A new study has found that spending as little as 15 minutes a day outside in bright sunlight could help prevent myopia in children.

The study was published on Aug. 13 in the Journal of the American Medical Association (JAMA). It was conducted as part of the Shanghai Time Outside to Reduce Myopia trial—a school-based cluster randomized clinical trial that followed school children in Shanghai for two years between October 2016 and December 2018.

The study aimed to explore different sun exposure patterns and their association with the progression of myopia, or nearsightedness, in children.

Myopia is a common condition in which the eyes have trouble focusing light correctly, causing distant objects to appear blurry while close-up vision remains clear. This happens because the eyeball is too long or the cornea is too curved. According to the American Academy of Ophthalmology, myopia currently affects nearly 30 percent of the U.S. population, and its prevalence is increasing, with nearly half the population expected to be affected by 2050.

Study Details

The JAMA study was a one-year prospective cohort that followed 2,976 children who were approximately 7 (7.2) years of age and split almost evenly between boys (48.8 percent) and girls (51.2 percent).

The children wore smartwatches to track their time spent outdoors and exposure to sunlight. They consistently wore the smartwatches for at least six hours a day over a minimum of 90 days during the study period, which lasted from December 2017 to December 2018. Additionally, none of the participants had myopia when the study began.

Twelve different patterns of outdoor exposure were examined, focusing on how long the children spent outside and the intensity of the sunlight during that time, as measured in lux—a measure of light’s intensity. The students spent an average of 90 minutes outdoors daily, and their average light intensity exposure was 2,345 lux (units of illuminance). Most of the time spent outdoors (74.9 percent) involved the children spending at least 15 minutes outside at a time.

Decrease in Myopic Shift

The study authors wanted to find which of the patterns of outdoor exposure could reduce the “myopic shift,” which is the progressive worsening of nearsightedness or myopia. They found that the outdoor exposure patterns in which the children spent a minimum of 15 minutes outdoors with a sunlight intensity of 2,000 lux or more were associated with less myopic shift, meaning these conditions helped reduce myopia progression over the one-year study period.

The authors concluded that in addition to the total time spent outdoors being critical—so are the specific patterns of exposure—especially continuous exposure for at least 15 minutes in bright sunlight.

Everyday Lux Examples

A minimum of 2,000 lux was found necessary to slow myopia progression in the children. As a reference, an article in Oxford Academic states that a full moon at its highest in the sky in mid-latitudes produces approximately 0.05–0.2 lux. By contrast, the light intensity on an overcast day is approximately 1,000 lux, full daylight (not direct sun) is 10,000 to 25,000, and direct sunlight is 32,000 to 130,000 lux, according to Ao[n²].

Myopia in Children

The study states that myopia has become a significant global health issue that has seen a rise in cases over the past three decades, especially in East and Southeast Asia, where nearly 90 percent of young people are affected.

“The trend of earlier onset of myopia among school-aged children suggests an increasingly serious epidemic of high myopia and pathologic myopia in the future,” the study stated.

It goes on to say that growing rates of myopia increase the risk of conditions that can endanger vision, such as glaucoma, cataracts, and myopic maculopathy, which impact quality of life and increase health care costs for those affected.

The authors also stated that the children in their study spent less time outdoors than expected and spent less time outdoors than children in other countries such as Australia and the UK even though they were exposed to similar light intensities while outside. The authors said that children in China may spend less time outside because they spend more time in school and on homework in addition to lifestyle and sociocultural reasons.

Conclusion

While the study offers valuable insights, the authors acknowledge several limitations. Since the research was limited to children in Shanghai, the results may not apply to other regions. Additionally, the one-year duration might not be long enough to fully capture the relationship between outdoor exposure and myopic shift.

They also note that using smartwatches could have underestimated light levels, concluding that more research is needed to confirm their findings over extended periods and in different populations and settings.

Ultimately, the authors suggest that to effectively prevent myopia, efforts should focus both on the total time children spend outdoors and ensuring they are exposed to sufficiently intense sunlight for at least 15 minutes at a time.