Circadian changes in vascular sympathetic activity in ambulant subjects.

In 10 ambulant subjects we studied the circadian changes in sympathetic vasomotor control as assessed by the spectral power of the 0.1-Hz low-frequency component of systolic arterial pressure variability measured with a Millar phi 3F tip transducer. The low-frequency component was higher during the daytime, while the subjects were performing light physical activity, and lower during the night, thus paralleling the circadian systolic blood pressure pattern. However, the morning low-frequency rise preceded the blood pressure increase by about 3 h, suggesting that vasometer control and blood pressure control are at least partly related to different mechanisms.