The idea that reading in dim light ruins your eyes isn’t my favourite wives' tale about “leisure activities” causing blindness, nor is it the most obscene! In any case, it’s simply not true.
I’ll begin my expose´ with a brief explanation of how we see.
The eyes are equipped to catch the light reflected off, or generated by, objects in our world. When light enters our eyes, it’s focussed by the front layers of the eye onto the retina – a delicate layer of just a few rows of cells, less than half a millimeter thick – which sits at the back of the eye.
The light that falls on the retina is referred to as the image. The retina begins the process of decoding the image and sorting this into information that tells our brains about its brightness, colour, shape, size, and movement. The information, in the form of neural signals, is then passed back to the brain, which further processes the data before bringing it to the attention of our conscious mind.
Some animals, such as owls, have retinas that are specialised for seeing even the tiniest amount of light. Put simply, they see clearly in conditions that we consider pitch black because their retinas contain “rod” detectors. These rods are very sensitive, but can’t decode colour (owls are colour blind). There are close to 60,000 of these rods per square millimetre of retina, which translates to owls' incredible sensitivity and sharpness of vision – called acuity.
Humans also have many rods, which is why we’re able to see when driving at night when there’s very little light around. But rods become useless in bright or even normal light levels. That’s why we also have “cones”, which are much better in daylight and allow us to see colour. These cones can be found throughout the retina, with the greatest number in the centre.
When you turn off the lights, your “night vision” gradually kicks in over six or seven minutes, as you stop using your cones and start using your rods.
There are no rods at the centre of the human retina which gives rise to the fact that we have low sensitivity to dim light in our central vision. That’s why, when you go outside on a clear night and look directly up at a faraway star, you won’t be able to see it. You can only see a star by looking to the side of it, thus using your rods.
On the flipside, in order to read, we practically only use the cones in the centre of our vision. To test this for yourself, try reading a column on the screen while looking just to the side of that column – impossible.
So reading in dim light – or reading at all – is possible when there is enough light around for the cones to pick up a signal.
Your eyes won’t be harmed but you may give yourself a headache. This is because, from an evolutionary perspective, the eyes weren’t designed for straining to see close-up objects for sustained periods. They are much better suited for looking out into the distance over fields of buffalos (not that many of us have that luxury in our modern-day lives).
Of course, eyestrain – the feeling of tired or aching eyes and headache – may indicate that you need glasses, or perhaps the glasses you’re wearing may need an update. If you are concerned about the health of your eyes, see your optometrist for a check up.