LONDON (Reuters) - People who do not get enough sleep are more than twice as likely to die of heart disease, according to a large British study released on Monday.

Although the reasons are unclear, researchers said lack of sleep appeared to be linked to increased blood pressure, which is known to raise the risk of heart attacks and stroke.

A 17-year analysis of 10,000 government workers showed those who cut their sleeping from seven hours a night to five or less faced a 1.7-fold increased risk in mortality from all causes and more than double the risk of cardiovascular death.

The findings highlight a danger in busy modern lifestyles, Francesco Cappuccio, professor of cardiovascular medicine at the University of Warwick's medical school, told the annual conference of the British Sleep Society in Cambridge.

"A third of the population of the UK and over 40 percent in the U.S. regularly sleep less than five hours a night, so it is not a trivial problem," he said in a telephone interview.

"The current pressures in society to cut out sleep, in order to squeeze in more, may not be a good idea -- particularly if you go below five hours."

Previous research has highlighted the potential health risks of shift work and disrupted sleep. But the study by Cappuccio and colleagues, which was supported by British government and U.S. funding, is the first to link duration of sleep and mortality rates.

The study looked at sleep patterns of participants aged 35-55 years at two points in their lives -- 1985-88 and 1992-93 -- and then tracked their mortality rates until 2004.

The results were adjusted to take account of other possible risk factors such as initial age, sex, smoking and alcohol consumption, body mass index, blood pressure and cholesterol.

The correlation with cardiovascular risk in those who slept less in the 1990s than in the 1980s was clear but, curiously, there was also a higher mortality rate in people who increased their sleeping to more than nine hours.

In this case, however, there was no cardiovascular link and Cappuccio said it was possible that longer sleeping could be related to other health problems such as depression or cancer-related fatigue.

"In terms of prevention, our findings indicate that consistently sleeping around seven hours per night is optimal for health," he said.