STUDY OBJECTIVES: We aimed to determine whether the endogenous circadian system modulates blood pressure (BP) and its hemodynamic mechanisms during the post-exercise period.
METHODS: Ten healthy adults (mean age: 24±2 [SD] years; 4 males, 6 females) completed a 30-hour circadian protocol in dim light (<8 lux), while all behaviors and measurements were evenly distributed across the 24-hour circadian cycle. Participants underwent five recurring 6-hour cycles of 2-hour sleep opportunities and 4-hour standardized wake episodes. After 90 minutes of awakening during each wake episode, participants performed 30 minutes of moderate-intensity aerobic exercise at 40% heart-rate-reserve. Systolic, diastolic, and mean BP and hemodynamic determinants derived from the beat-to-beat BP waveform (finger photoplethysmography) were measured just before and 30 minutes after the conclusion of each exercise bout. Arm and leg blood flows were assessed using vascular ultrasound. Endogenous circadian phases were determined using each participant's dim-light melatonin onset. Mixed-model cosinor analyses were used to test for the presence of circadian rhythmicity (with significance set at p<0.05).
RESULTS: Systolic and mean BP reduction post-exercise had significant circadian rhythms, with the greatest decreases at the circadian phase corresponding to ~1 PM (-10 mmHg and -9 mmHg), respectively. Systemic vascular resistance and leg blood flow also had significant circadian rhythms, with the lowest resistance and greatest blood flow at ~11 AM.
CONCLUSIONS: In young, healthy adults, the circadian system modulates the post-exercise BP reduction, likely due to skeletal muscle vasodilatory mechanisms.
