Objective: The influence of shifting the work-sleep cycle on the circadian rhythm of cardiac autonomic activity was investigated by the spectral analysis of heart rate variability (HRV).
Methods: The subjects were 10 healthy Japanese female nurses aged 33+/-3 (S.D.) years. The subjects underwent ambulatory 24-h electrocardiogram (ECG) recordings on the days of day shift (working from 08:00 to 17:00 h) and night shift (working from 21:40 to 08:40 h). Variables of the frequency domain of HRV were calculated for three activity states (work, awake but not working, and sleep).
Results: The mean values of HRV variables over 24 h were not different between day shift and night shift. For both shifts, variables related to the sympathetic control (low frequency component in normalized units and low/high frequency component ratio) were the largest during the work period and the smallest during the sleep period, while an opposite order was present for variables related to the vagal control (high frequency component in absolute value and normalized units). HRV variables in each activity state were not different between the two shifts.
Conclusions: The circadian rhythm of cardiac autonomic activity is largely modified by the level of physical activity regardless of the clock hour, which may have clinical implications when studying the circadian fluctuations of the onset of cardiovascular disease in shift workers.