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Seeing the Light: Why Sun Exposure May Be Good for Your Eyes

Posted By Mark Sisson On June 28, 2011 @ 7:00 am In Sun Exposure | 136 Comments

Having immersed myself in all things Primal for so long, I find myself viewing nearly everything through the prism of human evolution. Is this food, activity, environmental stimulus, or social more an evolutionary novelty? If so, might it possibly conflict with or impede our pursuit of good health? Is it benign? An improvement, even?

Grok logic will only get you so far [7]. It’ll give you a nudge in the right direction – that is, headed straight to honest inquiry and further research – but it’s not enough. You shouldn’t rest on your laurels if Grok logic suggests what you’re doing is right, and you shouldn’t make big changes just because Grok logic suggests you’re doing something wrong. Instead, use those insights to generate hypotheses, then try to explore them. Research, read, ask more questions. At least, that’s what I try to do. It’s awfully tempting to just go with conjecture (especially if it turns out to be right on a fairly regular basis!).

That little preamble was just my way of setting up yet another question with roots in evolutionary conjecture: does the avoidance of sunlight via indoor living, sunglasses, and general heliophobia have an impact on eyesight, and more specifically nearsightnedness? Going purely by Grok logic and what we know about sunlight’s interaction with other aspects of our health, I think it’s a reasonable question. To whit:

Sunlight and skin – Sunlight exposure is required for vitamin D synthesis [8]. When UVB hits our exposed skin, vitamin D is synthesized and distributed throughout our body. Vitamin D is an essential pro-hormone, necessary for musculoskeletal health, immune system robustness, as well as protection from heart disease and cancer.

Sunlight and circadian rhythm – We need exposure to light at certain times of the day in order to regulate our circadian rhythms [9]. Without daytime/morning light, or with too much evening light, our internal clocks – and general health – go awry.

Given those two extremely basic, widely-accepted interactions between sunlight and our bodies, coupled with the fact that the eye’s express function is to interact directly with light, I think Grok logic regarding the sun and our eye health might be onto something. But we can’t be sure, remember, without confirming through other sources.

So let’s look into those other sources.

I’m sure you’ve heard of myopia. You may have it yourself or know someone who does. In case you don’t, myopia is nearsightedness, which is characterized by blurry vision when looking at distant objects. If it weren’t so easily countered with prescription eyeglasses, myopia would probably be classified as a public health epidemic. It’s that common, and it’s getting worse.

In fact, the latest statistics [10] indicate that 41.6% of Americans aged 12-54 suffer from myopia, way up from 25% in the early 1970s. That’s an awfully big percentage of the tribe that can’t throw a spear, shoot an arrow, spot prey, or see the enemy coming from afar. That’s a ton of squinters who require assistance. In other words, if myopia were just an unfortunate part of growing old (to the ripe old age of 12!), we probably wouldn’t have made it this long.

No, there’s probably an environmental component to the rise of myopia. Genetics could play a part in determining susceptibility to myopia, and probably do [11], but an environmental factor is likely to be a trigger for the “myopia gene’s” expression. Could sunlight be just such an environmental factor?

Kathryn Rose, a visual disorder researcher, thinks so [12]. First, she points to the weak or inconsistent epidemiology that attempts to link time spent on the computer, watching television, reading, and studying to the development of myopia, instead suggesting that the real problem is lack of sunlight. In cases where digital media usage or inside work appears to be associated with myopia, Rose thinks it’s actually a measure of displaced outdoor time.

Then she points toward the epidemiology exploring the link between time spent outdoors and myopia prevention, which is much stronger. Let’s take a look at a few studies:

In Chinese school children [13], myopia progression was inversely correlated with outdoor activity.

Near work (studying, reading) did not correlate with myopia progression, but American kids who played fewer sports outdoors had more myopia [14].

In Taiwanese rural children [15], outdoor activities might be “an important protecting factor for myopia.”

In teens from Singapore [16], outdoor activity appeared to protect against myopia progression.

Parental myopia status interacts with risk, too, though. In one study [17], kids with two myopic parents were at the greatest risk of developing myopia themselves, more so if they did not engage in outdoor sports. Kids with no myopic parents and who played a lot of sports outside had the lowest risk. Genetic predisposition expressed by an environmental trigger, anyone?

Of course, any good Primal thinker knows that epidemiology [18], like Grok logic, only goes so far. It’s certainly interesting, and it can inspire new avenues of inquiry, but science cannot live on epidemiology alone. You need something else to look into, like perhaps a physiological mechanism. Rose’s proposed mechanism was retinal dopamine [19], a “known stimulator of eye growth whose release is stimulated by light.” A lack of retinal dopamine – from avoiding the outdoors – means excessive eye growth. This is bad, for the eye is a delicate, extremely complex structure with many components, and a lot can go wrong if those components grow faster and bigger than they’re supposed to grow. Like the progression of myopia, which is characterized by excessive eye growth.

But wait – isn’t excessive amounts of light one of the big issues with modern living? Even if we stay indoors most of the day working, browsing, or watching TV, we’re still parked in front of a screen beaming light into our eyes and we’re still immersed in artificial overhead lighting. If all that light is enough to disrupt our circadian rhythms and ruin our sleep patterns, why isn’t it enough to stimulate retinal dopamine release?

