We tested whether there is a circadian variation in baroreflex function measured as sensitivity and effectiveness, and whether it aligned with circadian variations in systolic blood pressure (BP). Eleven healthy adults (24 ± 2 years; 4 males and 7 females) completed a 30-h circadian protocol incorporating five recurring, identical, 6-h cycles of 4-h standardized wakefulness and 2-h sleep opportunities. Spontaneous beat-to-beat BP (finger photoplethysmography) and R-R intervals (electrocardiogram) were recorded at rest for 10 min during each wakefulness period, to assess cardiac baroreflex sensitivity (BRS) and effectiveness index (BEI). Vascular endothelial function (VEF) was assessed as flow-mediated dilation of the brachial artery. Dim light melatonin onset was used as a circadian phase marker. Systolic BP exhibited a significant trough at ~1:40 am (p = 0.02). This trough closely aligned with the nocturnal peak of cardiac BEI that occurred at ~1 am (p = 0.02). BRS showed no significant variation. VEF exhibited a significant peak at ~10 am In healthy adults, the nocturnal peak in BEI is possibly aligned with the circadian trough in systolic BP. It remains to be determined whether shifting one would shift the other in a similar direction, and whether the alignment holds in people with disease.
