Staring at things at close range, such as smartphones and TVs, is having a detrimental effect on children's eyesight.
According to data compiled by the Education, Culture, Sports, Science and Technology Ministry, children with uncorrected vision lower than 0.1 account for about 30 per cent of primary school students and 50 per cent of middle school students.
The figures show a rapid increase from those in the early 1980s when they were between 15 per cent to 20 per cent for primary school students, and between 30 per cent to 40 per cent for middle school students.
Although there is a hereditary element in shortsightedness, it is believed that lifestyle has a bigger impact on eyesight deterioration.
"Children are increasingly looking at things at close range when studying or watching TV. Nowadays there are also many children who stare at smartphones and game consoles," said Takuya Kikuchi, director of Kikuchi Eye Clinic in Kawasaki.
It is important to increase the amount of time they spend playing outdoors, so they can get used to looking at things some distance away.
Although children who need eyeglasses should wear them, some parents believe wearing glasses causes the eyes to become weaker. This is regarded as a common misunderstanding.
If children do not use glasses and spend their time staring into the distance at blurred images, this can affect their understanding during school lessons and their motor skills may become sluggish. There are glasses with strong frames available to keep them firmly in place even during sports activities.
When children keep wearing the same pair of glasses, their eyesight may still change over a number of years. This is because eyesight is related to the size of the eyeball, and as children grow their eyeballs become larger. Some children do not notice the changes in their eyesight.
Therefore Kikuchi advises: "Children should have eye tests once a year at an ophthalmology clinic. It is also important for parents to notice any changes in their children's eyesight."
There has been some research showing how eyesight deterioration can be slowed by using special bifocals or nighttime contact lenses. Although this does not cure shortsightedness, it is a good idea to consult an ophthalmologist.
Not only children's eyes but also adults' eyes can suffer from staring closely at things. On trains, many people stare at their smartphones, and computers are essential tools in the workplace.
"There is a possibility this may be the cause of various problems, including shortsightedness and retinal disease," said Rui Hiramatsu, head of the ophthalmology department of Higashi-Omiya General Hospital in Saitama.
Smartphones and personal computers are considered to be the cause of age-related macular degeneration, an incurable disease that causes black patches or distortion in the field of vision. This disease is caused by damage to the macula lutea, a tiny area located in the centre of the retina. Blue light, which is emitted from smartphones, PCs and TVs, is said to cause more damage to the macula lutea than other coloured lights.
Shortsightedness can progress further the more we look at things at a short distance. Smartphones and PCs tend to be used at close range, causing eyesight deterioration.
When an object is being stared at for a long time, the number of times the eyes blink is reduced. This decreases the secretion of tears, causing what is known as dry eye, which can result in pain and blurred vision.
The more people stare at smartphones and PCs at closer range, the greater the damage to their eyes. It is important to look at things from as far away as possible. In particular, smartphones are devices that we hold in our hands and stare at closely. However, Hiramatsu suggests the devices should be kept at least 40 centimeters away from the eyes.
Strong light emitted from a computer monitor can cause significant damage to the retina, so it is better to reduce screen brightness to about half. To prevent dry eye, it is important to know that we should consciously blink our eyes and should not stare at devices while lying down.
To alleviate temporary symptoms of nearsightedness and dry eye, it can be effective to stare leisurely at some point in the distance for about three minutes, and placing a warm towel over eyes to stimulate the secretion of tears.