Internal Clock Affects Heart Rhythm Patterns

Finding could lead to better understanding of heart disease

TUESDAY, Dec. 28, 2004 (HealthDayNews) -- In a scientific first, researchers have found that the body's internal clock, known as the circadian cycle, affects heart rate, independent of a person's sleep/wake cycle or other behavioral influences.

The finding may help scientists better understand the causes of heart disease and lead to improved treatments, the researchers said.

Physicists from Boston University and physiologists from Brigham and Women's Hospital in Boston studied five healthy volunteers who spent 10 days in dimly lit rooms that isolated them from any stimuli or time cues.

The researchers gradually shifted the volunteers' behavior patterns until the volunteers had a 28-hour day -- spending about 19 hours awake and nine hours sleeping. This 28-hour day schedule was maintained for seven days.

The researchers found that the volunteers' core body temperatures, which were monitored to measure their internal circadian phases, continued to function on an approximate 24-hour period. This showed that their sleep/wake cycles had been separated from their circadian cycles.

An analysis of the volunteers' heartbeat dynamics showed a significant circadian rhythm influence. This included a notable response at the internal circadian phase corresponding to 10 a.m. That's the time of day when people with heart disease most often suffer cardiac events such as heart attacks.

"We are tempted to speculate that if the same circadian effect occurs in people with diseased hearts, then this may contribute to the day/night pattern of cardiac events," Steven Shea, director of Brigham and Women's Hospital's medical chronobiology program and associate professor of medicine at Harvard Medical School, said in a prepared statement.

"But this was only a study on healthy subjects, and, therefore, we are a long way from making clinical recommendations. Further studies could, however, provide insight to the underlying cause of the disease and to therapies that might work better by being timed to the specific phases of the body clock," Shea added.

The study appears in this week's issue of the Proceedings of the National Academy of Sciences.

More information

Northwestern University has more about circadian rhythms.

SOURCE: Proceedings of the National Academy of Sciences, news release, Dec. 27-31, 2004
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