Higher habitual lying time is inversely associated with vagal-related heart rate variability outcomes in younger adults.

Heart rate variability (HRV) is a well-established noninvasive marker of autonomic cardiac control. We test whether time spent sitting (negatively) versus lying (positively) influences vagal HRV outcomes. HRV (10 min supine electrocardiogram) and free-living postures (dual-accelerometer configuration, 7 days) were measured in 31 young healthy adults (15♀, age: 23 ± 3 years). Habitual lying (66 ± 61 min/day), but not sitting time (558 ± 109 min/day), total sedentary time (623 ± 132 min/day), nor step counts (10 752 ± 3200 steps/day; all,  0.090), was associated with root mean square of successive cardiac interval differences (ρ = -0.409,  0.022) and normalized high-frequency HRV (ρ = -0.361,  = 0.046). These findings document a paradoxical negative impact of waking lying time on cardioautonomic function.   Using a multi-accelerometer configuration, we demonstrated that more habitual waking time lying, but not sitting or total sedentary time, was associated with worse vagally mediated cardiac control.