Modern life takes a toll on bodies. It's easy to tick off the uniquely 21st-century diseases that plague humans today: obesity, heart disease, diabetes. But those are the visible afflictions, the ones that show up on expanding bellies and skyrocketing death rates. Out of sight, another epidemic is silently raging: myopia, or nearsightedness. Between the 1970s and the early aughts, the incidence of myopia in the US nearly doubled, to 42 percent. Myopia’s rise has been the starkest in Asia; one survey in Korea found a rate as high as 96 percent among teenagers.

Clearly, something is going on. But scientists can’t agree on exactly what. Being constantly tethered to devices and books indoors might be part of it: Based on a handful of large epidemiological studies on myopia, spending time outdoors—especially in early childhood—reduces the onset of myopia. (So nerds and glasses? It’s true.) But what exactly about the outdoors helps? Is it the bright sunlight or how eyes focus on objects far away outside or something else entirely?

The exact answer matters, because just shooing kids outside has downsides, too. As Thomas Norton, a vision scientist at the University of Alabama at Birmingham, puts it, “You don’t want to trade myopia for skin cancer.”

The confusion around myopia is even more complicated because scientists can't agree on how to study it. You can’t just take healthy children and deliberately make them nearsighted (obviously). So the best you can do is induce myopia in animals—usually chickens, tree shrews, or monkeys. “This is a controversial area as to what is the best model for human myopia,” says Ian Morgan, a retired vision researcher at Australian National University. What works to prevent myopia in one model doesn’t seem to work in another. In other words, to find a cure for myopia, scientists need to understand the nitty gritty of animal models.

Neuroscientists discovered the classic animal model for myopia by accident in the 1970s, when they were sewing one eye shut in newborn monkeys to study the development of the brain’s visual system. It certainly did mess up their brains, but interestingly, the surgery also interfered with eyeball growth. Babies—human and monkey alike—are born farsighted, with their eyeballs slightly squashed. The eyeball elongates through childhood, and it knows to stop growing when the image on the retina is in focus. But sew the eye shut and it never sees a clear image; it keeps growing and growing until the eyeball is too long, aka nearsighted. “To become normal sighted is an incredible feat of controlled eye growth,” says Ian Flitcroft, an ophthalmologist in Dublin researching myopia.

Around the same time as the eye-sewing experiments, neuroscientists figured out they could do the same in chickens and tree shrews—much easier to keep in the lab than monkeys. And instead of sewing the eyelid shut, they could just put what looks like half a ping pong ball over the eye. This “form deprivation” model of myopia has inspired some fascinating science. In 2010, for example, Morgan’s collaborators found that exposure to bright light could reverse this type of induced myopia in chickens. Further experiments pinned down the mechanism, too: Light activates the neurotransmitter dopamine, which prevents the eyes from growing longer.

But! It may already be obvious that kids who get myopia don't walk around their eyelids sewn shut or covered with ping pong balls. So other scientists are inducing myopia by putting a set of lenses over the eyes of chickens. The lens induces an artificial error, moving the image on the retina to somewhere slightly behind the actual retina—so the eye will elongate (aka become myopic) as it attempts to find the clear image on the retina. But experiments using that model contradict those using the eye-closing methods: The results aren't as clear that bright light helps reverse nearsightedness.

Why is still unclear. Norton points out that with form deprivation, the eye never received any signal to stop growing, so the sudden strong bright light signal might be enough to stave off myopia. But if that light signal needs to fight against a competing signal—whatever made the eye grow too long in the first place, like that specially-designed lens—the treatment may be be less effective. Form deprivation gives scientists a mechanism, but the other model suggests treating myopia in the real world may be more complicated. “It’s a question of whether you’re trying to learn what’s going on or whether you want to jump to a cure,” he says.

Without understanding that competing signal, scientists can't nail down the cause of the recent rise in nearsightedness—whether it's focusing on your phone nine inches from your face all the time, how light interacts with our circadian rhythms to influence eye growth, or none of the above. But they may be able to land on a cure anyway. Morgan has since begun experiments in China to get children to study in translucent classrooms under the sunlight. Other scientists have found that special contact lenses or eye drops also seem to help slow the progression of myopia in children. The animal models have illuminated how the eye develops into myopia, but the search for a cure will have to come from human studies.