Scientists in the US have disproved the old wives' tale that sleeping too much makes you fat.
Sleeping more than nine hours a night may actually suppress genetic influences on body weight, according to a new study on sleep and body mass index (BMI).
Meanwhile, getting too little sleep was associated with being heavier.
The study looked at over 1,000 sets of twins and found that sleeping less than seven hours a night was linked with both increased BMI and greater genetic influences on BMI.
Previous research has shown that genetic influences include things like glucose metabolism, energy use, fatty acid storage and satiety.
In this study, the heritability of BMI was twice as high for the short sleepers than for twins who slept over nine hours a night.
"The results suggest that shorter sleep provides a more permissive environment for the expression of obesity related genes," said Professor Nathaniel Watson of the University of Washington, who led the study.
"Or it may be that extended sleep is protective by suppressing expression of obesity genes."
The research team determined that for twins sleeping less than seven hours, genetic influences accounted for 70% of the differences in BMI, with common environment accounting for just 4% and unique environment 26%.
For twins who got an average of more than nine hours of sleep, genetic factors were attributed to 32% of weight variations, with common environment accounting for 51% and unique environment 17%.