London, March 25 (IANS): Myopia, or short-sightedness, has assumed 'epidemic' proportions in the last 30 years, with some countries reporting prevalence of 80-90 percent - and the reason could simply be lack of adequate light in classrooms.

Richard Hobday, an authority on sunlight and health, has in a new study compared myopia to bone disease rickets, caused by the lack of Vitamin D that you get from sunlight.

Hobday has revived a century-old theory that suggests myopia could also be linked to the lack of sunlight - or at least adequate ambient light.

Myopia is today a global health problem, with countries like Singapore, Taiwan and China reporting rates as high as 80-90 percent among children leaving secondary schools. In the west, too, rates are increasing.

The cause and means of preventing of myopia are still unclear, despite 150 years of scientific research.

Hobday compares the history myopia with rickets. In the 17th century, rickets was common among children in England and reached epidemic levels.

A remedy proved elusive until the 1920s, when scientists found that a lack of sunlight, resulting in Vitamin D deficiency, was the cause of rickets.

Myopia, like rickets, is a seasonal condition which appears to get worse in winter. Recent research on myopia has revived the old theory from the 1890s that school children who spend more time outdoors have lower levels of myopia.

However, unlike rickets, low ambient light levels rather than low Vitamin D levels seem to be the deciding factor in myopia, Hobday says.

In the late 19th century it was believed that high daylight levels in schools could prevent myopia. Education departments built classrooms with large windows to try to stop children becoming short-sighted.

But this theory lost traction in the 1960s, and myopia was thought to be an inherited condition.

Hobday believes it is time to revisit the theory. Though evidence that daylight in classrooms prevents myopia is lacking, he says: "It has not been investigated properly since the connection was first made in the 1860s. But, given the rapid increase in myopia among school children worldwide, this should be revisited."