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Age-related modifications of heart rate during sleep-wakefulness cycle.

Twenty-four-hour monitoring of heart rate is currently used in cardiological practice, but it has been rarely applied to investigate the autonomic nervous system functioning. In the present study we investigated the heart rate of 275 healthy subjects, aged 15-69 years, divided into 11 groups by 5-year periods of age. Our results suggest that the balance between parasympathetic and sympathetic activities changes with advancing age. The parasympathetic activity becomes stronger and during sleep loses its power to modulate bradycardia. The general conclusion of our study is that all the investigations on cardiac autonomic functions are to be performed on age-matched basis.