Nightlight for sleeping children may lead to myopia, study hints

Apparent link is found using recollections of parents about their kids

May 13, 1999|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

A team of doctors in Philadelphia says the nightlights and regular lamps many parents leave on in their young children's bedrooms at night may contribute to nearsightedness as the kids grow up.

In a study in today's edition of the British journal Nature, the doctors report that nighttime light exposure before age 2 appeared to be linked to as much as a fivefold increase in nearsightedness (also called myopia) later on.

Only 10 percent of the children in the study who slept in the dark before age 2 became nearsighted, compared with 34 percent who had nightlights and 55 percent who slept with the light on.

"We're not saying the light causes myopia. We're just noting an association we found in a rather small set of children," said Dr. Graham E. Quinn, the lead author of the study and a pediatric ophthalmologist at the Children's Hospital of Philadelphia.

Apparently, he said, "the eye needs a period of rest to grow and develop in a normal way, just like the rest of the body does. The sensible outcome of an observation such as this is that parents should afford their children a period of darkness at night."

Other eye experts weren't sure what to conclude from the report.

Dr. Karl Kupfer, director of the National Eye Institute, which provided the primary financing for the Philadelphia study, said the findings are preliminary.

"Further evaluation and replication of the results from this observational study will be needed before we can conclude that eliminating room lighting during sleep in infancy can affect the development of myopia," he said.

At the Wilmer Eye Institute, a branch of Johns Hopkins Medical Institutions in Baltimore, Dr. David G. Hunter, assistant professor of pediatric ophthalmology, said the study "really asks a question rather than answers one."

Even so, he called the results "pretty impressive. I consider it sort of a gold nugget in the dirt, a sign that we need to do more digging to find out if there is really something there."

In the meantime, he said, there is little risk in turning out the lights for children younger than 2. Childhood fears of the dark typically begin after that age.

Nearsightedness -- an inability to focus clearly on distant objects -- affects 25 percent to 30 percent of the population in the United States. It is caused by abnormal growth of the eyeball -- an elongation that causes light to focus in front of the light receptors on the retina, the rear surface of the eye's interior.

Although nearsightedness can usually be corrected with glasses, severe myopia can have more serious vision consequences. Glaucoma, macular degeneration and retinal detachment are more common in people with high degrees of myopia.

"Myopia is a very strong risk factor for blindness in adulthood," Quinn said. If restricting nighttime light exposure in children under 2 could avoid myopia later on, "the impact on public health might be quite dramatic."

The Philadelphia study was reported in Nature as "scientific correspondence," a peer-reviewed section reserved for short papers with unusual findings. The research was led by Quinn and Dr. Richard A. Stone, professor of ophthalmology at the University of Pennsylvania's Scheie Eye Institute.

Traditional studies into the causes of myopia have focused on the effects of excessive focusing on nearby objects. Quinn said recent studies of families and twins have also found evidence of genetic factors at work. Economic status may play a role, because myopia is more common among higher-income families.

Quinn said the relative importance of these factors and environmental influences such as light exposure remain a mystery. But in animals, at least, light exposure clearly seems to play a role.

In decades of research with chicks and sheep, Stone has shown that varying the hours of light and dark during a 24-hour period can have significant impact on eye growth and nearsightedness. The animals' eyes grew in the light, and stopped growing in the dark.

Given those results, Quinn said, "We thought it reasonable to ask if one of the reasons we see more myopia in our more urban, literate, industrialized population is increased light exposure."

The team surveyed parents of 479 children during visits to Quinn's eye practice. The children were aged 2 to 16 at the time of the survey. Their average age was 8.

On a one-page questionnaire, parents were asked about their children's use of sunglasses, about lighting levels in their homes, in day care and schools, and whether their children slept with room lighting, a nightlight or in darkness before the age of 2.

Only nighttime bedroom lighting in children under 2 showed a clear correlation with nearsightedness.

Quinn acknowledged that the questionnaire was an imprecise tool.

"We're asking parents of an 8-year-old to remember what happened when they [the children] were 2 and under," he said. "It's a weakness in the study, of course."

Also unclear is just how exposure to light early in life might alter the growth of the eyes.

"The papers I've looked at theorize some change in the chemicals present in the eye that somehow regulate growth in the eye," Hunter said. "You could argue that the light would adjust the levels of certain growth factors to put them out of whack."

Eye growth also seems to be guided to some extent by focus. Perhaps leaving the lights on produces some change in eye growth related to attempts to focus on objects, instead of resting.

In any case, Quinn warned parents that "some very ill babies can't sleep in the dark. Those parents should do what's right for the baby first, and worry about myopia later."

Pub Date: 5/13/99

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