Endogenous circadian rhythm in an index of cardiac vulnerability independent of changes in behavior

Kun Hu, Plamen Ch Ivanov, Michael F. Hilton, Zhi Chen, R. Timothy Ayers, H. Eugene Stanley, Steven A. Shea

Research output: Contribution to journalArticlepeer-review

120 Scopus citations


There exists a robust day/night pattern in the incidence of adverse cardiac events with a peak at ≈<10 a.m. This peak traditionally has been attributed to day/night patterns in behaviors affecting cardiac function in vulnerable individuals. However, influences from the endogenous circadian pacemaker independent from behaviors may also affect cardiac control. Heartbeat dynamics under healthy conditions exhibit robust complex fluctuations characterized by self-similar temporal structures, which break down under pathologic conditions. We hypothesize that these dynamical features of the healthy human heartbeat have an endogenous circadian rhythm that brings the features closer to those observed under pathologic conditions at the endogenous circadian phase corresponding to ≈10 a.m. We investigate heartbeat dynamics in healthy subjects recorded throughout a 10-day protocol wherein the sleep/wake and behavior cycles are desynchronized from the endogenous circadian cycle, enabling assessment of circadian factors while controlling for behavior-related factors. We demonstrate that the scaling exponent characterizing temporal correlations in heartbeat dynamics does exhibit a significant circadian rhythm (with a sharp peak at the circadian phase corresponding to ≈10 a.m.), which is independent from scheduled behaviors and mean heart rate. Cardiac dynamics under pathologic conditions such as congestive heart failure also are associated with a larger value of the scaling exponent of the interbeat interval. Thus, the sharp peak in the scaling exponent at the circadian phase coinciding with the period of highest cardiac vulnerability observed in epidemiological studies suggests that endogenous circadian-mediated influences on cardiac control may be involved in the day/night pattern of adverse cardiac events in vulnerable individuals.

Original languageEnglish (US)
Pages (from-to)18223-18227
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number52
StatePublished - Dec 28 2004
Externally publishedYes


  • Correlations
  • Detrended fluctuation analysis
  • Forced desynchrony protocol
  • Heartbeat dynamics
  • Scaling

ASJC Scopus subject areas

  • General


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