Is there any evidence that watching television for too long or while sitting too close weakens the eyes (leading to myopia, for example) ?
There are two theories about this myth which I've heard:
- Watching TV causes eye strain, eye strain damages the eyes
- The TV set emits "cathode rays" which damage the eye directly
Let's debunk them in order
Eye strain theory
It's a well known fact that watching TV causes strain (tiredness) in the eyes. Any ergonomics source will tell you that. The problem here is that eye strain causes are many - TV, computer, books, driving, etc.
Another problem is that if you are short sighted and your sight is uncorrected (typical case would be a child with uncorrected eyesight), then the strain gets worse. This leads to confirmation bias (i.e. "My son had eye strain and then was diagnosed with myopia, therefore eye strain causes myopia" whereas the the causal relation is the opposite).
So, yes, TV can cause eye strain, as many different other things - this partially debunks the myth because TVs don't have any particular status here.
Does eye strain cause myopia? No. Eye strain might precipitate the onset in some cases, but there are no ways to prevent myopia. Furthermore, it's quite certain that myopia is of genetic origin. See the following references: 1, 2, 3, 4
So we can see that the eye strain theory does not stand on its feet.
Cathode rays theory
This is easily debunked. Yes, old school CR tubes did use beams of electrons to generate images. The thing is, these beams are tightly focused on the screen in order to have a focused image. This is obtained through different means, but basically the electrons hit phosphors in the screen which is a wire mesh - conveniently it is also a Faraday cage which prevents them from going anywhere.
The actual image is made of phosphor generated light. It's just light :-)
That said - there is a small amount of X-rays being emitted, but it's tightly regulated to super-low levels and it is widely considered innocuous.
According to the Straight Dope, in ordinary circumstances watching too much TV or sitting too close will only lead to eye fatigue, not eye damage. Some studies claimed that exposure at too young an age can cause myopia, but at the time of writing (1992), there was no definite proof.
No, watching too much television does not directly cause myopia.
Walter Wood has debunked the theory that too much near work causes myopia.
In his book The Ultimate Unification of Diet Health and Disease he makes the following points/observations:
In a study published in January 2011 80 7-11-year olds were randomly assigned into intervention or control groups. The children in both groups spent similar amounts of time in near vision activities, while the children in the intervention group spent less time in middle -vision activities and significantly more time in outdoor activities (13.7 hrs vs. 6.2 hrs). The annual myopic progression in the intervention group was significantly slower, 0.38D, than in the control group, 0.52D.. No relationship was found between the amount of near work and the rate of myopic progression.
In a report published in March 2011, researchers analyzed the visual activities of myopes and emmetropes (non-myopes) before, at and after the onset of myopia.. Hours per week spent reading, playing video games or using a computer didn't differ between the two groups before myopia onset, which the authors acknowledge "argue against a major causative role for near work" in myopia development. Hours per week spent in outdoor activity were far fewer (1.1 vs. 1.8 hrs. per week) for the children who became myopic..
A study entitled "Myopia, Lifestyle, and Schooling in the Students of Chinese Ethnicity in Singapore and Sydney" compared the prevalence of myopia in children 6-7 years of age in Sydney and Singapore. In Sydney the prevalence of myopia among these children was found to be only 3.3%, while in Singapore myopia rates were immensely higher, 29.1%, almost 10 times higher than the rate observed in Sydney.. Moreover, the children in Sydney read MORE and did MORE total near work than the children in Singapore. The only difference was, the children in Sydney spent significantly more times outdoors (13.75 hours per week) than the children in Singapore (3.05 hours per week) - almost five times more spent in outdoor activity than in Singapore.
Watching too much television or playing too many video games might indirectly reduce the amount of time you spend outdoors and may thus increase your risk of developing myopia.
I believe the theory that too much close work (such as watching too much television) causes nearsightedness was born out of the idea promoted by Dr. Bates and his followers who claim that near work causes strain on the eye and that stress and strain are the primary drivers of myopia - which is unsubstantiated and has been debunked numerous times. Also, I think this assumption likely stems from the fact that activities that normally increase near work also decrease our exposure to the sun. However, the lack of sun exposure has been consistently shown to be correlated with myopic progression whereas excessive close work was not.
@Sklivvz wrote in his answer:
"..but there are no ways to prevent myopia. Furthermore, it's quite certain that myopia is of genetic origin."
While genetics plays a role in everything that happens in our body the notion that "there are no ways to prevent myopia" is false and clearly refuted by the study Singapore and Sydney I linked to. The authors of the study state that parents of both groups had virtually the same amount of myopia - yet the rate was nearly 10 times higher in Singapore. So while genetics may influence how our bodies respond to the lack of sunlight, with sufficient sun exposure we could reduce the prevalence of myopia.
Focusing on anything for an extended period of time can cause eye fatigue. Older eyes tend to recover from the fatigue more slowly, which may lead to some perceived vision loss during the recovery time.
There's no real evidence to suggest focusing on a TV is any worse than anything else.