Hours glued to TV screens and electronic devices and not enough time spent outdoors is creating an 'epidemic' of eye conditions among children and teenagers, according to growing body of evidence.
It used to be thought that your chances of being short-sighted were largely down to genetics - now it seems children's lifestyles can play a key role.
The latest study published by Ulster University last week, found that the rate of short-sightedness among young people has doubled over the past 50 years. Twenty-three per cent of British 12 and 13-year-olds now suffer from myopia - the medical term for short-sightedness, which causes distant objects to appear blurred, while close objects can be seen clearly - compared to 10 per cent in the Sixties.
In East Asian countries, it is worse - with up to 90 per cent of schoolchildren short-sighted.
The problem occurs when the eye grows beyond its normal length, so instead of looking like a ping-pong ball, it is egg-shaped.
This affects how light enters the eye - rather than light rays reaching the retina at the back of the eye, they focus in front of it, resulting in distant objects appearing blurred.
This rise in short-sightedness is being blamed in part on a lack of daylight. Dopamine, a chemical messenger in the brain, is important for healthy eye growth and is released in the retina by daylight, explains David Allamby, an ophthalmologist and medical director of Focus eye clinic, London.
'Not having enough daylight may cause the eye to grow in an uncontrolled manner,' he says. 'It not only means children are having to wear glasses or contact lenses, with all the stigma that can bring, but being short-sighted increases the risk of a range of serious eye conditions.'
Study found going outside for an extra 40 minutes a day reduced short-sightedness in children.
These include a detached retina. With short-sightedness, the retina can become so stretched that it detaches, and if this is not treated, it can cause blindness. A detached retina is ten times more likely if you are short-sighted.
Glaucoma, a build-up of pressure in the eye which damages the optic nerve, and cataracts - cloudy patches that develop in the lens of your eye that can cause blurred or misty vision - are also more common in people with short-sight.
Myopia runs in families and, for years, it was thought that too much close-up work was to blame as it alters the development of the eyeball as it tries to focus on close-up images.
Now research shows that going outdoors even for short periods of time can make a huge difference.
One Chinese study found going outside for an extra 40 minutes a day reduced short-sightedness in children.
in the three-year trial following 900 children aged six and seven, one group was given an extra 40-minute outdoor class to end the school day. Of those, 30 per cent were short-sighted by the age of nine or ten compared with 40 per cent of those who stayed indoors.
Ashwin Reddy, a consultant paediatric ophthalmologist at Moorfields Eye Hospital and The Portland Hospital, both in London, says: 'For the past five years, I have been advising my patients to spend more time outdoors to help with their eye health. It can prevent the progression of myopia.'
Meanwhile, prolonged use of electronic devices such as iPads and smart phones can cause eye strain and dry, burning and itching eyes.
A 2014 survey by the American Optometric Association found that 80 per cent of children aged between ten and 17 reported burning, itchy or tired eyes after using electronic devices for long periods.
The problem is that when we use screens, we blink less. Blinking lubricates eyes and, while people normally blink every three to four seconds, when staring at a screen it is only every ten to 12 seconds, so eyes become dry and sore.
Mark Rosenfield, a professor at the College of Optometry at the State University of New York, explains: 'These are all symptoms of digital eye strain, a temporary condition caused by prolonged use of technology.
'Additional symptoms may include headaches, fatigue, loss of focus, blurred vision, double vision, and head and neck pain.'
Laboratory studies show these symptoms are significantly greater when viewing digital screens compared with books.
'There is a difference in how we blink when viewing digital screens versus printed material,' Professor Rosenfield explains. 'A higher number of incomplete blinks [where the upper eyelid fails to cover the front of the eye completely] has been found in our lab when viewing electronic screens.'
While there is no evidence yet that use of digital devices themselves rather than lack of sunlight causes permanent damage to eyes, 'it may be years before we see their full effect'
Figures from Ofcom show the average eight to 11-year-old now spends 4.5 hours a day in front of a screen, and among teenagers this rises to 6.5 hours - and experts fear this trend is only going to get worse.
As well as getting outside, Professor Rosenfield recommends not holding devices closer than 40cm (16in).
'The size of the print on smartphones is often smaller than in printed materials, with the result that users hold their phones closer,' he says.
'The closer the object, the more the eyes have to work to focus.'
He also warns that while there is no evidence yet that use of digital devices themselves rather than lack of sunlight causes permanent damage to eyes, it may be years before we see their full effect. 'These devices are relatively new, so the long-term effects may not yet be evident,' he warns.
Children's diet may be playing a role in their poorer eye health, too.
Omega 3, a nutrient found in oily fish and flaxseeds, is key to eye health but many children don't have enough of it, says Mr Allamby.
He explains: 'Omega 3 is important for reducing tear evaporation from the eyes.
'Without it, eyes can become dry and sore. And we know the proportion of Omega 3 in the diet is now significantly less.'
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