Autonomic nerve activity shows circadian variation. Therefore, we put forward the hypothesis that the responses of heart rate (HR) and high-frequency (HF) power of HR variability to exercise would be different between early morning and daytime exercise. We performed ergometer constant load exercise tests [50 watts (low), 100 watts (high load)] in the early morning and during the day in 6 healthy volunteers. The HR response was fitted to an exponential hyperbolic sine function: HR= α*e-β*t *sinh(ω*t)+γ. In this equation, the β/ω ratio was inversely correlated with the intensity of the HR response. HF power was determined using a recently developed algorithm with high time-resolution power. There were no significant differences in HR, HF power or systolic blood pressure (BP) pressure before exercise between early morning and daytime exercise with either the 50 or 100 watt loads. During exercise, there were no significant differences in maximal HR or maximal systolic BP between early morning and daytime exercise with either 50 or 100 watt loads. For high-load exercise, the β/ω ratio was significantly lower in early morning exercise (mean±SD, 0.945±0.02) than in daytime exercise (0.987±0.03). Similarly, for 100 watt exercise, HF power of HR variability was significantly lower in early morning exercise (0.94±0.52 msec/Hz1/2) than in daytime exercise (1.26±0.74 msec/Hz1/2). In conclusion, the present study demonstrated that a lower β/ω ratio in the HR response was associated with lower HF power of HR variability in early morning high-load exercise compared to that in daytime exercise, indicating that the heart rate responded more intensely to early morning exercise than to daytime exercise with a high load due, at least partly, to pronounced suppression of parasympathetic nerve activity.
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