U. WARWICK (UK)—People who sleep for fewer than six hours each night are 12 percent more likely to die prematurely than those who get the recommended 6-8 hours, according to a research review.

The study, published in the journal Sleep, also suggests that consistently getting too much sleep (more than 9 hours a night) could be a cause for concern. While it does not in itself increase the risk of death, sleeping too much can be a significant marker of potentially fatal illnesses.

The researchers examined the relationship between the level of habitual duration of sleep and mortality by reviewing 16 prospective studies from the U.K., U.S., and countries in Europe and East Asia. The study included more than 1.3 million participants, followed up for up to 25 years, with more than 100,000 deaths recorded.

The study points to evidence of a direct link between both short (fewer than 6 hours sleep a night) and long (9 hours or more) duration of sleep and an increased chance of dying prematurely, compared to those who sleep 6-8 hours a night on average.

“Whilst short sleep may represent a cause of ill-health, long sleep is believed to represent more an indicator of ill-health,” says Francesco Cappuccio, leader of the Sleep, Health and Society Programme at the University of Warwick in the U.K.

“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,” adds Cappuccio. “On the other hand, the deterioration of our health status is often accompanied by an extension of our sleeping time.

“Consistently sleeping 6 to 8 hours per night may be optimal for health. The duration of sleep should be regarded as an additional behavioral risk factor, or risk marker, influenced by the environment and possibly amenable to change through both education and counseling as well as through measures of public health aimed at favorable modifications of the physical and working environments ” says Cappuccio.

The work was conducted by researchers at Warwick in collaboration with the Federico II University Medical School in Naples, Italy.

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