shorter and less consistent sleep, older age, and higher BMI. These findings provide the first large-scale characterization of HRV-CV and support its potential as a biomarker for behavioral monitoring and risk stratification.