Research suggests that sleeping five hours a night or less is linked with a higher risk of coronary heart disease.

Adequate daily sleep should not be considered a luxury, but an important component of a healthy lifestyle Najib Ayas, sleep specialist

Surprisingly, spending more than the usual eight hours in bed also seems to be bad for you.

In a study, women who slept for nine hours or more a night were also at higher risk.

Sleep is the latest lifestyle factor to be associated with coronary heart disease.

The condition is caused by the narrowing of the coronary arteries, a common cause of heart attacks.

Lack of exercise, smoking and an unhealthy diet are all known to play a role.

Studies have shown that sleep deprivation increases blood pressure and affects hormone and blood sugar levels, which could have an impact on the heart.

Burn out

The authors behind the data - drawn from a major study of the health of nearly 122,000 US nurses - say modern lifestyles are to blame.

"This research sends an important message to a population that is spending more and more time working and staying up late watching television or using the internet," said team leader Najib Ayas of Brigham and Women's Hospital in Boston.

"Adequate daily sleep should not be considered a luxury, but an important component of a healthy lifestyle."

Sleep: What's normal?

Most people sleep for 6.5-8.5 hours per night Very few people can function on less than 5 hours Five million Britons have their quality of life impaired by sleep disorders such as insomnia, sleep apnoea and snoring

Experts disagree on how much sleep you need to stay healthy. The idea that you can train yourself to sleep for just a few hours a night is a matter of debate.

According to Dr Adrian Williams of the sleep clinic at St Thomas' Hospital in London, the national average is 8.1 hours.

The latest data, published in the Archives of Internal Medicine, is more evidence that sleep serves a very important function, he said.

"The idea that we can train ourselves to do with less seems to hold less and less water," he told BBC News Online.

However, Professor Jim Horne, a sleep expert at Loughborough University, said there was no evidence from animal studies that lack of sleep had a detrimental effect on the heart.

He said there could be other explanations for the link between coronary heart disease and sleep patterns.

Stress, for example, is known to disrupt sleep. People sleeping for very long periods could have other physical problems that predispose them to heart disease, he added.