Does spending time outdoors really matter? According to a series of studies, it absolutely does—especially where the health of children’s eyes is concerned. Ophthalmologists and eye health researchers around the world have been trying to determine why myopia (nearsightedness) rates have skyrocketed, doubling over the last century. They now think the answer lies in exposure to sunlight.

Myopia, according to the National Eye Institute, is a disorder; more specifically, a “refractive error where close objects appear clearly, but distant objects appear blurry.” This is caused when the eye is slightly elongated or the cornea is too steep, thus causing light rays to focus in front of the retina instead of on it. Myopia typically rears its head in children who are either school-aged or adolescent, as these are the times when the eye is growing. 

Originally, myopia was thought to develop in kids who read too much and/or in those who had parents with the disorder. Today’s researchers, however, realize that bookwormish tendencies are not in fact a cause of nearsightedness; rather, the culprits are genetics and, according to new research, not spending sufficient time in sunlight.

Dr. Donald Mutti, of the Ohio State University College of Optometry, explains, “Brighter visible light outside stimulates the release of dopamine from the retina, and that dopamine slows down the growth of the eye.” Dr. Mutti and his colleagues have long studied the ways in which playing sports and spending time outdoors affects children’s eye health. They recently found that “children who are genetically predisposed to nearsightedness are three times less likely to need glasses if they spend at least 14 hours a week outdoors, compared to children who spend less than five hours a week outdoors.” The reason is still somewhat indeterminate—Mutti explains that UVB rays create vitamin D, which may benefit the eyes, but that the theory still needs more testing.

What’s more, Dr. Ian Morgan, of Australian National University and Sun Yat-sen University in China, along with his colleagues, has found that children become very myopic in East and Southeast Asia, where they’re subjected to intensive schooling and typically get outside for less than an hour per day. Dr. Morgan says recent controlled trials, which have introduced a mandated two-hour outside period per day for children, halved the rate of new myopia cases.

Dr. Morgan goes on to explain that 10,000 lux—lux is a unit of light measurement or intensity on a surface—is necessary to protect eyes’ health. Both Morgan and Mutti say that even on cloudy days, natural light intensity could be sufficient to provide kids with enough lux. And Nature put lux into perspective by stating that even the brightest of offices or classrooms emit only about 500 lux. This means that staying indoors most of the day will never expose eyes to enough dopamine stimulation to maintain optimal eye health.  

Interestingly, it is not the intensity of sunlight exposure, nor the outdoor activity, that matters when it comes to children’s eye health. One recent study found that reading outside or having a picnic was just as effective for myopia prevention as playing sports.

Chalk it up as being one more reason to encourage your child to take a break from studying or playing video games or watching TV, and to go outside and play. After all, the health of their eyes depends on it. Just don’t forget the sunscreen and sunglasses!