The chances of suffering heart problems are not equal throughout the day. Heart attacks occur more often around 10 o'clock in the morning than any other time, a peak that previously was attributed to daily behavior patterns getting underway. A report published online this week by the Proceedings of the National Academy of Sciences indicates that the heartbeats of healthy people, too, exhibit strong circadian rhythms, which could help explain the morning crest of adverse cardiac events.

Steven A. Shea of Harvard Medical School and his colleagues studied healthy individuals between the ages of 20 and 33 years old. The volunteers lived in individual suites for 10 days and had their regular daily patterns disrupted. They were monitored closely and asked to adhere to specific sleep and wake cycles while having their heartbeats monitored using electrocardiograms. The team discovered that a feature of the subjects' heartbeats known as the scaling exponent, which is a statistical classification of beats over time, displayed a significant 24-hour rhythm regardless of daily activities. What is more, the peak occurred between nine and 11 a.m.

According to the report, the findings suggest that "the underlying mechanism of cardiac regulation is strongly influenced by the endogenous circadian pacemaker." And because higher scaling exponents are associated with heart disease, the scientists posit that this fundamental pattern could influence vulnerable people, such as those suffering from congestive heart failure, and contribute to the pattern of early morning heart attacks observed in epidemiological studies.