(CNN) -- While the belief that prolonged close-up activities like reading and playing computer games cause short sightedness (myopia) is popularly held, new research indicates that a deficiency of sunlight is the true culprit.
Kathryn Rose, a leading international researcher of visual disorders at the University of Sydney's Faculty of Health Sciences, talks to CNN about her study.
CNN: In this digital age, do you believe that using handheld digital games, texting, playing games on mobile phones and computer desktops have contributed to an increase in children's myopia? In other words, is there a direct cause and effect?
Rose: Many epidemiological studies have examined the issue of computer, video and television based activities on the eye health of children and have found no association between time spent using digital media and the development of myopia. While it is an intuitive association to make, it simply has not been confirmed. There are some studies that have found an association between the time spent doing near-work activities such as reading and studying, but the results are inconsistent and marginal. There may be an indirect link where children have substituted large amounts of time spent outside with these activities.
CNN: At the time one of your studies was published, there was a belief that outdoor light had a positive effect on vision. Since then, have you learned anything more? Is there a connection between outdoor activities and perhaps a decrease in myopia in children?
Rose: This finding has now been confirmed in a number of other studies from the U.S., Singapore and China. Our hypothesis that the mechanism of the effect of light was mediated by retinal dopamine, a known inhibitor of eye growth whose release is stimulated by light, has also been supported by animal experiments. All of these studies confirm a consistent link between the time spent outdoors and the prevention of myopia, possibly crucially mediated by the at least ten-fold increase in light levels between indoor lighting and being outside. So yes, it is highly likely that there is a direct connection between time spent outside and preventing myopia.
One of the observations from the studies conducted so far is that the effect of light on the prevention of the development of myopia may have a threshold effect, that is both the level of light required and the duration of light exposure may need to reach critical amounts before light has its preventive effect.
In the latter case, the epidemiological studies that have examined children's exposure to outdoors have consistently found a preventative effect for between 10-14 hours outside per week in addition to any hours spent outside during school time, while 3-6 hours per week has not been associated with any effect.
CNN: In terms of the outdoor light threshold of 10-14 hours/week to prevent myopia. Is this effect minimized if a child is wearing sunglasses? I imagine parents are worried about the brightness of light hurting their child's eyes.
Rose: There is no evidence either way that wearing glasses has no effect or hinders the protective effect of sunlight, but this has not been studied systematically.
Young children tend not to routinely wear sunglasses, possibly due to high rates of breakage and general intolerance to wearing glasses. In Australia wearing hats is compulsory at primary school and is encouraged for outdoor activities, but again this seems not to have reduced the protective effect of being outdoors. This may be because even with a hat the level of illumination outside is still so high.
CNN: Have you found differences in myopia among children in different countries? If so, what are the reasons behind the differences?
Rose: Differences in the prevalence of myopia have been found between countries, between the same ethnic group located in different countries, between the same ethnic groups located in the same country but differentiated by urban versus rural living conditions and/or by level of academic achievement, and finally between generations of families. When these observations are linked to rapid rises in myopia prevalence, particularly in highly urbanized populations who have a particular focus on academic success at a young age, the very strongly likelihood is that the rise in myopia prevalence is being driven by environmental factors. Time spent outside is likely to be a major component of these environmental differences.
CNN: You mentioned that several studies have been done that compare myopia in kids in different countries. Did you see any interesting differences based on countries or regions? For example, in Scandinavia where you have less light year-round do you find kids with higher rates of myopia?
Rose: The differences between countries seem to relate to high levels of myopia in highly urbanized, academically focused countries with an emphasis on early competitive education systems versus low levels in less urbanized and/or countries with more emphasis on learning through play at an early age.
Scandinavian countries seem no more myopic or otherwise than other more southern European countries. In part this may be lifestyle as people with limited sunlight during winter seem to get outside for long hours during the summer which may balance the short available sunlight hours in the winter. However, there is some limited evidence that in these countries, the rate of eye growth advances more in winter and slows in the summer.