The circadian clock is an endogenous biological timekeeper that responds to environmental changes and governs various physiological processes over a 24-hour cycle. Blood pressure (BP) variation is thought to be controlled by the circadian clock, but few studies have examined circadian control of BP in humans. Moreover, it is unknown whether nighttime BP dipping is driven by the circadian system or by external factors. We investigated whether the circadian system drives 24-hour rhythms in BP, including nighttime BP dipping, using a 30-hour constant routine (CR) protocol. The CR protocol controls for external factors, allowing circadian rhythms to be isolated and measured, by having participants lie in a semi-recumbent posture in dim light (<10 lux) at a constant temperature, consume isocaloric snacks every 2 hours, and maintain wakefulness. To measure the BP rhythm, ambulatory BP was measured every 30 minutes (SpaceLabs 90227), and to measure the central circadian rhythm, core body temperature was measured every 10 seconds using an ingestible, wireless sensor (HQInc Core Body Temperature Wireless Data Record and Sensor). To date, 17 normotensive African American participants (13 females and 4 males), with a mean age of 37 (± 11.3) years and body mass index (BMI) of 32.5 kg/m
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, have completed the study. Approximately 59% of participants (10 of 17) had non-dipping systolic BP at screening, defined as a <10% decrease in mean systolic BP from daytime to nighttime. Under constant conditions, 94% of participants (16 of 17) had a non-dipping BP phenotype. Median systolic BP dipping was 0.8% for females and 2.2% for males. There was a robust rhythm in participants’ core body temperature but not BP, suggesting that the circadian clock may not contribute substantially to a nighttime decrease in BP in normotensive African Americans. Instead, the non-dipping BP phenotype is likely more so a result of behavioral and/or physiological sleep-related processes. Future research and interventions for non-dipping BP may need to target these underlying behavioral and physiological processes.
Effect of circadian system disruption on the concentration and daily oscillations of cortisol, progesterone, melatonin, serotonin, growth hormone, and core body temperature in periparturient dairy cattle
