In a new study from Japan, men who often worked more than 60 hours a week and got five hours or less of sleep a night had twice the risk of suffering a heart attack.
The study builds on and confirms previous research, says study author Dr. Ying Liu, a research fellow at the National Cancer Center in Tokyo and a graduate student at Kyushu University in Fukuoka. It appears in the July issue of Occupational and Environmental Medicine."Previous studies in the United States and Europe, as well as in Japan, have suggested that long work hours and insufficient sleep are related to heart attack, and our findings confirm their results," Liu says.
The findings from the Japanese study probably apply to American men as well, Liu says. However, he's not sure whether women are at the same risk if they work to excess and sleep too little because not enough research has been done on them.
In the United States, full-time workers average a 42.9 hour work week, according to the latest figures from the U.S. Bureau of Labor Statistics, although many workers regularly log 60 hours or more.
Liu and his colleagues evaluated 260 men, aged 40 to 79, who had been admitted to a hospital for a heart attack for the first time. All survived the attack. The researchers compared this group with 445 men with no history of heart attacks who were in the same age range and lived in the same areas.
Study subjects provided information about their usual work week, their sleep habits, and their days off during the past month and the past year. They also discussed any medical conditions that might boost their heart attack risk, such as high blood pressure and high blood cholesterol.
Even when the researchers controlled for the known risk factors, they found that lack of sleep and long hours were associated with heart attack. Those who worked 60-plus hours a week -- not uncommon in Japan -- had double the heart attack risk of those who worked 40 hours or less. Also, those who suffered heart attacks often slept for five or fewer hours a night.
"The joint effect of work hours and lack of sleep is the unique point of our study," Liu says.
A U.S. sleep expert calls the study findings interesting, but not surprising.
"The conclusions reflect what we clinically think to be true," says Dr. Susan Sprau, an associate clinical professor of medicine at the University of California, Los Angeles, and a sleep disorders expert at Santa Monica-UCLA Medical Center.
Adequate sleep is crucial for good health, she says, adding that most adults need eight hours of sleep to function best.
Another American expert, cardiologist Dr. Thomas Pickering, calls the study findings "very plausible."
Many experts have talked about the negative effects of stress from too much work and too little sleep, Pickering says. "But here is some fairly concrete evidence that it does matter, that some of us are working ourselves into the ground," he says.
Exactly how the lack of sleep and excess work hours trigger a heart attack isn't known for sure. Liu speculates both can increase blood pressure and heart rate, and the chronic stress can induce heart function abnormalities.
Sprau agrees that several theories could explain how lack of sleep and excess work do a double whammy on the heart. They include the "sympathetic drive" theory, in which the activity of the sympathetic nervous system -- the so-called "fight or flight" response -- might increase to a point where it triggers a heart attack. Or, insufficient sleep might have an adverse effect on blood platelets, which promote clotting, she says.
Is there any way to undo the damage if a hectic work schedule is unavoidable and there's no time to get enough sleep?
"You can compensate by taking more days rest if possible, or eat more healthfully to offset the risk," Liu says. "But I don't know whether doing more exercise should be recommended. Maybe moderate exercise, such as walking or jogging, is good for a person with a sedentary job. But it may not be good for one who is tired out with [manual] labor."
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