Endogenous circadian rhythm in human motor activity uncoupled from circadian influences on cardiac dynamics

Plamen Ch Ivanov, Kun Hu, Michael F. Hilton, Steven A. Shea, H. Eugene Stanley

Research output: Contribution to journalArticlepeer-review

100 Scopus citations


The endogenous circadian pacemaker influences key physiologic functions, such as body temperature and heart rate, and is normally synchronized with the sleep/wake cycle. Epidemiological studies demonstrate a 24-h pattern in adverse cardiovascular events with a peak at ≈10 a.m. It is unknown whether this pattern in cardiac risk is caused by a day/night pattern of behaviors, including activity level and/or influences from the internal circadian pacemaker. We recently found that a scaling index of cardiac vulnerability has an endogenous circadian peak at the circadian phase corresponding to ≈10 a.m., which conceivably could contribute to the morning peak in cardiac risk. Here, we test whether this endogenous circadian influence on cardiac dynamics is caused by circadian-mediated changes in motor activity or whether activity and heart rate dynamics are decoupled across the circadian cycle. We analyze high-frequency recordings of motion from young healthy subjects during two complementary protocols that decouple the sleep/wake cycle from the circadian cycle while controlling scheduled behaviors. We find that static activity properties (mean and standard deviation) exhibit significant circadian rhythms with a peak at the circadian phase corresponding to 5-9 p.m. (≈9 h later than the peak in the scale-invariant index of heartbeat fluctuations). In contrast, dynamic characteristics of the temporal scale-invariant organization of activity fluctuations (long-range correlations) do not exhibit a circadian rhythm. These findings suggest that endogenous circadian-mediated activity variations are not responsible for the endogenous circadian rhythm in the scale-invariant structure of heartbeat fluctuations and likely do not contribute to the increase in cardiac risk at ≈10 a.m.

Original languageEnglish (US)
Pages (from-to)20702-20707
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number52
StatePublished - Dec 26 2007
Externally publishedYes


  • Cardiac vulnerability
  • Circadian pacemaker
  • Locomotor activity
  • Scale invariance

ASJC Scopus subject areas

  • General


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