Most experts agree that reading in low light does not damage your eyes. It can cause eye strain, however, which has a number of unpleasant temporary symptoms and possibly some long-term effects.
When the room light is low, your eye adjusts in several ways. First, the rod and cone cells on the retina begin to produce more light-sensitive chemicals. These light-sensitive chemicals are the first step in detecting the light, converting it to an electrical signal and transmitting that electrical signal to the brain. Second, the iris muscles relax, which causes the opening of your eye, the pupil, to become very large. This allows your eye to collect as much light as possible. Finally, the nerve cells in the retina adapt so that they can work in low light. These three changes take about 20 minutes to 2 hours, but they increase your sensitivity to low light by about 10,000 times.
When you read, your eye must be able to focus an image of the words onto your retina. To do this, the iris, as well as the muscles that control the shape of your lens, must contract to keep the focused image on the retina. If you read in low light, your visual muscles get mixed signals: Relax to collect the most light, but at the same time, contract to maintain the focused image. When that object is poorly lit, focusing becomes even more difficult because the contrast between the words and the page is not as great, which decreases the eye's ability to distinguish visual detail. That ability is called visual acuity. Your eyes have to work harder to separate the words from the page, which strains your eye muscles. Consider this to be strenuous exercise for your eye muscles. So your eye muscles will ache, much as your arm muscles and leg muscles become sore after strenuous exercise.
When your eyes are working this hard for a long period of time, the strain may cause a number of physical effects. Symptoms of eye strain include sore eyeballs, headaches, back and neck aches, drooping eyelids and blurred vision. Because you often don't blink enough when focusing on a single object, you may also experience uncomfortable dryness in your eyes. None of this damages your eyes, and all of it eventually goes away after you stop straining them. Many eye doctors leave it at that, but some note that eye strain may contribute to nearsightedness. Most people who are nearsighted were born that way, but there is evidence that prolonged eye strain can make it worse.
If you are comfortable reading with a flashlight (or other low light) and don't experience any of the above symptoms of eye strain, it's probably fine for you to read this way. It's certainly easier on your eyes to read in good light, however. You can also avoid eye strain when you're reading by blinking frequently and taking a moment to focus on something out the window or across the room every 15 to 30 minutes.
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