Why Do Heart Attacks Occur Most Frequently Between 9 And 11 Am?

Studying five healthy volunteers for 10-day periods in pioneering efforts to ultimately answer this question, a collaboration of Boston University physicists and Harvard physiologists has found evidence that the body's circadian clock (a part of the brain that regulates daily biological activities) influences patterns in the heart's "interbeat intervals," the lengths of time between successive heartbeats. At around 10AM for all the healthy individuals, the values of successive interbeat intervals displayed increased signs of randomness, statistically resembling that seen in previous studies of individuals with heart disease.

In their studies, the researchers took special care to isolate the effects of a person's internal circadian clock (which has a 24.2-hour rhythm, marked by a regular rise and fall of body temperature) from the effects of behavior (such as physical activity and a person's wake/sleep time) or external stimuli (such as the rising or setting of the sun). Towards these ends, the researchers made sure to "desynchronize" the individuals' internal body clocks from these other factors by keeping the volunteers in a dimly lit room and by varying their sleep and wake times from day to day while keeping activity levels relatively constant.

The researchers next plan to explore how an individual's behavior may interact with the circadian clock to influence the correlations in interbeat intervals. The researchers have not yet studied patients with heart disease and are far from being able to make clinical recommendations. However, their further research may obtain insights into the underlying causes of increased cardiac risk and could lead to improved therapy, such as more appropriately timed medication to coincide with phases of the body clock. (Hu et al., Proceedings of the National Academy of Sciences, December 28, 2004; contact Plamen Ch. Ivanov, Boston University, 617-353-3891, plamen@argento.bu.edu; Steven Shea, Harvard Medical School, 617-732-5013, sshea@hms.harvard.edu)