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Major study associates shift work with illness

Risk of heart attack, stroke increases with night work

By: Sharon Kirkey

Posted: 07/27/2012 1:00 AM | Comments: 0

Shift work -- especially working nights -- increases the risk of heart attack and stroke, according to a Canadian-led study billed as the largest of its kind.

About a third of Canada's full-time labour force does shift work -- working evening and nights, rotating shifts, split shifts and any other schedule that isn't nine-to-five.

Until now, the data on whether shift work heightens heart attack and stroke risk have been controversial, with some studies showing an increased risk and others no association whatsoever.

For their new study, the international research team pooled results from 34 studies involving more than two million people. They found shift work was associated with a 23 per cent increase in the risk of heart attack, a five per cent increase in the risk of stroke and a 24 per cent increase in the risk of unstable angina, coronary artery disease and other "coronary events."

Night work was associated with the sharpest increase in risk -- 41 per cent -- for major vascular problems.

When extrapolated to the Canadian population, "about one in 14 heart attacks and just under one in 60 strokes are directly related to shift work," said senior investigator Dr. Daniel Hackam, an associate professor and clinical pharmacologist at Western University in London, Ont.

The findings were reached after other factors that could skew the results were taken into account, the authors said.

"One in three adult Canadians who are employed full time are shift workers, so it's a highly prevalent employment condition," Hackam said. "And we're going toward a 24/7 society -- there's even talk of banks being open 24 hours a day.

"We don't think shift work is going to go away."

The team reviewed studies published from the 1960s to 2012 involving 2,011,935 people. They found heart attacks and ischemic strokes -- where a blood clot lodges in a vessel in the brain, squeezing off blood flow -- were more common among shift workers than other people.

Published in the British Medical Journal, the study is the "largest synthesis of shift work and vascular risk reported to date," according to the authors.

They haven't proved cause and effect, merely an association. Just how the erratic schedules of shift work might heighten heart attack and stroke risk isn't clear, but several mechanisms might be at play. "We may just not be hard-wired to be working in the night," Hackam said.

Shift work disrupts circadian rhythm -- the body's 24-hour sleep-wake cycle. Blood pressure, heart rate and even cholesterol levels move to low levels during sleep.

"That's just not the case in someone who is awake and engaged in work," Hackam said. "Their blood pressures are going to be higher, their heart rates are going to be higher, their cholesterol and (blood) clotting factors -- they're going to be exposed to cardiovascular risk while the rest of us are sleeping and repairing our bodies." According to the authors, even a single overnight shift is enough to increase blood pressure, and insomnia itself is a risk factor for heart attack.

Our bodies also aren't genetically adapted to eating in the middle of the night. "Most of us are fasting while we sleep and not accumulating fat and carbohydrates at night," Hackam explained. Night eating can lead to spikes in blood glucose and insulin, which are toxic to the arteries and increase the risk of diabetes.

Another factor may be exposure to electric light at night -- a "relatively recent phenomenon," Hackam said. Lab experiments have shown within seconds of exposure to bright light at night, melatonin -- a hormone that lowers blood pressure -- drops dramatically, essentially to undetectable levels, Hackam said.

The study's major strength is its sheer size, said Dr. Andrews Wielgosz, a professor of medicine at the University of Ottawa and a spokesman for the Heart and Stroke Foundation. "It's a powerful observation," he said. "But at the end of the day, it's still an observation, which doesn't explain what's going on."

-- Postmedia News

Republished from the Winnipeg Free Press print edition July 27, 2012 A13

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