People who sleep less than six hours a night have significantly higher rates of heart disease and stroke, and are more likely to die of those causes.
That's the conclusion of a new analysis that combined the results of previously published research papers on the sleep habits and health of almost a half-million people in eight countries.
Lead author Francesco Cappuccio of the University of Warwick Medical School in Britain says the researchers didn’t find any significant health problems in people sleeping between six and eight hours a night. However, "when we go below and above, we find that the risk increases quite sharply," he explained in a telephone interview.
"On the low end, short sleep predicts a number of conditions including cardiovascular disease, and we feel short sleep may cause those conditions by a number of mechanisms."
Cappuccio says those mechanisms have been identified in other studies, where people were forced to go without sleep. Sleep deprivation leads to a variety of measurable physiological changes.
"Your blood pressure will go up and your cholesterol will go up. Your cortisol will go up, which is a stress hormone. And all these conditions - if you think you can extrapolate to being sustained for many years - are likely to cause the outcomes we detect," he said.
Cappuccio’s study also found that people who sleep a lot - more than nine hours a night - are also more likely to have heart disease and stroke, and are more likely to die from those conditions. But he says, however, that there is no obvious cause for that, so he thinks it may be that other underlying conditions are leading some people to sleep longer. Cancer might be one cause. A mental disorder could also be the issue.
"You may be fatigued, tired, and you may tend to sleep longer. And that may be an early sign of depression that people often don't pick up."
As a result, he says, patients and their doctors should explore the possibility that an apparently unrelated condition is causing longer-than-normal sleep patterns.