AHA Scientific Sessions 2026 Abstract · Nov 2, 2026
Nocturnal Heart Rate Variability Across Age, Sex, and Resting Heart Rate in a Multinational Cohort of 4 Million Smart Ring Users
By Kentta T
Background: Heart rate variability (HRV) is an established marker of cardiovascular health, with reduced HRV reflecting diminished vagal modulation and associated with adverse cardiovascular outcomes. Resting heart rate is a known confounder of HRV but has not been explicitly stratified in prior population studies. This research aimed to characterize nocturnal HRV across age, sex, and resting heart rate in a population-scale cohort of consumer wearable users. Methods: De-identified nightly HRV data were aggregated from 4,056,059 Oura Ring users (2,936,936 female, 1,119,123 male) over the most recent seven nights of use per user (May 2026). Inclusion criteria were self-reported age ≥ 20 years and sleep duration ≥ 4 hours. Oura Ring estimates inter-beat intervals (IBIs) from photoplethysmographic (PPG) signals and computes the root mean square of successive differences (rMSSD) across 5-minute segments throughout each night; the mean across segments represents nightly cardiac parasympathetic activity. Rate-corrected HRV was calculated as crMSSD = 100 х rMSSD / mean IBI, and log2-transformed to stabilize variance. Data were summarized as percentile distributions stratified by sex, 10-year age group, and average nocturnal heart rate bin (40-49, 50-59, 60-69, and 70+ bpm). Results: Nocturnal HRV declined monotonically with age and with increasing nocturnal heart rate in both sexes (Figure). Median rMSSD in the 20-29 age group ranged from 101 ms at 40-49 bpm to 29 ms at 70+ bpm; within any heart rate bin, median rMSSD declined approximately 2-fold from the 20-29 to the 60-69 age group. When stratified by sex at matched heart rate (Figure, bottom; 50-59 bpm bin shown), median log2(crMSSD) was higher in females than males across all adult age groups, with the difference modestly widening in later life. Conclusions: In a population-scale cohort of over 4 million smart ring users, nocturnal HRV declined monotonically with age and with elevated nocturnal heart rate, consistent with reduced cardiac vagal modulation over the lifespan. At matched nocturnal heart rate, females showed higher rate-corrected HRV than males across the adult lifespan, including in older age groups. Because resting heart rate differs systematically by sex, raw HRV comparisons are confounded by mean heart rate. Rate stratification at population scale isolates the underlying sex effect, supporting consistently higher parasympathetic HRV in females.