Does Chronotype Influence Diurnal Changes in Cardiovascular Disease Risk Factors? A Preliminary Analysis
Authors: Cherubini Joshua , Stone Jenna , Badour Matthew , MacDonald Dr. Maureen 
Published: May 2024

Introduction: Humans experience diurnal fluctuations in the prevalence of cardiovascular events which is likely an output of endogenous circadian influence. Relatedly, vascular endothelial cell function, measured by conduit artery responsiveness to hyperemia during a flow-mediated dilation test, is supposed to follow a diurnal rhythm that is moderated by an individual’s chronotype. We therefore sought to examine the extent to which diurnal changes in other cardiovascular risk factors, such as blood pressure, cardiac chronotropy, and circulating blood leukocytes, were related to chronotype in humans. A moderating influence of chronotype on diurnal changes in the aforementioned parameters was hypothesized given prior research associating a preference for evening wakefulness with an adverse cardiometabolic phenotype. Methods: All data are presented as mean ± standard deviation. This abstract includes secondary data from a preliminary sample of 16 young (19 ± 2 years), healthy males and females. Participants completed a Munich Chronotype Questionnaire (MCQ) to quantify diurnal preference for morning or evening activity. The corrected mid-sleep time on free days (MSFSC) was calculated from the MCQ and interpreted to represent an index for morning or evening chronotype. Participants attended data collection sessions in the morning (09:16 ± 01:27 hh:mm) and again in the evening (16:19 ± 00:31 hh:mm) under resting conditions for the quantification of blood pressure and enumeration of circulating leukocytes from venous blood. Results: The sample included in this analysis had a slight preference for evening wakefulness (MSFSC = 04:58 ± 01:17 hh:mm). A significant and robust diurnal variation was found among circulating venous leukocytes (P < 0.001) which measured higher in the afternoon (2.06×106 ± 3.67×105 cells/mL blood) than morning (1.61×106 ± 4.68×105 cells/mL blood). The diurnal variation in circulating venous leukocytes was independent (β = -9.87×104; R2 = 0.19; P = 0.186) of individual chronotype, as were individual diurnal changes of all other hemodynamic parameters included in this analysis (all P > 0.05). More participants will be recruited to satisfy statistical power for primary outcomes. Discussion: These preliminary results from healthy, young adults indicate that individual disposition towards morning or evening wakefulness may not exert a profound influence over diurnal changes in several cardiovascular disease risk factors under resting conditions. Funding Sources: The study was funded by the Natural Sciences and Engineering Research Council of Canada (DG: 20011033) held by Dr. MJM). JMC and MIB were supported by Ontario Graduate Scholarships. JCS was supported by the Natural Sciences and Engineering Research Council of Canada Graduate Scholarships – Master’s. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.