FRIDAY, Jan. 30, 2009 (HealthDay News) -- Kids who spend more time outside -- and away from the television set -- are less likely to develop myopia, the inability to see things clearly at a distance.
The new report, from researchers in Boston, doesn't determine whether too much indoor activity actually causes poor eyesight. And even if it does, researchers haven't pinpointed what the exact mechanism might be.
Still, "it would seem prudent to encourage outdoor activities -- not necessarily sports -- for all growing children and young adults in order to reduce the progression of myopia," said Howard C. Howland, a professor of neurobiology and behavior at Cornell University.
About a third of Americans suffer from myopia, said study author Jane Gwiazda, director of research at The New England College of Optometry. The rates are much higher in some parts of Asia.
The condition seems to be caused by both genetics and the environment, Gwiazda said. The condition is more common in people who engage in a lot of "near work" due to their jobs, she said.
The study authors gave questionnaires to the parents of 191 children who were at an average age of 13.3 years. Among other things, the researchers asked about the children's time spent using the computer, reading for pleasure and watching TV.
The children's eyesight was tested annually.
The findings were published in the January issue of Optometry and Vision Science.
The children who developed myopia -- also known as nearsightedness -- spent less time in outdoor activities, an average of 8.3 hours a week compared to 12.6 hours among the other children.
Those with myopia also watched more television (12.5 hours vs. 8.4 hours a week).
What's going on? "One possibility is that all the hours spent viewing objects at a distance rather than up close, as happens outdoors, provides a 'stop' signal to block myopia progression," Gwiazda said. "Outdoor exposure also may be beneficial, because sunlight causes the pupil to constrict, resulting in a larger depth of focus -- the range in which objects appear clear -- and less image blur that's associated with myopia development."
In other words, the eye may see more clearly outside in the sunlight and avoid developing myopia.
Looking at things farther away may be another benefit of outdoor activities. "We know a great deal about what causes myopia in animals, including primates," said Howland. "Images that are focused behind the retina cause the eye to grow in length, making the animal more myopic. Generally speaking, one can prevent animals from becoming myopic if they are provided with sufficient opportunity to see distant objects."
In popular culture, bookworms and nerds are often depicted as wearing glasses. Some studies have indeed shown a connection between heavy reading and myopia, Gwiazda said. But the new research doesn't confirm that link.
"In our study, children with more hours of outdoor activity do not necessarily spend less time reading and using computers," Gwiazda said.
To learn more about myopia, visit the U.S. National Institutes of Health.