People who get too much sleep are at greater risk of an early death than those who get too little, research suggests. Experts found those who sleep for more than eight hours a night have greater mortality risk than those who sleep for less than seven hours.

The study, involving more than 3.3 million people around the world, found sleeping for too long raised the risk of heart disease and strokes. The scientists - from the universities of Keele, Manchester, Leeds and East Anglia - said excessive sleep should be considered a "marker" of poor health.

One explanation could be getting too much sleep means people get too little exercise, raising their risk of heart problems. But it is more likely that people who sleep for too long are already suffering from undiagnosed problems.

‘Long sleep duration may be associated with an increased risk of cardiovascular disease because of comorbidities that lead to fatigue, such as chronic inflammatory disorders and anaemia,’ they wrote in the Journal of the American Heart Association, ‘Depressive symptoms, low socioeconomic status, unemployment and low physical activity are also associated with long sleep duration.’

The lowest risk was shown for those who slept between seven and eight hours per day, the researchers said. For people who got less sleep, the risk of disease and death rose gradually, but not enough to be statistically significant. But for people who got more than this, the effect was dramatic.

People who slept for nine hours a night had a 14 percent increased mortality risk, among those who got 10 hours the risk went up 30 percent and those who slept for 11 hours were 47 percent more likely to die an early death. People who got 10 hours or more were also at a 56 percent increased risk of dying from a stroke and 49 percent increased risk of dying from cardiovascular disease.

The researchers combined the results of 74 previous studies to produce the results.

Lead researcher Dr Chun Shing Kwok of Keele University said: ‘Our study has an important public health impact in that it shows that excessive sleep is a marker of elevated cardiovascular risk. Our findings have important implications as clinicians should have greater consideration for exploring sleep duration and quality during consultations.

‘If excessive sleep patterns are found, particularly prolonged durations of eight hours or more, then clinicians should consider screening for adverse cardiovascular risk factors and obstructive sleep apnoea, which is a serious sleep disorder that occurs when a person's breathing is interrupted during sleep.’