If you were ever caught reading in low light, or using a torch under the bedcovers to read after lights-out, your parents might well have warned you that straining your eyes would damage your eyesight. Or perhaps you used to hear that it’s easy to spot the studious children at school because they were the ones who had spent so long with their head in a book that they had to wear glasses.
Whatever you heard, the warning that people shouldn’t read regularly in dim light is a familiar one. Carry out a quick internet search, though, and you’ll discover that this is apparently a myth. End of story? Not quite. When you dig a little deeper and look at the scientific evidence, the story becomes much more complex.
Let’s look at the basics first. Short-sightedness or myopia means that a person can easily see things that are close up, but objects in the distance such as the number on a bus or the menu board in a restaurant look blurry. Wearing glasses or contact lenses solves the problem, but it doesn’t answer the question of why some people develop short-sightedness in childhood while others don’t.
Our eyes are cleverly designed to adjust to different light levels. If you are trying to read in the gloom your pupils dilate in order to take in more light through the lens onto your retinas. Cells in your retina, called rods and cones, use this light to provide information to the brain about what you can see. If you are in a dark room, for instance, when you just wake up, this process allows you to become gradually accustomed to what initially feels like pitch-black darkness. If you switch a light on, it feels unbearably bright until your pupils have had time to readjust once more.
The same happens if you strain to read a book in dim light. Your eyes do adjust, but some people find the strain gives them a headache. Likewise when you look at something close-up like a book or some sewing, the eye adjusts, muscles lengthen the area known as the vitreous chamber – the gelatinous bulk of the eyeball that lies between the lens and the retina.
Unfortunately, there is no convenient set of studies that have examined the long-term effects of reading in the dark. So we have to look at studies that looked at different factors and try to piece together the information.
Most of the research and the debate in short-sightedness has focused on the effects of repeatedly looking at things up close, what researchers call close work, rather than the effects of reading in poor light.
A British study last year, for example, found that close work could influence the onset of short-sightedness in adults but this was not nearly as important as factors such as birth weight or expectant mothers smoking during pregnancy.
Other regions of the world have a higher prevalence of short-sightedness, for instance, as many as 80-90% of school-leavers have myopia in parts of East and South East Asia, leading researchers to wonder whether the long hours children spend studying could cause the problem.
However, any geographical differences in rates of myopia could reflect genetic differences, and there is plenty of evidence that the genes you inherit from your parents are a major factor in short-sightedness. If both your parents are short-sighted there is a 40% chance that you are too. If your parents both have good vision, that risk drops to just a 10% chance.
The classic way to estimate the extent to which genes underlie a condition or illness is to compare identical and non-identical twins. A twin study conducted in the UK demonstrated that 86% of the spread of people’s eyesight scores could be explained by genetic factors. But as the authors of this study point out, this doesn’t mean we should ignore the effect of the environment.