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FRIDAY, April 8 (HealthDay News) -- The human body has an independent circadian rhythm of blood pressure (BP), which peaks around 9 p.m., according to a study published online April 7 in the Circulation Research.
Steven A. Shea, Ph.D., from Brigham and Women's Hospital in Boston, and colleagues investigated the existence of an endogenous circadian rhythm of BP. They assessed BP in 28 normotensive adults across three complementary, multiday, in-laboratory protocols: a 38-hour constant routine, a 196-hour forced desynchrony protocol with seven sleep/wake cycles, and a 240-hour desynchrony protocol with 12 sleep/wake cycles. Behavioral and environmental influences were controlled and/or uniformly distributed across the circadian cycles. The core body temperature was used to determine circadian phases.
The investigators found significant circadian rhythms in systolic and diastolic BP in all three protocols, with the rhythm profile almost identical in all protocols. The three systolic and three diastolic peaks occurred at a circadian phase corresponding to approximately 9 p.m. Systolic BP had peak-to-trough amplitudes of 3 to 6 mm Hg, and diastolic BP, 2 to 3 mm Hg. The rhythm of BP seemed to be unrelated to phase differences between the circadian rhythms of BP and other variables, including cardiac vagal tone, heart rate, cortisol, catecholamines, or urine flow.
"There exists a robust endogenous circadian rhythm in BP. The highest BP occurred at the circadian time corresponding to ~9 p.m., suggesting that the endogenous BP rhythm is unlikely to underlie the well-documented morning peak in adverse cardiovascular events," the authors write.
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