Synchronous recording of ECG from the second lead, phonocardiograph from the Erb's point and right carotid sphygmogram was performed in 20 healthy men aged 20 to 25 years four times during a 24-hour period: at 9:00 a.m., at 4:00 p.m., at 2:00 a.m. after awakening, and at 2:00 a.m. during all-night activity. The obtained polycardiograms were analysed by the method of Weissler calculating the measured and corrected left ventricular contraction periods. An increase was observed in the dependence of the diastole time (D), total electromechanical contraction time (QS2) and the left ventricular ejection time (LVET) on the heart rate (HR) in the night hours. Moreover, the functional reserve of the heart increased during the night as demonstrated by prolongation of the D time and decrease of QS2 in the heart cycle (R-R). It was found also that during the night hours the LVET and QS2 values and the pre-ejection period (PEP) were significantly greater independently of the degree of activity than in the morning and afternoon (p less than 0.05).