English and Italian researchers investigating the relationship between duration of sleep and mortality from all causes, have performed a systematic review of 16 relevant studies. The studies looked at a total of 1.4 million people, following them up for between 4 and 25 years.
It emerged that people who generally slept for less than six hours per night, were 12% more likely to die prematurely, than those who consistently managed six to eight hours. Longer sleep durations (exceeding nine hours) were associated with a 30% greater risk of death.
(Sleep Duration and All-Cause Mortality: A Systematic Review and Meta-Analysis of Prospective Studies. Sleep Journal May 2010.)