Do natural light levels make a baby born in June more likely to be nearsighted, or myopic, than if he or she had been born in December? While scientists think genetic factors play the strongest role in nearsightedness, a number of studies show that light exposure before and just after birth generates biological signals that influence the development of the eye's ability to focus and refract light properly.

Research had suggested that the influence of light on vision development in this perinatal period might occur through mother-baby biological signals before birth, or through the baby's direct exposure after birth, or both. Because the effect of light levels on myopia, if any, was likely to be slight, a large population study was needed to further explore the question.

Yossi Mandel, MD, and his research team found a suitable study group in candidates for Israeli military service who were medically evaluated between 2000 and 2004. All 276,911 participants (157,663 male, 119,248 female) were born in Israel, ensuring their exposure to the same seasonal light variations. Myopia prevalence rates, determined by visual examination and defined at three levels of severity, were: mild, 18.8 percent; moderate, 8.7 percent; and severe, 2.4 percent.

The risk of moderate and severe myopia varied with seasonal levels of light, or photoperiods, with highest rates in babies born in June/July and lowest in December/January. These correlations were considered highly statistically significant. The findings were adjusted for other known myopia risk factors, such as gender, education level, and father's country of origin. Mild myopia was not associated with season of birth or perinatal light exposure.

Myopia allows people to see close-up objects clearly while distant objects appear blurred. The condition is on the rise worldwide, but causes remain uncertain. Treating myopia in the United States costs more than $4.6 billion per year, and people with the condition are at higher risk for retinal detachment and other complications.

Since the link to seasonal birth date was most pronounced for severe myopia, Dr. Mandel concludes: "It seems reasonable to assume that only a fraction of the population might be genetically prone to develop myopia if exposed to environmental risk factors such as a long perinatal photoperiod. Further exploration of the mechanisms underlying the effect of light on the development and progression of myopia in humans is needed to devise effective preventive measures."

This research was published in the April 2008 issue of Ophthalmology, the journal of the American Academy of Ophthalmology.