Not too little sleep, yet not too much, the experts advise
Getting less than six hours sleep a night can lead to an early grave, UK and Italian researchers have warned.
They said people regularly having such little sleep were 12% more likely to die over a 25-year period than those who got an "ideal" six to eight hours.
They also found an association between sleeping for more than nine hours and early death, although that much sleep may merely be a marker of ill health.
Sleep journal reports the findings, based on 1.5m people in 16 studies.
The study looked at the relationship between sleep and mortality by reviewing earlier studies from the UK, US and European and East Asian countries.
Premature death from all causes was linked to getting either too little or too much sleep outside of the "ideal" six to eight hours per night.
But while a lack of sleep may be a direct cause of ill health, ultimately leading to an earlier death, too much sleep may merely be a marker of ill health already, the UK and Italian researchers believe.
Professor Francesco Cappuccio, leader of the Sleep, Health and Society Programme at the UK's University of Warwick, said: "Modern society has seen a gradual reduction in the average amount of sleep people take and this pattern is more common amongst full-time workers, suggesting that it may be due to societal pressures for longer working hours and more shift-work.
"On the other hand, the deterioration of our health status is often accompanied by an extension of our sleeping time."
If the link between a lack of sleep and death is truly causal, it would equate to over 6.3 million attributable deaths in the UK in people over 16 years of age.
Prof Cappuccio said more work was needed to understand exactly why sleep seemed to be so important for good health.
Professor Jim Horne, of the Loughborough Sleep Research Centre, said other factors may be involved rather than sleep per se.
"Sleep is just a litmus paper to physical and mental health. Sleep is affected by many diseases and conditions, including depression," he said.
And getting improved sleep may not make someone better or live longer, he said.
"But having less than five hours a night suggests something is probably not right.
"Five hours is insufficient for most people and being drowsy in the day increases your risk of having an accident if driving or operating dangerous machinery."