Sleeping too much does not make one fat -- in fact, sleeping more than 9 hours a night may suppress genetic influences on body weight, U.S. researchers say.

Principal investigator Dr. Nathaniel Watson of the University of Washington and colleagues said the study looked at 1,088 pairs of twins and found sleeping less than 7 hours a night was associated with both increased body mass index and greater genetic influences on BMI.

The study, published in the journal Sleep, found the heritability of BMI was twice as high for short sleepers than for twins who slept longer than 9 hours a night.

Watson and colleagues determined that for twins sleeping less than 7 hours, genetic influences accounted for 70 percent of the differences in BMI, with common environment accounting for just 4 percent and unique environment 26 percent.

For twins averaging more than 9 hours of sleep, genetic factors accounted for 32 percent of weight variations, with common environment accounting for 51 percent and unique environment 17 percent, the study said.

"The results suggest that shorter sleep provides a more permissive environment for the expression of obesity related genes," Watson said in a statement. "Or it may be that extended sleep is protective by suppressing expression of obesity genes." (UPI)