阳光是最好的验光师

by Sandra Aamodt

Why is nearsightedness[近视] so common in the modern world? In the early 1970s, 25% of Americans were nearsighted; three decades later, the rate had risen to 42%, and similar increases have occurred around the world. There is significant evidence that the trait is inherited[遗传的], so you might wonder why our myopic[近视的] ancestors werent just removed from the gene pool long ago, when they blundered into[无意中遇到] a hungry lion. Well, although genes do influence our fates, they are not the only factors.

The rapid increase in nearsightedness appears to be due to a characteristic of modern life: more and more time spent indoors under artificial[人造的] lights. Humans brains and eyes originated long ago, when we spent most of our waking hours in the sun. The process of development takes advantage of such reliable[可靠的] features of the environment, which then may become necessary for normal growth.

Researchers suspect that bright outdoor light helps childrens developing eyes maintain the correct distance between the lens[晶状体] and the retina[视网膜]. Dim indoor lighting doesnt seem to provide the same kind of feedback. As a result, when children spend too many hours inside, their eyes fail to grow correctly.

One study compared six and seven-year-old children of Chinese ethnicity[种族] living in Sydney with those living in Singapore. The rate of nearsightedness in Singapore (29%) was nearly nine times higher than in Sydney. The children in Sydney spent on average nearly 14 hours per week outside, compared with just three hours per week in Singapore.

Similarly, another study by scholars at Ohio State University found that, among American children with two myopic parents, those who spent at least two hours per day outdoors were four times less likely to be nearsighted than those who spent less than one hour per day outside.

I n s h o r t , t h e b i o l o g i c a l mechanism[机制] that kept our vision sharp for thousands of sunny years has, under new environmental conditions, driven visual development off course. This capacity for previously well-adapted genes can account for many imperfections[缺陷]. Brain wiring that effortlessly recognizes faces and animals can be thrown off by letters and numbers, leading to reading difficulties. A restless[好动的] nature was once helpful to people who needed to find food sources in the wild, but in todays classrooms, its often classified as ADHD(attention deficit hyperactivity disorder[注意缺陷多动障碍]).

Luckily, there is a simple way to lower the risk of nearsightedness: Spend more time outside! If you are going to read a book, do it outdoors!

为什么近视在现代社会中如此常见？在二十世纪七十年代初，25%的美国人患有近视；三十年之后，这个比例上升到42%，在全球范围内也有同样的上升趋势。有明显证据表明近视会遗传，于是你也许会想，当我们的近视祖先撞上一只饥饿的狮子时，为什么这样还不足以将这种基因从基因库中淘汰？尽管基因确实会影响我们的命运，但它并不是唯一因素。

近视的飙升似乎是由现代生活的特性引起的：我们在室内人造光源下呆的时间越来越长了。人类的大脑和眼睛很早以前就形成了，当时我们清醒的时间大多在太阳底下度过。这种进化过程利用了环境中这些可靠的因素——在当时，这可能是正常发展的必要条件。

研究人员认为，室外的明亮光线能帮助孩子们正在发育的眼睛保持晶状体和视网膜之间的正确距离。昏暗的室内光线似乎不能带来同样的效果。因此，当孩子们在室内呆得太久，他们的眼睛就不能长好。

一份研究报告对生活在悉尼和新加坡的6-7岁华裔儿童进行了比较。新加坡的近视率（29%）比悉尼高出近九倍。悉尼儿童平均每周在户外呆14小时左右，新加坡儿童只有3小时。

另一项由（美国）俄亥俄州立大学的学者展开的研究同样发现，在父母都患有近视的美国儿童中，每天呆在户外不少于两小时的儿童的近视可能性要比每天户外活动不足一小时的儿童低四倍多。

简而言之，在新的环境因素下，数千年来让我们在阳光下保持好视力的生物机制，如今令视觉发展偏离了正轨。之前能较好地适应环境的基因正是许多缺陷的成因。能轻松识别人脸和动物的大脑回路在面对字母和数字时找不着北，从而导致阅读困难。好动的天性曾经为需要在野外觅食的人们带来裨益，但在今天的课堂上，它通常会被归结为小儿多动症（ADHD）。

幸运的是，还有一个简单的方法能够降低近视风险：多花点时间呆在户外吧！如果你打算看书，那就去室外看吧！