Background/Purpose: Reduced heart rate variability (HRV) has been shown to be associated with a risk of cardiovascular disease. The combined influence of shift work and menstrual cycle on HRV in nurses has not been studied. The aim of this study was to examine the effects of the menstrual cycle within shift patterns on HRV in nurses, using a within-subjects design and a multivariable analysis to control covariates. Methods: Twelve healthy, young, female shift nurses volunteered to have repeat measures of female sex hormones, 24-hour physical activity, and HRV at three points, during their menses, follicular, and luteal phases. Results: Normal cyclic variations in endogenous sex hormones levels were shown; however, no significant correlations were found between estrogen levels and HRV variables. Results demonstrated that the high frequency (HF) was lower in the follicular phase than in the luteal phases; however, the low frequency (LF) or LF to HF ratio (LF/HF) was significantly higher in the follicular phase than in the luteal phase during sleeping periods after night or day work. Conclusion: The endogenous sex hormones levels were shown normal cyclic variation. Under the effects of shift work, the diminished parasympathetic activity and the increased sympathetic activity were shown in the follicular phase compared with the luteal phase. This result may serve as a reference to explain why shift workers have high risk of cardiovascular disease.
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