Why Are Some Children Shortsighted?

The eyeball and the muscles which control the eye adjust the vision of the eye. When children read and write for quite a long time with their eyes very close to the books or notebooks, their eyes will feel tired. And some children like to read books when they are walking or lying in bed, or read in poor light .If they do this, after a period of time ,the muscles of the eye will feel tired and they will suffer eyestrain and have trouble focussing . At the same time, some children who are shortsighted inherit the problem from their parents or grandparents . those children who are shortsighted should wear glasses to correct defects of vision as soon as possible.