Diurnal variations in vagal and sympathetic cardiac control.

To investigate the diurnal variation in autonomic cardiac control, the magnitudes of the power spectral components of supine and standing heart rate variability were measured during controlled respiration (15 breaths/min). Examination was performed hourly between 0700 and 2300 h in eight male subjects whose activities and food intake were controlled for 24 h in the laboratory. The respiratory component (0.25 Hz) was greater in the morning than in the late afternoon (P less than 0.05) and decreased 30 min after food intake (P less than 0.01) in the supine position, but it was unaffected by the time of day or food intake while in the standing position. The Mayer wave component (0.03-0.15 Hz) did not change with the time of day, but it increased 90 min after food intake in both supine and standing positions (P less than 0.01 and P less than 0.05, respectively). These data suggest that supine vagal cardiac control during the waking period increases in the morning and decreases 30 min after food intake and that sympathetic cardiac control increases 90 min after food intake.