One look at school children in China today will show one common glaring thing -- most of them are wearing spectacles.

Upon closer look, the reason why they are wearing them is because of nearsightedness, which means they can only clearly see objects within a few feet from their eyes.

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The numbers back up this ugly truth. Nearly nine out of ten high school students in China's biggest city Shanghai suffer from myopia, the medical term for nearsightedness, the state-run news outlet Xinhua reported.

Researchers are still trying to determine the exact cause of rising nearsightedness among China's youth.

A study released this week in the journal Ophthalmology offers a hint: myopia is almost twice as usual in an area of a middle-income province as it is in another part of a poorer province.

The data back up previous studies that show a rise in the incidence of nearsightedness is connected to education, higher incomes and urbanization. The researchers did say it is still not clear how economic differences affect nearsighted children.

A Chinese eye expert and leading researcher of nearsightedness, Dr. He Mingguang, offers an educated guess on the spread of myopia.

He blames the schooling system, because the heavy homework load is putting too much stress on students' eyes. The doctor adds a typical 10-year-old China has a meal on weeknights and then buckles down to studies from 7 until 11:30 in the evening or midnight.

Dr. He says competition in society is driving drastic changes to habits in China. School children are drilled to burn the midnight oil to pass the nation's make-or-break college entrance test, called the "gaokao."

He said Chinese parents were worried if their children did not make the university gaokao, or entry test, they will fail later in life.

In 2008, another study, which zeroed in on ethnic Chinese students in Sydney and Singapore, shows more time spent outdoors in the sun may help prevent myopia.

Students included in the study in both countries had similar study periods. But, the results showed 29 percent of students in Singapore had myopia compared to only 3 percent in Sydney.