The study, published in the journal Sleep, found those sleeping five or fewer hours -- or nine or more hours -- were significantly more likely to be diagnosed with diabetes over the next 10 years than those who reported sleeping seven hours. The higher rates held even after adjusting for variables such as physical activity, depression, alcohol consumption, ethnicity, education, age, obesity and history of hypertension, the researchers found.

The study's follow-up period took place from l982 to l992 after the first National Health and Nutrition Examination Survey involved 8,992 subjects ages 32 to 86.

"If short sleep duration functions to increase insulin resistance and decrease glucose tolerance, then interventions that increase the amount and improve the quality of sleep could potentially serve as treatments and as primary preventative measures for diabetes," study author James Gangwisch of Columbia University in New York said in a statement.

It is unknown why sleeping longer than nine hours contributes to diabetes, but it may be the increased time in bed is needed to compensate for poor sleep quality, Gangwisch says.