Heart rate variability and heart rate fragmentation among shift workers: The Multi-Ethnic Study of Atherosclerosis

Night shift work is linked with cardiovascular disease (CVD), although underlying mechanisms remain unclear. Heart rate variability (HRV) is a well-established biomarker, but heart rate fragmentation (HRF) may offer additional insights. We conducted a study to compare heart rate metrics, including HRV and HRF, across different types of shift schedules, and to evaluate the influence of sleep duration. In this cross-sectional study, we included 845 participants (mean age 64; 51% women) from the Multi-Ethnic Study of Atherosclerosis Sleep study. Heart rate metrics were compared between the day shift (reference), night/rotating shift, and other shift workers using adjusted linear regression. HRV was measured from overnight bipolar (Electrocardiogram) ECG. HRV (log transformed) and HRF values were reported as % difference with 95% confidence intervals (CIs). Analyses were stratified by sleep duration (short <7 h, recommended 7-<9 h, and long ≥9 h), and tested for interaction. Night/rotating shift workers had higher HRF than day shift workers, particularly a greater percentage of normal-to-normal intervals in alternation segments (PAS) (% difference 11.9, 95% CI 1.8, 22.0). We found differences based on sleep duration for PAS. The difference was most pronounced in workers with short sleep (% difference 20.4, 95% CI 3.0 to 37.9), somewhat smaller in those with recommended sleep (% difference 14.2, 95% CI -1.0 to 29.5), and reversed in long sleepers (% difference -10.8, 95% CI -32.4 to 10.8; interaction p = 0.06) Interaction terms were assessed at p < 0.10. HRV metrics showed minimal differences and no consistent patterns across shift groups. Night shift work is related to higher HRF, and sleep duration influences these findings, which may reflect circadian misalignment and autonomic dysregulation.