Nocturnal variation in human sympathetic baroreflex sensitivity.

To determine whether or not there are nocturnal variations in sympathetic baroreflex sensitivity (BRS), we measured spontaneous sympathetic BRSs in eight normal subjects (average 24.5 years old) between 2300 and 0700 h. Electrocardiogram, blood pressure, polysomnography, and muscle sympathetic nerve activity (MSA) using microneurography were recorded. We defined cardiac 'baroreflex sequences' as those that contain three or more adjacent pulses, with the systolic blood pressure and the subsequent pulse interval either continuously increased or decreased. A similar analysis was applied to sympathetic BRSs. We selected three or more adjacent pulses during which diastolic blood pressures continuously increased or decreased. Total activity in MSA was defined as burst per minute x burst amplitude and we calculated the regression coefficients between the diastolic blood pressure and the subsequent total activities in MSA. The regression coefficients were classified as either negative or positive ones. When they were less than zero, we termed them 'baroreflex sequences'. Cardiac and sympathetic BRSs were estimated from the average slope of the baroreflex sequences. Sympathetic BRS was significantly lower during sleep than while subjects were awake in the evening (P < 0.05), and it remained low after the subjects woke up in the morning (P < 0.05). Conversely, cardiac BRS had a tendency to increase during sleep in the night, but not statistically significant. This sympathetic BRS pattern may contribute to diurnal haemodynamic variables and may account, at least in part, for the connection between circadian rhythm and cardiovascular disease.