陽光對視力的幫助

賈慶文

hy is nearsightedness 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， so you might wonder why our myopic ancestors werent just removed from the gene pool long ago， when they blundered into a hungry lion. 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 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 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 and the retina. 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 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.

In short， the biological mechanism 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. Brain wiring that effortlessly recognizes faces and animals can be thrown off by letters and numbers， leading to reading difficulties. A restless 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）.

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.

什么近視在現代社會中如此常見？在20世紀70年代初，25%的美國人患有近視；30年之后，這個比例上升到42%，在全球范圍內也有同樣的上升趨勢。有明顯證據表明近視會遺傳，于是你也會納悶，當我們近視的祖先撞上一只饑餓的獅子時，為什么沒有從基因庫中淘汰呢？盡管基因確實會影響我們的命運，但它并不是唯一因素。

近視的飆升似乎是由現代生活的特性引起的：我們在室內人造光源下待的時間越來越長了。人的大腦和眼睛很早以前就形成了，當時我們醒著的時間大多在陽光底下度過。這種進化過程利用了環境中可靠的因素——在當時，這可能是正常發展的必要條件。

研究人員認為，室外的明亮光線能幫助孩子們正在發育的眼睛保持晶狀體與視網膜之間的正確距離。昏暗的室內光線似乎不能帶來同樣的效果。因此，當孩子們在室內待得太久，他們的眼睛就不能發育好。

一份研究報告對生活在悉尼和新加坡的6～7歲華裔兒童進行了比較。新加坡的近視率為29%，比悉尼高出近九倍。悉尼兒童平均每星期在戶外待14小時左右，新加坡兒童只有三小時。

另一項由美國俄亥俄州立大學的學者展開的研究同樣發現，在父母雙方都患有近視的美國兒童中，每天待在戶外至少兩小時的兒童患近視的概率是每天戶外活動不足一小時的兒童的四分之一。

簡而言之，在新的環境條件下，數千年來讓我們在陽光下保持良好視力的生物機制，如今令視覺發展偏離了正軌。之前能較好地適應環境的基因正是許多缺陷的成因。能輕松識別人臉和動物的大腦回路在面對字母和數字時找不著北，從而導致閱讀困難。好動的天性曾經為需要在野外覓食的人們帶來裨益，但在今天的課堂上，它通常會被歸結為小兒多動癥（ADHD）。

幸運的是，還有一個簡單的方法能夠降低近視風險：多花點時間待在戶外吧！如果你打算看書，那就去室外看吧！