Growing up, slightly misguided common wisdom has an astounding influence on how we view and manage eye health.
“If you cross your eyes, they’ll stay that way.”
“If you sit to close to the TV, you’ll ruin your eyes.”
“If you eat carrots, you’ll see better.”
But among them all, there’s an old wive’s tale that seems to persist into adulthood with the most tenacity. It is that reading in dim light—or reading too much—can damage your vision.
I’d like to say that all myths are rooted in some shallow soil of fact, whether true or (much more commonly) false. I can’t say that though, because I can’t piece together where the one about crossing your eyes came from. Did adults hate when kids did that so much that they made it up whole cloth? If so, it’s fun to think they made up something horrible and barely related to add weight to it. “I hate when this kid makes that face so much I’m gonna make him paranoid about a permanent disability.”
Your eyes have muscles, and they get tired too
I do believe the one about reading in dim light has good intentions. When we read, our eyes are doing three things that involve muscular effort:
- They focus the lens by contracting and making it a little more spherical.
- They constrict the pupil to make it a little bit smaller.
- And they go a little bit cross-eyed to keep an object that’s in the center between both eyes from going double.
When we read in dim lighting, or if reading for a very long time in good lighting, there’s a decent amount of cumulative muscle strain over that amount of time. It requires more concentrated effort and slower reading in dim light, and less effort in bright light. This adds up over a long enough time.
The best comparison for this might be to imagine holding a half gallon jug of milk. If you’re holding it a little in front of you for 10 seconds while you clear a space on the top shelf of the fridge, you won’t even notice. If someone said, “hold that jug of milk in front of you for two hours,” you’d say, “No! Get out of my house.” But for this example, if you did try to do it for two hours, that milk is going to get pretty heavy because your arm muscles will get tired. A little bit of exertion over a very long time can cause as much fatigue as a ton of exertion over a brief period.
Despite short-term effects, eye strain is not permanent
When you read in dim light, there’s a little more exertion there. So if you normally read for an hour without difficulty, you may find that an hour with poor lighting leaves your eyes feeling exhausted. On the other side of the coin, if you read in good lighting for six hours when you normally read only for an hour, your eyes will feel exhausted again. Eye strain like this even has a name. It’s called asthenopia. And when something feels bad, it’s not a big leap to assume it must be bad for you—or in this case, bad for your eyes.
There’s another contributor to the uneasiness of reading in dim light or reading for a long time. It is that hard-worked muscles often keep working hard even when you’ve stopped trying to use them. Back to the example of holding milk for two hours because of a bully. When you do finally put it down you’ll notice something about your arm: It’ll feel kind of tight. It’s almost like it wants to stay in the same spot in front of you. Your hand muscles will also feel a little tight and clenched even though you’ve set the milk down. That mild amount of muscle spasm or tonic contraction is a normal phenomenon and it happens in our eyes too.
Your eyes are awesome at bouncing back
When you’ve been focusing hard or for a long time up close, you may notice when you look across the room it will be out of focus for a bit. Muscles relax over brief timeframes, eye muscles especially. So it may be only seconds till your focus comes back. Stiffened lenses of eyes in the late 30s through the 40s can sometimes take a minute or two to come back into focus. Again, this seems like it must be bad for your eyes.
Luckily, your eyes were made to be used and they have pretty impressive resilience. Since you use them a lot, you’ll notice these little quirks. A cyclist will notice tight leg muscles, or someone who knits may notice tight hand muscles. With eyes, though, we’re all in the same boat. We will all experience the symptoms that come from using our eye muscles to the point of fatigue, whether it’s from dim light or from long hours. But, in this case, they always bounce back and it won’t lead to early vision loss or damage of any kind. Unlike the once-upon-a-time warning of the disciplinarian whose curmudgeonly legacy lives on to this day, your eyes will not get stuck that way.