May 13, 1999 Children who sleep with a light on in their bedrooms at night before the age of 2 may be at significantly higher risk of developing myopia - near-sightedness - when they become older than children who sleep as infants in the dark at night, according to a collaborative study by researchers at the University of Pennsylvania Medical Center and The Children's Hospital of Philadelphia. A report on the findings will appear in the May 13 issue of Nature.
The team's results showed that, of children aged 2 to 16 who had slept in darkness before age two, 10 percent were myopic at the time of the study. Of children who had slept with a night light on before age 2, 34 percent were myopic. And of children who had slept at night with a room light on before age 2, 55 percent were myopic - more than a five-fold increase over the children who slept in darkness during early childhood.
"Our findings suggest that the absence of a nightly period of full darkness in early childhood may be an important risk factor in the future development of near-sightedness," says Richard A. Stone, MD, a professor of ophthalmology at Penn's Scheie Eye Institute and senior author on the study. "The study does not establish that nighttime lighting during early childhood is a direct cause of myopia, and there are undoubtedly other risk factors. Still, it would seem advisable for infants and young children to sleep at night without artificial lighting in the bedroom until further research can evaluate all the implications of our results."
Near-sightedness is more than a minor inconvenience to be corrected with glasses or contact lenses, the scientists emphasize.
"Especially in the more severe degrees, myopia itself is a leading risk factor for acquired blindness, putting individuals at increased risk for retinal detachment, retinal degeneration, and glaucoma," says Graham E. Quinn, MD, a pediatric ophthalmologist at The Children's Hospital of Philadelphia and lead author on the study. "The earliest years of life appear to represent a critical time in the proper growth and development of the eye, so this strong association, not noted before, between a daily dark period - nighttime - during infancy and later near-sightedness in children may well have significant clinical ramifications."
Large numbers of people are affected by the problem of near-sightedness. In the United States, at least 25 percent of the population is myopic, and in Asia the proportion of people with myopia is even higher.
"This is an extension of myopia research in animals done by Dr. Stone and others," said Carl Kupfer, MD, director of the National Eye Institute, one of the Federal Government's National Institutes of Health and the agency that primarily funded the study. "The investigators have reported an association between ambient light exposure during sleep before age two and myopia. Additional studies are needed to determine whether eliminating such light exposure during sleep in early childhood can affect the development of this common form of refractive error."
Beyond its immediate clinical implications, the study also offers a novel explanation for the increasing prevalence of myopia over the last two centuries, as populations shifted from agricultural to urban, industrialized environments. Many clinicians and investigators have theorized that so-called nearwork - reading and other close-at-hand occupations - is responsible for the increase. While not bearing directly on the nearwork hypothesis, the current findings suggest that the greater ambient nighttime light levels associated with industrialization may be a leading factor in the high incidence of myopia in developed nations.
The study was conducted using a survey instrument. The parents of 479 children aged 2 to 16 - the median age was 8 - were asked whether their children slept with room lighting, with a night light, or in darkness before the age of 2. They were also asked to report on the current nighttime lighting conditions for the same children. Other questions addressed the lighting in various rooms of the home, in day care or school settings, and in the geographical region in which the child lived. The use of sunglasses was also assessed.
The investigation was designed to be a clinical extension of results from basic laboratory research in chicks demonstrating that the relative proportions of light and dark during the 24-hour day greatly affected eye growth and refractive development.
An association was found only between current refraction and nighttime lighting before age 2. No association was found between current refraction and room lighting during sleep at the children's current ages. The investigators chose the cut-off age of 2 years because the eye grows particularly rapidly before this. Whether or not the specific age of 2 years actually defines the precise end of the susceptibility period to nighttime illumination can only be known from future research.
Coauthors on the paper with Stone and Quinn are biostatistician Maureen G. Maguire, PhD, at Penn's Scheie Eye Institute and Chai H. Shin, MD, a clinical fellow at Children's Hospital. Primary funding for the investigators' myopia research has been provided by the National Eye Institute. Additional support has been provided by Research to Prevent Blindness, the Pennsylvania Lions Sight Conservation and Eye Research Foundation, Inc., and the Ethel Brown Foerderer Fund for Excellence at The Children's Hospital of Philadelphia.
The University of Pennsylvania Medical Center's sponsored research and training ranks second in the United States based on grant support from the National Institutes of Health, the primary funder of biomedical research and training in the nation -- $201 million in federal fiscal year 1998. In addition, the institution continued to maintain the largest absolute growth in funding for research and training among all 125 medical schools in the country since 1991. The Children's Hospital of Philadelphia, the nation's first children's hospital, is a leader in patient care, education and research. This 406-bed multispecialty hospital provides comprehensive pediatric services to children from before birth through age 19.
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