Lack of sleep can more than double the risk of death from heart disease, according to a large British study, among the first to link duration of sleep and mortality.

But too much sleep can also be harmful, according to the study, which was released on Monday.

The team of researchers from the University of Warwick and University College London presented the findings to the British Sleep Society in Cambridge on Monday.

The findings, which will be published in the journal SLEEP, are based on a 17-year analysis of how sleep patterns affected the mortality of 10,308 civil servants.

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

The researchers also took into account other possible factors such age, sex, marital status, employment grade, smoking status, physical activity, alcohol consumption, self-rated health, body mass index, blood pressure, cholesterol, and other possible physical ailments.

Once the team adjusted for these factors, they were able to isolate the effect that changes in sleeping patterns over the span of five years had on mortality rates some 11 to 17 years later.

Those who cut their sleeping time from seven hours -- which is considered the optimal amount of time for an adult, to five hours or fewer faced a 1.7-fold increased risk of death from all causes.

They also faced more than double the risk of death from a cardiovascular problem.

"Fewer hours sleep and greater levels of sleep disturbance have become widespread in industrialised societies," Franceso Cappuccio, professor of cardiovascular medicine at the University of Warwick's medical school, said in a written statement.

"This change, largely the result of sleep curtailment to create more time for leisure and shift-work, has meant that reports of fatigue, tiredness and excessive daytime sleepiness are more common than a few decades ago. Sleep represents the daily process of physiological restitution and recovery, and lack of sleep has far-reaching effects," Cappuccio said in the statement posted on the university's website.

But curiously, the researchers also found that too much sleep also increased mortality.

Those who showed an increase in sleep to eight hours or more a night were more than twice as likely to die than those who did not change their slumbering habits.

However, the increased mortality was predominantly from non-cardiovascular diseases.

It was possible, Cappuccio said, that longer sleeping was related to depression, low socioeconomic status and cancer-related fatigue.

"Short sleep has been shown to be a risk factor for weight gain, hypertension and Type 2 diabetes sometimes leading to mortality but in contrast to the short sleep-mortality association it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated," Cappuccio said.

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