Why outdoor time matters in myopia development

Originally Published: June 21, 2015
Updated: August 5, 2021

There’s no doubt that the simplest lifestyle intervention we can advise for both our at-risk pre-myopes and progressing myopes is to spend more time outdoors. This data first came to prominence in Rose et al’s 2008 paper, published in Ophthalmology.[1] Over 4000 children (half aged 12 and the other half aged 6) participating in the Sydney Myopia Survey demonstrated that 12 year olds who spent more time outdoors and less time on near work had the most hyperopic refractions, with the reverse also being true – more near work and less outdoor time means a lower level of plus. An important finding, though, was that outdoor time didn't directly trade for near work time - even the children with high levels of near work got the benefit of more time spent outdoors. This relationship was not shown for the 6 year old group. There was also no association between indoor sport and myopia, so the question begs – what defines the beneficial effect of outdoor time?

Outdoor light exposure - UV or brightness?

On discussion with savvy parents in clinical practice, there is a concern raised about UV exposure, especially here in Australia. One parent recently asked about indoor light – both her and her husband are myopic, and with her six-year-old only demonstrating R&L +0.25, he’s mostly likely heading the same way.[2] This mum asked if she should ask for her son to be sat next to the window in class? Is brighter better for indoor light? And what were the implications for an at-risk pre-myope wearing sunglasses, even if she couldn’t get her six year old to wear them yet?

It would appear that brightness rather than wavelength and hence UV exposure is the likely stronger link with myopia, with light levels outdoors being up to 500 times brighter than indoor light. Here's what we know about light and brightness, and UV protection.

1. Animal models show that chickens, when reared under strong indoor light without UV, showed dose-dependent propensity to retard development of form deprivation myopia.[3]
2. Myopic adults have been shown to have lower levels of long term ocular UV exposure, as measured by conjunctival autofluorescence, compared to non-myopic adults. This may be long-term evidence of these myopes spending less time outdoors, though complicated by potential use of UV blocking spectacle lenses and contact lenses.[4]
3. Outdoor light levels are 11 to 43 times higher (11,000 to 18,000 lux) than indoors (100-150 lux). Even under tree shade, wearing a hat and wearing sunglasses, the higher lux levels protective of myopia (>1000 lux) were exceeded by 2-8 times.[5]
4. Genetics and environment can interact - children of myopic parents spent less time outdoors in the Sydney Myopia Study.[1] Lower amounts of outdoor time could be a consequence as well as a developmental factor in myopia.

So for our savvy concerned parent, it’s likely that sitting closer to the window will be helpful to her son, even though clear glass does block a majority of incident UVA and UVB light. Wearing sunglasses blocks UV exposure and reduces brightness, but on a sunny day it’s likely that an 85% tint sunglass lens is still able to let through as much brightness, and likely more than typical bright domestic light.

What other mechanism could be involved?

The alternative viewpoint is that of Ian Flitcroft, who describes how the dioptric environment varies dramatically indoors.[6] Sitting at this computer while I type, looking at the screen, my keyboard and fingers induce hyperopic blur on my inferior retina while the world beyond my screen induces myopic defocus. Several dioptres of variation from the optical infinity across my office to at least 2-3D of blur from my keyboard is possible. However once you’re outdoors and everything is beyond 3m distance, your retina enjoys a full field of optical infinity.

Given the strong understanding of how the peripheral retina drives eye growth in both animal and human models, the benefit of outdoor time is likely to be a combination of the dioptric environment as well as brightness.

How much outdoor time is needed?

A meta-analysis of studies on the role of outdoor time in myopia development found that less than 13 hours a week (just under 2 hours a day) was associated with the highest odds ratio for incident myopia. On the basis of five studies investigating a dose response, they found that "an increase of 76 min/day, was needed to obtain a 50% reduction in incident myopia, while an increase of 1 hr/day or 7 hr/week will result in a 45% reduction in incident myopia compared with controls."[7] This is a simple, effective and achievable intervention which also may have positive effects on other child health factors such as reduction in body mass index (BMI), less sedentary behaviours, and other general health outcomes.