Children have long been encouraged to go outdoors to play - to improve their mood, get exercise and break the hypnotic spell of mobile phones, television and video games. But now there's another important reason - it may help them avoid vision problems.
Two studies published recently in Ophthalmology, the journal of the American Academy of Ophthalmology, say that time spent outdoors blunts the chances of children developing nearsightedness (myopia) and slows its progress in those with myopia.
"It's not the time spent reading, just that they spend more time indoors and are not getting good light needed in early childhood," said David G. Hunter, ophthalmologist-in-chief at Boston Children's Hospital and the academy's clinical spokesman.
"There's something about outdoor light that's good for the eyes. But there's nothing in these two papers that says children shouldn't be reading.
Maybe they should read outside."
With nearsightedness, more distant objects appear blurry due to light in the elongated eye focusing in front of the retina. With farsightedness, or hyperopia, the person sees distant objects clearly with close ones blurry. In that case, due to a shortened eyeball, light travelling through the lens never does focus before hitting the retina with the focus point behind the retina.
A recent study from Taiwan says myopia has become a public health issue worldwide in recent years, making it a priority issue for the World Health Organization's Vision 2020 initiative.
"Early onset of myopia is associated with high myopia in adult life," the study says. "High myopia is a significant public health problem because of its association with increased risk of several ocular diseases, including cataract, glaucoma, retinal detachment, myopic retinal degeneration, visual impairment and blindness."
That reveals the urgency of postponing the onset of myopia as long as possible and retarding progression to keep children "at low myopic status" until adulthood. "Recent evidence suggests that increased outdoor activities and reduced long-term near work could help prevent myopia," it says.
In the study, one suburban Taiwanese school adopted a program to encourage 333 students to go outdoors during recess. The school serving as the control group generally had its 228 students remain indoors.
After one year, new onset of myopia was significantly lower in the group that took recess outside when compared with the control group that stayed indoors. Researchers also measured significantly less progression of myopia in the outdoor group. Hunter said the numbers weren't dramatic, but enough to prove the advantages of children spending more time during the school day, or any day, in daylight.
The study goes on to encourage schools to adopt policies to allow children to spend recess time outside.
"Because children spend a lot of time in school, intervention from the educational system is a direct and practical approach to tackle the increasing prevalence of myopia," it states.
Hunter said the retina is thought to control eyeball growth, depending on the type of light hitting its neurotransmitters. The quality and colour of indoor light hitting the retina "somehow is contributing to stimulating eye growth."
Before recent research on the importance of natural light on eyesight, Hunter said, he had little to recommend to parents whose children had myopia. Now he does: "I encourage with more rigour and confidence for their kids to spend more time outdoors."