Circadian rhythm in cardiac baroreflex effectiveness contributes to low systolic blood pressure during the biological night in young healthy adults

Introduction: The endogenous circadian system modulates sympathetic activity, plasma catecholamines and resting systolic blood pressure (BP). However, the timing of circadian peaks and troughs of catecholamines and sympathetic activity do not match the circadian peak and trough in systolic BP. As the baroreflex is essential for rapid modifications of heart rate and sympathetic nerve activity to maintain systolic BP, we tested whether the circadian system modulates the cardiac baroreflex system in healthy young adults and whether such a system could explain the circadian variations in systolic BP. Methods: Nine (4 males and 5 females) healthy adults (age: 23±SD2 y) completed a 30-hour circadian ‘forced desynchrony’ protocol in continuous dim light (<8 lux), throughout which all behaviors and measurements were evenly distributed across the 24-hour cycle. Participants underwent five recurring 6-hour cycles of 2-hour sleep opportunities and 4-hour standardized wake periods. Circadian phases of measurements were determined relative to the dim light melatonin onset, calculated from salivary melatonin. Spontaneous beat-to-beat systolic BP (finger plethysmography) and electrocardiogram were recorded for 10 minutes every waking period at rest with participants in the supine position. We assessed standard cardiac baroreflex sensitivity (BRS) (ms/mmHg) using the sequential method and the baroreflex effectiveness index (BEI) as the proportion between the number of systolic BP changes followed by reflex changes in heart rate and the total number of systolic BP changes in the same time window. Circadian variation was tested using a mixed model ANOVA with circadian phase as a fixed factor and subject as a random factor with significance set as p<0.05. Results: Systolic BP had a circadian rhythm (p=0.028) with a trough at ~midnight and a peak at ~5 pm. Cardiac BEI had a circadian rhythm (p=0.025) with a peak at ~midnight and through at ~8 am, while BRS did not have a significant circadian rhythm (p>0.05). Conclusion: In young, healthy adults, the endogenous circadian system impacts spontaneous baroreflex effectiveness but not baroreflex sensitivity. The number of times the cardiac baroreflex system acts on BP changes appears to be more important than the sensitivity of this system in determining the BP decline during the biological night. This finding indicates that the baroreflex system may be a promising system to study in the pathophysiology of the non-dipping pattern of BP in people with hypertension. R01HL163232; R35HL155681, OCTRI-UL1TR002369; AHA 24CDA1267757. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.