Age, sex and underlying heart rate as determinants of nocturnal heart rate variability among wearable smart ring users

Abstract Background Heart rate variability (HRV) is an important marker of overall health and especially cardiovascular health. Decreased HRV has been associated with reduced vagal control of heart rate and with adverse outcome in several studies. Purpose The purpose of this study was to investigate the impact of demographic factors (age and sex) and underlying heart rate on nocturnal HRV among wearable smart ring users. Methods De-identified data was gathered from 104,431 wearable smart ring users over the course of one week (640,911 nights; 65% men and 35% women; age 42.6±12.1 years). Inclusion criteria included self-reported age of ≥20 years and sleep duration ≥4 hours. The ring estimates heart rate and HRV with photoplethysmographic (PPG) sensors and calculates the root mean square of successive differences (rMSSD) between adjacent inter-beat-intervals (IBI) for 5-minute segments throughout each night. The average HRV in these segments was then taken to represent the cardiac parasympathetic activity among each user. In addition, rate-corrected HRV was calculated by dividing the rMSSD value with mean IBI (crMSSD = 100 * rMSSD/IBI). Results Nocturnal HRV was significantly dependent on age and underlying heart rate (Figure). The observed rMSSD values declined with age (P