Shift work — especially working nights — increases the risk of heart attack and stroke, according to a Canadian-led study that is being billed as the largest-ever 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 has 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 that shift work was associated with a 23 per cent increase in the risk of a 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 held after other factors that could skew the results were taken into account, such as smoking — meaning that other unhealthy behaviours couldn't explain the association, 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 towards a 24/7 society — there's even some talks 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 that 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.

The risk for any one person is modest but, given shift work is so widespread, the overall risks for the population are high, Hackam said.

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 hardwired 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 all return to low levels during sleep while the body rests.

"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.