Night-lights no cause of myopia, study concludes

By Poul Olson

Constant light does not cause nearsightedness in infants, according to vision scientists at Yerkes and the College of Optometry at the University of Houston, who report recent findings in the May edition of Investigative Ophthalmology and Visual Science.

Co-authored by Yerkes’ Dolores Bradley and Houston’s Earl Smith, the study challenges the conclusions of a 1999 investigation that found infants who sleep with night-lights or overhead lights left on are five times more likely to develop myopia than those who sleep in the dark.

In their study, Bradley and Smith examined nine infant rhesus monkeys, between one and four weeks of age, that were reared in around-the-clock room light for six months. The researchers compared these monkeys to 23 other infant monkeys reared with balanced periods of light and dark. The monkey life stage examined in this study closely corresponds to the first two years of human life, during which the human eye undergoes its most intense developmental period.

The scientists tested the infants’ eyes for evidence of myopia using retinoscopy, keratometry and A-scan ultrasonography. They found no significant differences in corneal radius, refractive error or axial dimensions of individual ocular components between the eyes of the monkeys exposed to constant light and those of the controls.

“Parents should not worry,” Bradley said. “Having lights on at night does not cause myopia.”
The 1999 study, completed at the University of Pennsylvania and published in the June 1999 edition of Science, identified a higher incidence of myopia among children who slept with night-lights or overhead lights before age 2 compared to those who did not. The study was based on retrospective accounts from the parents of nearsighted children.

“What’s well-established is that myopic parents tend to have myopic children,” Bradley said. “By the same token, there’s a high correlation between nearsighted parents and those who turn on night-lights. Myopic parents prefer to have night-lights on and are more likely to have nearsighted kids.”
The most common vision disorder, myopia results from an elongation of the eyeball. Near-sighted people have difficulty seeing distant objects because images fail to focus directly
on the retina.

In addition to genetics, environment likely plays a role in the development of myopia. “Visual experience, especially early in life, is a factor in both normal and abnormal refractive development,” said Bradley.

Bradley and Smith believe the night-light study reaffirms the strength of the rhesus model for studying myopia. Tradition-ally, researchers have used chickens to model nearsightedness because of their relatively low cost and the rapidity of their eyes’ development.

The model is limited, however, by the fact that chickens lack the binocular visual system characteristic of the human visual system.

“Chickens provide a means for evaluating and refining basic hypotheses, but their eyes respond very differently to the stimuli affecting eye development,” Bradley said. “Rhesus monkeys are ideal models for studying myopia because their eyes develop in much the same way as those in people.”

Currently, Yerkes and Houston’s College of Optometry are the only institutions in the world that use Old World monkeys (like the rhesus macaque) to model eye development.

Bradley and Smith co-authored the study with Yerkes investigators Alcides Fernandes and Ronald Boothe and Houston’s Li-Fang Hung. Investigative Ophthalmology and Visual Science is published by the Association for Research in Vision and Ophthalmology, the international association for clinical vision scientists.