A Circadian Rhythm in Heart Rate Variability Contributes to the Increased Cardiac Sympathovagal Response to Awakening in the Morning

Abstract

Morning hours are associated with a heightened risk of adverse cardiovascular events. Recent evidence suggests that the sleep-wake cycle and endogenous circadian system modulate cardiac function in humans and may contribute to these epidemiological findings. The aim of the present study was to investigate the interaction between circadian and sleep-wake–dependent processes on heart rate variability (HRV). Fifteen diurnally active healthy young adults underwent a 72-h ultradian sleep-wake cycle (USW) procedure (alternating 60-min wake episodes in dim light and 60-min nap opportunities in total darkness) in time isolation. The present study revealed a significant main effect of sleep-wake–dependent and circadian processes on cardiac rhythmicity, as well as a significant interaction between these processes. Turning the lights off was associated with a rapid increase in mean RR interval and cardiac parasympathetic modulation (high-frequency [HF] power), whereas low-frequency (LF) power and sympathovagal balance (LF:HF ratio) were reduced (p ≤ .001). A significant circadian rhythm in mean RR interval and HRV components was observed throughout the wake and nap episodes (p ≤ .001). Sleep-to-wake transitions occurring in the morning were associated with maximal shifts towards sympathetic autonomic activation as compared to those occurring during the rest of the day. Namely, peak LF:HF ratio was observed in the morning, coincidental with peak salivary cortisol levels. These results contribute to our understanding of the observed increase in cardiovascular vulnerability after awakening in the morning. (Author correspondence: [email protected])

Keywords:

• Autonomic nervous system
• Circadian rhythms
• Heart rate variability
• Sleep-wake cycle

ACKNOWLEDGMENTS

This work was supported by an operating grant from the Canadian Institute of Health Research; and salary support from the Fonds de la recherche en santé du Québec (D.B.B.); the Standard Life Foundation (D.B.B.); and the Institut de Recherche Robert-Sauvé en Santé et en Sécurité du Travail (P.B.). We wish to thank the research participants and the staff and students of the Centre for Study and Treatment of Circadian Rhythms for their contributions to this investigation. We also thank Dr. Sylvie Rhéaume, Dr. Alain Solignac, and Abdelmadjid Azzoug, RN, for medical supervision; Dr. Claire-Dominique Walker for salivary cortisol assays; Francine Duquette for dietary information; Zia Choudhry and Manon Robert for their assistance on the sleep recordings; and Véronique Pagé for statistical advice.

Declaration of Interest: D.B.B. has received speaker honoraria and travel grants from Servier Canada. She is founder/CEO of Alpha Logik Consultants Inc. The remaining authors report no conflict of interest. The authors alone are responsible for the content and writing of the paper.