Aerobic fitness and exercise have been associated with improved cardiovascular health. Health effects of exercise may be associated with the circadian rhythm of the heart and may specifically reduced the risk of heart attacks during high-risk periods over the 24 hours. Linear and nonlinear heart rate variability (HRV) measures indicate the degree of heart rate modulation by the parasympathetic and sympathetic branches of the autonomic nervous system. Twenty-one sedentary middle-aged men

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... t an 8-week moderatevolume exercise program and 24-hour heart rate recordings were obtained prior and following the exercise period. Temporal dynamic changes over the 24-hour period were calculated using the complex correlation measure (CCM) derived from the Poincaré Plot and analyzed in 1-hour intervals. The maximum significant differences between pre and post moderatevolume exercise were between the morning hours of 8.30 to 9.30 AM (p= 0.011) and in the afternoon between 4.00 and 5.00 PM (p=0.021). In the morning the temporal dynamics (mean ± sd) increased from 0.1±0.02 to 0.15±0.06. In the afternoon period the increase was from 0.11±0.04 to 0.14±0.05. An increase in CCM indicates a more complex temporal dynamics of the heart rate and an improved parasympathetic influence.