The research, led by Leandro C. Brito and colleagues at Oregon Health & Science University, set out to answer a deceptively simple question: does the baroreflex system itself follow a circadian rhythm, and if so, does that rhythm line up with the circadian rhythm of blood pressure? Previous studies had hinted at a daily pattern in baroreflex function, with sensitivity appearing greatest around three in the morning in both normotensive and hypertensive individuals. But those studies measured people during their normal sleep-wake cycles, meaning the results were hopelessly entangled with behavior. Sleep, posture, meals, and physical activity all influence blood pressure and autonomic function, making it impossible to separate the effect of the internal clock from the effect of simply being asleep.

To untangle this knot, the team turned to one of the most powerful tools in human circadian physiology: the forced desynchrony protocol. In this design, volunteers live on a behavioral schedule that is too short for their internal circadian pacemaker to follow. Because the human circadian clock runs on an intrinsic period of roughly 24.1 hours and cannot entrain to cycles far outside its entrainment range, the internal rhythm is forced to free-run at its own pace while the behavioral cycle marches to a different drummer. When physiological measurements are later plotted against circadian phase rather than clock time, any rhythm that emerges must be driven by the endogenous circadian system rather than by sleep, meals, or activity.

Eleven healthy young adults, aged 20 to 39, completed a 30-hour laboratory stay built around five consecutive six-hour cycles, each containing four hours of wakefulness followed by a two-hour sleep opportunity. Every aspect of their environment was tightly controlled. Wake periods took place in dim light below eight lux, sleep occurred in total darkness, and no time cues were provided. Identical isocaloric meals of roughly 322 kilocalories, composed of about 35 percent fat, 50 percent carbohydrate, and 15 percent protein, were delivered every three hours. Participants remained supine except during meals, scheduled restroom visits, and supervised exercise. Polysomnography confirmed that they slept an average of 114 minutes during each two-hour window, with no significant differences in sleep duration across the protocol. Circadian phase was anchored to the dim light melatonin onset, the gold-standard marker of the biological clock, determined from hourly salivary samples.

During each wake period, the researchers recorded beat-to-beat blood pressure with finger photoplethysmography and cardiac electrical activity with a standard three-lead electrocardiogram. From these continuous signals they extracted two distinct measures of baroreflex function. The first, baroreflex sensitivity, quantifies how strongly the heart rate responds to a given change in systolic pressure, expressed as the slope of the relationship between the two. The second, the baroreflex effectiveness index, counts how often the baroreflex actually intervenes: it is the ratio of validated baroreflex sequences, in which pressure and heart interval change in the appropriate opposing directions across at least three consecutive cardiac cycles, to the total number of spontaneous systolic pressure sequences observed. Sensitivity measures the strength of the reflex when it fires; effectiveness measures how frequently it fires at all.

The results were striking in their asymmetry. Systolic blood pressure showed a clear circadian rhythm with a peak-to-trough amplitude of ten millimeters of mercury, bottoming out at a circadian phase corresponding to roughly 1:40 in the morning and peaking near 11:40 in the morning. Diastolic pressure showed no significant rhythm. Baroreflex sensitivity, perhaps surprisingly, also showed no significant circadian variation. But the baroreflex effectiveness index told a different story entirely. It peaked at a circadian phase corresponding to about one in the morning, averaging 34 percent above its overall mean, and fell to its lowest value, 48 percent below the mean, at around nine in the morning. In other words, the reflex was most active precisely when systolic pressure was at its circadian minimum.

Deeper analysis of the directionality of the reflex added an important nuance. When the researchers separated the data into up-up sequences, in which the reflex slows the heart in response to rising pressure, and down-down sequences, in which it accelerates the heart as pressure falls, they found that the nocturnal advantage came almost entirely from the up-up responses. During the circadian night, the baroreflex was 110 percent more effective at correcting increases in blood pressure than during the circadian morning. This asymmetry carries real clinical weight. Prior work has shown that in women with a family history of hypertension, the ability to correct pressure rises is selectively impaired, and in people with established hypertension, a deficient baroreflex response to pressure increases independently predicts death and adverse cardiovascular events.

The study also mapped the circadian rhythms of the vasculature. Brachial artery vascular conductance, an index of skeletal muscle vasodilation, peaked around two in the morning at 27 percent above its mean, immediately preceding the systolic pressure trough, and fell to its lowest point around six in the evening. Vascular endothelial function, measured as flow-mediated dilation of the brachial artery, peaked later, at around ten in the morning, 34 percent above the mean, and was lowest at six in the evening. Notably, this morning peak in endothelial function occurred about five hours earlier than previously reported in middle-aged adults studied with the same technique, raising the possibility that aging shifts the circadian phase of vascular protection. The authors suggest this morning peak may help explain why, paradoxically, adverse cardiovascular events cluster between six in the morning and noon in epidemiological studies.

Why did sensitivity fail to rhythm while effectiveness did? The authors offer several possibilities. Their participants were exceptionally healthy, young, and recreationally active, conditions that may have created a ceiling effect masking a true rhythm in sensitivity. Alternatively, sensitivity and effectiveness may act in counterphase, as earlier work by di Rienzo and colleagues proposed when the effectiveness index was first introduced. It is also possible that the circadian system exerts a stronger influence on the spontaneous engagement of the reflex than on its gain. Importantly, the findings held up when the total number of pressure sequences and validated baroreflex sequences were included as covariates, ruling out the trivial explanation that more opportunities to detect sequences arose at night.

The implications extend well beyond basic physiology. No currently available antihypertensive medication acts directly on the baroreflex, but non-pharmacological interventions, including exercise training, respiratory muscle training, and non-invasive vagal nerve stimulation, are known to improve baroreflex function. If the circadian system genuinely gates baroreflex effectiveness, these therapies might be timed to exploit that rhythm, opening a chronotherapy-based avenue for treating non-dipping blood pressure. The authors are careful to note the limits of their work: the sample was small and predominantly young and healthy, the 30-hour protocol offers only six-hour phase resolution, and the findings must be reproduced in older adults and in people with hypertension before they can inform clinical practice. Still, by rigorously isolating the internal clock from the noise of daily behavior, the study elevates the baroreflex from a passive bystander to a leading suspect in the mystery of nocturnal blood pressure regulation, and it hands researchers a concrete, testable target in the search for why some hearts fail to dip when night falls.

Subject of Research: Circadian variation in cardiac baroreflex effectiveness and its alignment with the nocturnal trough in systolic blood pressure in young healthy adults

Article Title: Nocturnal peak in cardiac baroreflex effectiveness aligns with nocturnal trough in systolic blood pressure in young healthy adults: Possible circadian effects

Article References: Brito, L. C., Jones, M. L., Chaudhary, N., Shea, S. A., & Thosar, S. S. (2026). Nocturnal peak in cardiac baroreflex effectiveness aligns with nocturnal trough in systolic blood pressure in young healthy adults: Possible circadian effects. Physiological Reports, 14(17), Article e71092. https://doi.org/10.14814/phy2.71092

Image Credits: AI Generated

DOI: 10.14814/phy2.71092

Keywords: circadian rhythm, baroreflex, blood pressure, nocturnal dipping, forced desynchrony, autonomic nervous system, vascular function, hypertension, melatonin, flow-mediated dilation, cardiovascular physiology, chronotherapy