<b>Supplementary Materials: </b><b>Heart Rate Variability Coefficient of Variation During Sleep as a Digital Biomarker That Reflects Behavior and Varies by Age and Sex</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 &gt;21,000 individuals, we document that five nights are sufficient for reliable seven-day estimates. Higher HRV-CV associated with greater alcohol consumption, lower activity, shorter and less consistent sleep, higher BMI, and older age. These findings provide the first large-scale characterization of HRV-CV and support its potential as a biomarker for behavioral monitoring and risk stratification.