New Study Finds Attacks Most Common in Morning Hours : Stress of Waking Up Can Be Hazardous to Your Heart

People are three times more likely to suffer heart attacks at 9 a.m. than at 11 p.m., probably because the stress of waking up somehow triggers changes in the body that cause the attacks, researchers said Wednesday.

The findings could improve understanding of what makes lethal blood clots lodge in the heart’s arteries and provide clues for preventing them, said Dr. James E. Muller, one of the researchers. “This represents a big new area to research,” he said.

The study found that heart attacks are more common between 6 a.m. and noon than at any other time of day. The incidence reaches a peak at 9 a.m. and then declines to a low point at 11 p.m.

The researchers theorize that the important factor is when people wake up, not the time of day, so that those who work night shifts might have the highest risk of heart attacks in the evening.

2,999 People Studied

The study, conducted by researchers at Boston’s Brigham and Women’s Hospital and Harvard Medical School, was based on an analysis of 2,999 heart attack victims. It was published in today’s issue of the New England Journal of Medicine.

Usually a heart attack--what doctors call a myocardial infarction--occurs when a blood clot blocks an artery that feeds the heart. The heart muscle is starved for oxygen, and some of it dies.

The researchers are not certain why heart attacks are more common in the morning, but they have several theories.

“Since we know that infarct is associated with a clot, the leading possibility would have to be that there is some variation in the tendency of the blood to clot during that time of day,” said Muller, the principal author of the study.

Other research suggests that levels of a natural blood thinner called heparin are lower in the morning, while blood platelets are more likely to clump together then.

Muller noted that the sympathetic nervous system, which prompts such things as increases in heartbeat during stress, is also less active during sleep.

“It begins to be activated in a very harsh manner, as we all know, when the alarm clock rings,” he said. “It could be related to some aspect of that stress in the morning.”

The researchers noted that 14 earlier studies have also found heart attacks to be more likely in the morning.