Does reading in the dark hurt your eyes?

When I was in second grade, I locked myself in the bathroom and read Beverly Cleary’s “Dear Mr. Henshaw” from back to front. The rush of it was enough that I started trying to fit longer and longer books into a single day and often found myself as a literate outlaw, reading after hours in a dark bedroom. Today, I wear corrective lenses, and I sometimes wonder how much Nancy Drew and the Hardy Boys are to blame.

Not much, as it turns out. Doctors have been trying for ages to debunk a myth that reading in dim lighting causes permanent damage to your eyes. But a debate lingers among some scientists who say that myopia (or nearsightedness) may be partially caused by environmental factors during childhood.

When the eye adjusts to lower levels of light, many reversible things happen. Muscles around the iris relax and allow the pupil to dilate so that more light will enter and hit the back of the eye. This rear area is where photoreceptor cells called rods and cones turn the light into meaningful information for the brain. As the light lowers, rods and cones enhance their ability to transform the light. These and other adaptive responses kick in once you’ve been sitting in the dark for a few hours, but return to normal in the light.

However, reading or focusing on close objects in the dark can put strain on the eyes. In dim lighting, the level of contrast decreases between black words written on a white page, and to read you may have to pull the book closer to your eyes. As you do this, the ciliary muscle around the lens of your eye contracts, reshaping it so that light flooding in is redirected to a focal point at the back of the eye. As all this eyeball adjusting occurs, many people report headaches and even nausea, the reason being they become tense from muscles that are working harder than normal to focus images; what’s causing the headache is not so much the darkness as staring at something very close to your face.

Most doctors say the discomfort is completely harmless, but a few say that young children may increase their chances of becoming nearsighted when they put too much strain on their eyes. They point to studies that have linked high incidences of myopia to cultures that encourage reading and formalized education among their youth.

One such study says that the strain of reading, and especially of reading in the dark, could give undeveloped eyes a signal to grow in the wrong way; how eye shape develops is important because nearsightedness occurs when the eye grows overly elongated. Almost all infants are born farsighted because their eyes have not yet grown to the right shape–one in which light penetrates the eye and lands on the point of highest acuity in the back of the eye (the fovea). During the first decade of life, the eyes change slightly in shape and size, significantly impacting how the eye focuses.
What scientists do not yet know for sure is which factors direct the eye during these crucial growing years. Most data show that genetics play a huge part, with the likelihood of being myopic increasing tremendously if your parents are as well.

But some preliminary animal research suggests that there could be an environmental element at work as well. It’s possible that while the eye is growing it’s experimenting with how to focus on the world around it, trying on new shapes to find out the best way to turn light into image; think of it as ocular puberty. Doctors theorize that if you strain the eye during this time by forcing it to focus on near objects, you could upset this process of growth.

Even if these findings are true, it is only relevant to the developing eye. Adults reading in the dark should be more concerned about why they didn’t pay their electricity bill than about potentially harming their eyes. But, in the remote chance that you are a child prodigy reading these lines from an unlit crib…well, then you might consider turning on a light.