Why outdoor activity reduces myopia?

Researchers found that the children participating in more outdoor activities are less likely to develop myopia (short- or near-sightedness). I've known this research for quite some time but never found a reasonable explanation. The following are the factors I can think of that may be relevant to the observed difference.

1. Indoor lighting is lacking in certain physical properties of light under which human eyes are least stressed. The property one may readily think of is light intensity, or more specifically, luminance. If this is a factor, adequate indoor lighting is essential in maintaining eye health.

Another property of outdoor light is its full spectrum. I personally feel less stressed on my eyes sitting under fluorescent than under incandescent lamps, an experience probably different from that of many people, although natural, outdoor light makes me feel the most comfortable (not directly in the sunlight though). Between fluorescent and incandescent, there's CFL (compact fluorescent lamp) that somewhat stresses my eyes. According to this Popular Mechanics article, the light spectra for various types of light sources are


(You may go to the linked page and click View Larger.) We see incandescent light is dramatically different from sunlight; it has way too much long wavelength (red) component, while sunlight has more or less even wavelength distribution, gradually growing stronger toward the short, light blue, side, and then weak on dark blue or purple. CFL, if the two spikes are excluded, fairly well matches the profile of sunlight. And according to an image on Wikipedia, regular fluorescent light is almost the same as CFL. (The Popular Mechanics image shows the two left side peaks at ca. 40nm shorter wavelengths than the Wikipedia image.)

So, if the light spectrum difference plays a role in reduction of myopia by outdoor activities, we may hypothesize that compact or regular fluorescent light is at least more healthful than old-style incandescent light, another reason for its elimination on top of energy inefficiency.

Incidentally, I feel more comfortable reading under a regular fluorescent light than a CFL. I think it may have to do with the even coverage of the light emitted along the whole length of the light tube. But it's just a conjecture.

2. Another factor that favors outdoor activities in terms of eye stress may be circulating air. If the components of air are ignored, the outdoor environment may be simulated by circulating indoor air with a fan. Otherwise, chemical and environmental analysis of air is needed. For instance, radon that almost always exists indoors will be at an undetectable level outdoors. It would be a pleasant surprise if fragrance of flowers and pollens of certain plants are also found to be conducive to eye health.

3. One last factor that may be relevant is the ease of seeing things far away when one stays outdoors. Since the eye is the most relaxed when seeing far, there're more opportunities for eye muscle relaxation when outdoors than indoors. To have this outdoor effect when indoors, one has to stay in a room with more, larger, windows, and frequently stop near work such as reading to look at far-away objects.

The discovery of the protective effect of being outdoors on eyes is a great, yet incomplete, step in fighting myopia. Science is about what, and then why, and how.

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[note] In discussion of light spectrum, we should not forget the UV light, which is part of sunlight, and is responsible for promoting synthesis of Vitamin D in our bodies and also raising the risk of skin cancer. According to this research, CFLs emit UV light, especially from the worn-out bulbs. The UV C, as opposed to the common A and B, travels a short distance before it's absorbed by air. These scientists study the negative aspect of UV, i.e. its effect of skin damage. But if outdoor light differs from indoor light in preventing myopia because of the trace amount of UV, we have yet one more reason to substitute CFLs, placed in an appropriate distance, for incandescent lamps.