DW: What happens to our eyes when they're exposed to screens for long periods of time — whether a computer, mobile phone or tablet?

Frank Schaeffel: These three things are quite different. A computer screen usually covers a large part of your visual field, because it's big, but a phone is much smaller. When talking about myopia (short sightedness), it makes a big difference whether you're looking at a big screen or a small one, like a cell phone.

But it actually doesn't matter whether you're looking at a book or a screen. What's critical is the viewing distance. For example, if you read a book from 20 cm (12 inches) away all day, you'll probably develop myopia pretty soon.

There's a lot of speculation about the risks of being exposed to too much blue light when looking at a screen – what's your view?

The amount of blue light you're exposed to when you go outside is so much greater than anything you'll get from a computer screen.

We do have certain cells in the eye — melanopsin ganglion cells — that respond to blue light, and they influence our sleeping patterns. So if you're looking at a screen in the evening, say from 8 p.m. to 10 p.m., it could affect your sleep quality.

The question is always how much blue light. Is it enough to stimulate this melanopsin system sufficiently to really affect your sleep quality? This is the only risk, I think.

What about people who complain they feel tired after looking at a computer screen, or that their eyes are blurring? What's happening then?

You have to look at the viewing distance of the subject to their screen. The problem is usually that they're reading small text. And if they are at a close distance, their eyes have to work harder to accommodate.

If you're working on an Excel spreadsheet, for example, you're concentrating on a small area on the screen for a long period of time. And your eye movements are much more limited than if you were walking around outside. It'd be the same if it was a textbook.

The thing is, now that most people use computers for work, they're spending a lot more time reading in general. The screen itself is not dangerous — it's the fact that it draws your attention for many hours.

Special glasses have come onto the market promising to "eliminate eye strain" and "improve sleep" for those glued to their screens. What do you make of these products?

What I have seen in the literature is that if people wear yellow-tinted glasses in the evening, their retinas are exposed to less blue light, and their sleep quality can improve.

And I haven't tested it on my phone yet, but there's an option to change the color of the light emitted by your smartphone in the evening, so that it's less blue. But I haven't actually seen any data on its effectiveness, and the energy that comes out in the blue light is not terribly high, so I'm not sure how effective it really is.

Do you have any other suggestions for ways we can minimize the risks of damaging our eyesight?

It's important not to get locked to some fixed target for many hours. It's a good idea to change your viewing distance occasionally — look outside the window for a few minutes, get up and walk around a bit.

In Taiwanese schools, for example, it's an official rule now that the children are only allowed to read for 30 minutes at a time, with a 10-minute break in between. And they also have to go out for at least one and a half hours per day. Since these rules were applied in 2012, there's already been quite a reduction in the development of myopia in children.

Frank Schaeffel is a professor of eye neurobiology at the University of Tübingen in Germany.