People who workin shifts are at heightened danger of heart disease and the risk increases with years they work in shifts, finds a Chinese study of more than 300,000 people.Shift work "can earn more profit, but it can also cause harm to the health of employees. Thus, employers should reduce shift work as much as possible," lead author Weihong Chen, a researcher in occupational and environmental health at Huazhong University of Science and Technology in Wuhan, was quoted as saying to the Health Day.While the reason is unknown, disruption in the normal sleep-wake cycle could increase stress. In the study, published in the journal Occupational Medicine, the team analysed data from 21 earlier studies involving over 320,000 people and nearly 20,000 cases of coronary heart disease.The study was not designed to prove the cause and effect, but the data showed shift workers were 13 per cent more likely to develop coronary heart disease than daytime workers.For every year spent working in shifts, there was a nearly one per cent increase in the risk of coronary heart disease, the report said.According to Weihong, employers should pay attention to staff members who are experiencing symptoms of heart problems as well as those with a family history of heart disease. Employers could provide health promotion, such as information on how to prevent and deal with ischemic heart disease, she said.Companies could also consider providing health check-ups to detect early signs of heart problems, Weihong said.