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Children born in June and July - the months with the most sunshine - are 25 percent more likely to become nearsighted (myopic) than those born in December or January, according to Israeli research just published in the on-line edition of the most prestigious eye medicine journal, Ophthalmology.
The cause is not astrological, according to Prof. Michael Belkin of the Goldschleger Eye Research Institute at Tel Aviv University's Sackler Faculty of Medicine and Sheba Medical Center at Tel Hashomer, and colleagues in the IDF, the Hebrew University of Jerusalem and the Technion-Israel Institute of Technology in Haifa. Rather, it is apparently that the amount of exposure to natural light at birth that affects the size and shape of the eyeball.
The more light a newborn is exposed to after birth, the more the eyeball lengthens, according to the study, causing images to be focused in the vitreous part inside the eye rather than on the retina at the back of the eye. Myopia is a refractive defect of the eye in which light produces the image focus in front of the retina. The nearsighted see nearby objects clearly, but distant objects appear blurred. The opposite defect is hyperopia (farsightedness), in which the eye is too short or the cornea is flat.
Aside from light exposure, animal studies have shown a mechanism affecting vision dependent on the amount of melatonin, a hormone released by the pineal gland in the brain during darkness. This hormone supervises, among other things, the growth of the eyeball; when there is less melatonin because of long summer days, the eye lengthens and myopia can result.
The retrospective study was conducted on 276,911 young people (167,000 of them male) aged 16 to 22 before induction into the army and during military service. Their vision was tested, and myopia was classified as mild (-0.75 diopters), moderate (-3.0 to -5.99) or severe (-6.0 and beyond). The researchers compared the degree of shortsightedness with the month of birth and found those born in the months with the most light were more likely to have a refractive index lower than -3 diopter).
In their article, called "Season of Birth, Natural Light and Myopia," Belkin along with Drs. Yossi Mandel, Itamar Grotto, Ran El-Yaniv, Eran Israeli, Uri Polat and Elisha Bartoov made sure that country of origin was not a factor; only Israeli-born young men and women were chosen for the study. They took into account birth complications, weather, infectious agents, socioeconomic status and education. Better-educated parents, they suggested, might prefer to have their babies born in the summer and time their pregnancies thusly, resulting in the possibility of a higher prevalence of myopia in those born in the summer. They also registered the eyesight of siblings born during other months. But the results showed no connection among these factors, except when a baby was born.
In Israel, there is a quite a lot of light even during the winter - with only 4.2 fewer daylight hours less than in the summer. The researchers suggested that the difference could be magnified by the "natural tendency to spend more time outdoors during summer. A previous study showed that even a small difference in the amount of light between Israeli summers and winters is enough to affect the newborn's melatonin system. It was not clear, however, whether the light directly affects eyeball length or if the mother's melatonin system is responsible for it, they wrote. Further investigation "of the mechanisms underlying the effects of light on the development and progression of myopia will be needed to devise effective preventive measures," the authors concluded.