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Morning Heart Attacks May Follow Body's Clock

Circadian Rhythms Influence the Heart, Study Shows
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WebMD Health News

Dec. 20, 2004 -- Ten o'clock in the morning is the most dangerous hour of the day for your heart, with more heart attacks and strokes occurring then than at any other time. The body's internal clock may be partly responsible, new research shows.

Heart attacks and strokes peak around 10 a.m. "These events do not occur randomly during the day," say experts, including Kun Hu of Boston University.

What explains the pattern? For many people, mornings hum with activity from the time their feet hit the floor to the start of work, school, or other commitments. Throw in a morning workout, job angst, or a rough commute, and you've got plenty of factors that could conceivably tax the heart, which might already be strained by poor diet and conditioning.

Or could something else also be at work? Hu's team looked past the mad pace of mornings to a more subtle influence: circadian rhythms. They are the body's natural pacemaker, which is governed by sleep-wake or light-dark cycles. This pacemaker controls functions such as sleep.

Hu and colleagues wanted to see if circadian rhythms affect the heart. They recruited five healthy young people (four men and one woman) about 25 years old.

Using electrocardiographs, the researchers monitored participants' heartbeats for two days. Then, they sent subjects into a kind of temporary twilight zone.

Participants spent 10 days in individualized, controlled suites. Instead of normal 24-hour days, they were put on a 28-hour cycle, sleeping for about 9.5 hours and staying up for 18 hours and 40 minutes at a stretch.

The volunteers had no way to tell time. Bright lights were eliminated and room temperatures remained steady. Activities were also limited. During wakeful periods, participants were only allowed to walk around the suite, sit, or rest.

The conditions weren't intended to drive participants crazy. Instead, the researchers wanted to force them out of their comfort zones, allowing their body clock to oscillate at its own rate.

Meanwhile, the electrocardiograms traced participants' heart rhythms. The researchers were especially interested to see that heartbeat fluctuations peaked sharply from 9 a.m.-11 a.m. (on the normal clock).

That's a "well-known window of cardiac vulnerability," say the researchers.

If the same thing happens in less healthy people, circadian rhythms could partly explain their midmorning heart attack risk.

That doesn't mean that your body clock is out to get you. But it might be the last straw for hearts in poor condition.

"It is likely that both factors [combine] to produce the morning peak in adverse cardiac events," say the researchers. Their study appears in the Dec. 28 issue of the Proceedings of the National Academy of Sciences of the United States of America.

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