People who work the graveyard and other non-9-to-5 shifts may face a higher risk of heart attacks, a Canadian review suggests.

Shift work disrupts our body clocks and can wreak havoc with work-life balance. But research on its association with health problems like high blood pressure, high cholesterol and Type 2 diabetes has been conflicted, partly because investigators use different methods, populations and definitions.

Thursday's issue of the British Medical Journal includes what an Ontario-led team called the "largest synthesis of shift work and vascular risk reported to date."

Paramedics and other emergency responders who work night shifts are among those who may face higher heart attack risks. (CBC)

After poring over the results of 34 studies on more than two million people, lead author Dr. Daniel Hackam, a clinical pharmacologist at the Stroke Prevention & Atherosclerosis Research Centre in London, Ont., and his co-authors concluded that shift work was associated with increased rates of vascular events such as heart attacks and ischemic strokes caused by lack of blood to the brain.

"Shift workers need to be vigilant about their cardiovascular health and get their cardiovascular risk factors checked on an annual basis through their primary care provider (e.g. cholesterol, blood pressure, glucose, waist circumference)," Hackam advised in an email.

"They should do everything they can to minimize their risks including smoking cessation, adopting a healthy diet, taking care to include physical exercise when they are not working, and recognizing the cardinal symptoms of cardiovascular disease (stroke and heart attacks)."

For employers, Hackam's suggestions included:

• Provide adequate breaks at work.
• Offer seminars on relaxation.
• Encourage employees to get their blood pressure and weight checked at work.

"All of these strategies are likely to be necessary to reduce the adverse consequences of shift work on cardiovascular health," he added.

In the study, shift work was not associated with increased death rates from any cause.

But working night shifts was associated with the steepest increase in risk for coronary events, 1.41 times, compared with working day shifts or the general population.

While the relative risks were "modest," the researchers said, there are implications for public policy and occupational medicine.

Lifestyle habits

Given that nearly a third, 32.8 per cent, of adults working in Canada were shift workers in the years 2009 and 2010, they estimated shift work could account for:

• 7 per cent of heart attacks.
• 7.3 per cent of coronary events.
• 1.6 per cent of ischemic strokes.

Shift work may select for people with worse lifestyle related habits like smoking and eating poor quality food, they acknowledged.

But considering socioeconomic status and unhealthy behaviours couldn't fully account for the increased risk.

The study's authors didn't have information about whether shift workers tended to be morning larks or night owls, so they couldn't assess if that mattered.

As with all observational studies, any cause-and-effect relationships can't be proven.

The investigators called for more research to try to determine which shift workers are most vulnerable and to look at how changing shifts might help heart health.

The research was funded by the Canadian Institutes for Health Research.