Low-frequency-to-high-frequency ratio (LF/HF) of heart rate variability has been used as a maker of stress or sympathetic predominance in cardiac autonomic activity. Laboratory studies, however, indicate that LF/HF increases with standing and decreases with lying. We hypothesized that LF/HF in ambulatory monitoring is associated with the ratio of time in lying position during the monitoring (lying ratio). We examined this hypothesis using 47,624 ambulatory 24-h electrocardiograms recorded with 3-axis accelerograms in a large database of Allostatic State Mapping by Ambulatory ECG Repository (ALLSTAR) project. We found that LF/HF decreases with advancing lying ratio even after adjustment for the effects of age.