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People who work irregular schedules or work outside of normal daytime hours are at higher risk of heart attack, stroke and other coronary events, according to a new study published this week in the British Medical Journal.

This study is not the first to show a link between shift work and heart health, but it is the largest-ever analysis of its kind. It pools together results from 34 previous studies on the topic, with a combined 2 million study participants from across the industrialized world, estimating that shift workers are at 23% greater risk of heart attacks than the other workers, 5% greater risk of ischemic strokes and 24% greater risk of all coronary events combined (a category that includes heart attack but not stroke). Shift workers also had slightly higher overall death rates than average, but those results were not statistically significant.

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In their analysis of shift work, the study’s authors included any regularly scheduled work outside of normal daytime hours — such as evening shifts, night shifts and early-morning shifts — as well as on-call or casual shifts, split shifts or irregular working hours, no matter what time of day that work typically occurred. In Canada (where many of the new study’s authors are based), about one-third of the workforce is engaged in shift work.

What’s not clear, however, is why those workers have worse heart health. Shift workers may be engaged in a wide range of industries, from retail to health care to transportation, and they may be highly skilled employees, like medical doctors, or relatively unskilled, like fast-food workers.

In their paper, the researchers write that shift work can disrupt sleep cycles and circadian rhythms, and that many night-shift workers in particular report insomnia, which is an independent risk factor for heart attack.

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But irregular working hours can also be a source of stress. Erratic schedules make it tougher for people to organize convenient child care, to keep doctor’s appointments or simply to plan leisure time with friends and family. Fluctuations in income that come from irregular shifts can also be stressful for anyone on a budget.

Studies like this are always complicated, however, in that it’s never completely obvious that shift work alone is the main culprit. Studies do typically try to adjust (using statistics) for other factors that may make shift workers different from other workers. But a woman who works as a grocery-store clerk may lead a very different life than one who works as an office receptionist, even if they are both 50-year-old nonsmokers with a high school diploma. In their research paper, the authors say they believe their results are not strongly biased by outside lifestyle factors, however, since adjustments for smoking and for socioeconomic status — measures of occupational class and education — did not alter their results very much.

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Most important, the researchers feel their findings have serious real-world implications. Of course, most people doing shift work won’t find it easy just to switch to a new work schedule. But even though we don’t know why shift work is linked to heart attack and stroke, the researchers write, people can take steps to protect themselves. “[P]eople who do shift work should be vigilant about risk-factor modification,” they write.

In other words, be extra careful to lead a healthy lifestyle: watch your diet and activity levels, keep your blood pressure in check and limit any smoking.

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