Children are spending more time indoors than ever before. At the same time, rates of nearsightedness are climbing around the world.

One source of these vision problems could be something missing from the light inside our homes, schools and daycare centers, an intriguing new study suggests. The missing ingredient? Indigo light.

Indigo light ranging from 419 to 446 nanometers was most effective and, remarkably, completely prevented myopia from developing.

The study found that exposure to a particular range of indigo light completely prevented myopia, or nearsightedness. The study was done on tree shrews, who, despite their squirrel-like appearance, are near-primates with eyes that share anatomical and optical features with human eyes.

The findings, by a team from the Cincinnati Children's Hospital Medical Center and the University of Alabama at Birmingham, raise the possibility that someday indoor lighting could be redesigned to help protect children's developing vision.

Myopia occurs when the eye grows too long from front to back, causing incoming light to focus in front of, rather than directly on, the retina. The result is that distant objects become blurry. While glasses and contact lenses can correct vision, severe myopia can increase the risk of retinal detachment, glaucoma and macular degeneration later in life.

The problem is growing. Some experts predict that nearly five billion people, that's half the world's population, could be nearsighted by 2050. Children who spend more time indoors and less time outdoors appear to be particularly vulnerable.

Standard white LED lighting contains relatively little of this portion of the spectrum. Sunlight, however, is rich in it.

To conduct the experiments, Rafael Grytz, a visual sciences expert at the University of Alabama at Birmingham, and his colleagues fitted tree shrews with tiny spectacles designed to create a powerful myopia-inducing signal in one eye. The other eye served as a comparison.

The researchers then exposed the animals to different wavelengths of light and carefully monitored changes in their eyes over time. They used a biometer to measure the axial length, or front-to-back dimension of the eye, and an auto refractor to measure changes in refraction.

The team was particularly interested in a light-sensitive receptor called Opson-5 or OPN5. Earlier experiments in mice had suggested that violet light near 380 nanometers could suppress myopia. But there was a problem: human lenses, like those of tree shrews, block most ultraviolet light below about 400 nanometers. That led the team to investigate slightly longer wavelengths.

They found that indigo light ranging from 419 to 446 nanometers was most effective and, remarkably, completely prevented myopia from developing in the tree shrew model. “Human lenses, and now we know tree shrew lenses, don't transmit a lot of light in the ultra-violet spectrum,” Lang explained in a press release. "They basically cut off most wavelengths below 400 nanometers.”

This finding pushed the scientists to explore slightly longer wavelengths that could still stimulate OPN5. Ultimately, they found that indigo light from 419 to 446 nanometers was the most effective at preventing myopia.

Children who spend less time outdoors appear to be particularly vulnerable. Reproducing more of sunlight's spectrum indoors may someday become another tool for keeping young eyes healthy.

Standard white LED lighting, the researchers note, contains relatively little of this portion of the spectrum. Sunlight, however, is rich in it. "We evolved outside in the full-spectrum light provided by our sun," researcher Richard Lang, director of research in the division of ophthalmology at Cincinnati Children's Hospital, said. "When we live inside, we don't get all the wavelengths the eye needs for normal refractive development, and so we get myopia. That's the basic message of this paper."

Parents should not rush out and buy specialty lights. The research was conducted in animals, and it remains unknown whether indigo-enriched lighting will prevent myopia in children. That's the researchers' next step. The team hopes to install specially designed lighting in daycare centers and follow children over time, comparing rates of myopia with those in facilities using conventional lighting.

For now, the results offer a possible explanation for why the outdoors appears to protect children's vision, and suggests that reproducing more of sunlight's spectrum indoors may someday become another tool for keeping young eyes healthy.

No one yet knows for certain whether changing indoor lighting will reduce myopia in children. If it does, something as ordinary as the light above a child's desk could eventually become part of protecting their eyesight.

The study is published in Cell Reports Medicine.