It’s the magnitude. Try looking up at the sun in the afternoon. I mean really give it a good, long look. You can’t do it (in fact, that is definitely bad for your eyesight!) for more than a second or two, tops. If you squint, you might make three. Now try the same with an illuminated lightbulb. It’s easy and nearly painless. It doesn’t compare. To quantify the massive gulf between sunlight and artificial light, let’s look at another study [20]. Researchers trying to study the link between light exposure and myopia exposed chicks to various amounts of light. Normal laboratory lighting was 500 lux, “intense” laboratory lighting was 15,000 lux, and sunlight was 30,000 lux. Only intense lab light and sunlight were able to retard the development of myopia, while normal lab lighting – which is still quite bright and very similar to standard office lighting conditions – did not adequately protect. Oh, and good news for you sunglass wearers: the chicks who were continuously exposed to bright lighting while wearing “translucent diffusers” also showed resistance to eye lengthening and myopia.

To get an idea of how many lux you can expect to “get” in various situations, check out the Wikipedia article on the subject [21]. Prepare to marvel at the insane brightness of the outdoors and the comparatively piddling illumination found indoors. Note that direct sunlight is ridiculously bright (up to 130,000 lux), while just being outside in “full daylight” will provide plenty of light for your retinal dopamine labs. No need to stare at the sun or avoid dark forests. Just be outdoors and the sun will take care of the rest. If you can see stuff, that means light is getting to your eyes, it’s from the sun (and thus bright enough) and you’re good to go.

Of course, us oldsters might be too far gone for sunlight to have an effect on nearsightedness. Myopia develops early (hence the inclusion of 12 year-olds in myopia statistics), so it’s absolutely crucial that kids get plenty of time outdoors. I’d say “as much as possible,” but if you want a specific number, Kathryn Rose suggests between 10-14 hours a week as a bare minimum. Barring that, I suppose you could blast your toddler in the face with a halogen bulb every couple hours. No, but really: let those kids get outside, get dirty [22], play with bugs [23], climb stuff, and get some sun [24]. Although the chick study showed that sunglasses may not be problematic, I don’t think kids need ‘em, and they might still interfere with normal eye development. They’d just fall off, anyway, unless you hooked the kid up with some Horace Grant-style goggles.

Given all that, I think it’s safe to say that sunlight exposure probably plays a role in the development of nearsightedness. It’s not the only player – physiology is rarely that simple – but it appears to be a major factor. Anyway, I think we’ll have a better idea in the coming years. My Pubmed [25] trawling pulled up a ton of very recent studies on the subject, all in the last few years or so, so we can probably expect more definitive [26] answers in the near future.

Are you nearsighted? Did you play a lot outdoors as a kid? What’s your family history of myopia – do your parents have it too? Let me know in the comment section!


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[7] Grok logic will only get you so far: http://www.marksdailyapple.com/when-science-trumps-grok/

[8] vitamin D synthesis: http://www.marksdailyapple.com/deconstructing-vitamin-d/

[9] circadian rhythms: http://www.marksdailyapple.com/how-light-affects-our-sleep/

[10] statistics: http://www.sciencedaily.com/releases/2009/12/091214163030.htm

[11] probably do: http://www.ncbi.nlm.nih.gov/pubmed/19587595

[12] thinks so: http://www.cnn.com/2011/HEALTH/06/01/myopia.causes/

[13] Chinese school children: http://www.ncbi.nlm.nih.gov/pubmed/21251384

[14] had more myopia: http://www.ncbi.nlm.nih.gov/pubmed/20926821

[15] Taiwanese rural children: http://www.ncbi.nlm.nih.gov/pubmed/20868261

[16] teens from Singapore: http://www.ncbi.nlm.nih.gov/pubmed/19211608

[17] one study: http://www.ncbi.nlm.nih.gov/pubmed/17652719

[18] epidemiology: http://en.wikipedia.org/wiki/Epidemiology

[19] retinal dopamine: http://archopht.ama-assn.org/cgi/content/full/126/4/527?maxtoshow=&hits=10&RESULTFORMAT=&fulltext=%22Myopia,+Lifestyle,+and+Schooling+in+Students+of+Chinese+Ethnicity+in+Singapore+and+Sydney%22&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT

[20] another study: http://www.iovs.org/content/50/11/5348.abstract?ijkey=17668e059cc997c70c579fc6387735cceae6842f&keytype2=tf_ipsecsha

[21] Wikipedia article on the subject: http://en.wikipedia.org/wiki/Lux

[22] get dirty: http://www.marksdailyapple.com/going-grubby-the-primal-benefits-of-dirt-dust-dishevelment/

[23] bugs: http://www.marksdailyapple.com/edible-bugs/

[24] get some sun: http://www.marksdailyapple.com/vitamin-d-sun-exposure-supplementation-and-doses/

[25] Pubmed: http://www.ncbi.nlm.nih.gov/pubmed/

[26] definitive: http://www.marksdailyapple.com/category/definitive-guide/?submit=view

[27] Learn More: http://www.primalblueprint.com/get-certified?utm_source=mda_footer&utm_medium=link&utm_campaign=mda_footer_cert_text

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