Shift work raises the risk of having a heart attack, a major review of working pattern health effects has shown.
Working unsocial hours was found to increase the chances of a heart attack by more than a fifth, with the risk of a stroke rising by 5%.
Shift patterns, especially night shifts, have long been linked to high blood pressure, high cholesterol levels and diabetes. But experts have disagreed about their association with heart and artery disease.
The new research, the largest analysis of shift work and heart risk to date, pulled together evidence from 34 studies involving more than two million people.
Coronary events - including heart attacks and angina chest pains - and strokes were more common among shift workers.
The chances of "any coronary event" were increased by 24%, and of heart attacks by 23%, while stroke risk was raised slightly by 5%.
Despite the relatively small effect, the impact on a large population could be significant, the authors pointed out.
In Canada, where a number of the researchers were based, 7% of heart attacks could be attributed to shift work.
The scientists, led by Dr Daniel Hackam, from Western University in Canada, wrote in the British Medical Journal: "Shift work is associated with vascular events, which may have implications for public policy and occupational medicine."
Night shifts were associated with the steepest increase in risk for coronary events, 41%. But research found no link between shift work and higher death rates from any cause.