outdoor time and nearsightedness in kids

While we have several proven ways to manage myopia progression in kids who are already nearsighted, there is growing interest in preventing or delaying the onset of myopia. Several studies have suggested that spending more time outdoors during childhood lowers the risk of developing myopia (1, 2). Outdoor time has been shown to be protective even in children with myopic parents (3). Increased time outside may prevent myopic progression in those who are already myopic, but more data is needed on that front, as study results have been conflicting.

Exactly how time outdoors is protective is uncertain. Some proposed mechanisms: brighter light, release of dopamine from cells in the retina, increased depth of field, less near work, higher vitamin D levels, greater physical activity, differing chromatic spectrum of light (4, 5).

The leading theory has to do with light intensity. There is greater light intensity outdoors, which leads to pupil constriction. That pupil constriction has a pinhole effect, increasing the depth of field and reducing image blur (6). The greater intensity of light outdoors also causes the retinal cells to release dopamine, which inhibits the signals for eye elongation (7). High light levels in chickens retarded the development of myopia in one study (8, 9).

Another line of thinking has to do with the visual environment of indoor spaces vs outdoor spaces. Indoor environments are more “dioptrically varied,” meaning there are objects at varying distances from the eye, and more objects that are closer, so a person is more likely to experience what’s called “peripheral hyperopic defocus” indoors. Peripheral hyperopic defocus is when images focus behind the retina in the periphery; that defocus is thought to stimulate the eye to elongate, or become more nearsighted. By contrast, outdoor environments are more “dioptrically flat,” with more objects at a distance, so there is a more uniform pattern of retinal focus (10).

So how do we know how much outdoor time does the trick? A meta analysis of 13 studies found that outdoor light exposure significantly reduced the incidence of myopia; it reduced myopia incidence by approximately 50%, slowed myopic spherical equivalent refraction progression by 32.9% and axial elongation by 24.9% for individuals in Asia. Daily outdoor light exposure of more than 120 min was the most effective intervention (11).

CLIFFSNOTES: Children at risk for developing myopia should spend at least two hours a day outside to help reduce the risk.