Sleeping less than four hours or more than eight hours a night increases the risk of dying from some types of coronary heart disease, such as heart attacks and unstable angina pectoris, researchers said.

"This is the single largest study that has looked at how sleep duration affects the risk of dying from cardiovascular disease. Our results show that enough, but not too much sleep is important for a healthy lifestyle," said Linn Beate Strand from the Norwegian University of Science and Technology.

Doctors asked 392,164 adults who came for a health check in Taiwan between 1998 and 2011 how long they usually slept at night. Of those, 711 of the participants died of cardiovascular disease during the follow-up period.

The findings show that the risk increased 50 percent for participants who slept fewer than four hours compared with participants who slept between six and eight hours.

Part of the difference might be explained by the fact that participants who slept the least had a less favourable risk profile for cardiovascular disease, researchers said, but even after adjusting for these factors, the risk of heart-related death increased by 36 percent.

Less sleep is associated with stress responses that increase heart rate, blood pressure and secretion of adrenaline, for example, all of which are known risk factors for cardiovascular disease, researchers said.

Sleep deprivation has also been associated with the secretion of inflammatory substances, and previous studies have shown an association between short sleep duration, decreased insulin sensitivity, obesity and diabetes, they said.

Participants who reported that they slept more than eight hours a night also had a 53 percent increased risk of dying from coronary heart disease compared with participants who slept between six and eight hours per night. After adjusting for other risk factors, the risk was still 28 percent higher.

The findings were published in the International Journal of Cardiology.

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