Outdoor time may protect kids from nearsightedness

Original research journal summary at Optometry and Vision Science Volume 86(1), January 2009, pp 2-3
Reuters' summary (Note this summary probably stretched by adding "exposure to sunlight may play a role")

Highlights
* Having myopic parents is a very minor risk factor
* So are near [distance] work, schooling, watching TV, working on computer screens
* Ethnicity plays no role
* Reasons for protection of outdoor time may be "lesser accommodative demands in outdoor environments (despite the considerable evidence that accommodation is not important), pupil constriction in the brighter light typical of outdoor environments resulting in greater depth of focus, or a direct effect of light exposure, perhaps mediated by release of a retinal transmitter such as dopamine, which is known to inhibit eye growth in certain circumstances."
* Outdoor time matters, regardless activity or inactivity; even reading outdoors is good
* "around 2 to 3 hours a day outside of school hours seems to be sufficient to markedly lower the risk of myopia"

My comments:

The sunlight comment is probably added by the Reuters summary. If sunlight exposure is not the key, then the researchers' speculation all come down to one factor: brighter white light. If that's the case, we may just need to make sure that the brightness of indoor environment matches that of outdoor and that indoor light covers the entire visible light spectrum. On the other hand, if sunlight is essential, then the UV component of the sunlight may be a critical factor and therefore Vitamin D may play an important role in protecting eyesight. Unfortunately we don't know if the summary author or the original researchers suggested the role of natural sunlight.

Having read that, I recollect my teenage years when my eye sight degraded at a typical alarming rate from about 10 years to 15 years of age, while I was strict in following all eye usage hygiene rules (no reading in dim areas or in supine position, etc.), perhaps except one, not stopping to rest when the eyes are tired in reading. At the same time, other children, including one habitually reading inside the quilt with a flashlight, were fine with their eyesight. We all spent a great deal of time outdoors, probably three hours a day. And in the southern city we lived (with about the same latitude as Houston, Texas) there's no shortage of sunlight, although it's almost always cloudy in winter.

(If a summary in Chinese is needed, search for "孩子在室外度过的时间长就不容易近视" on baidu.com, or click here. But note this summary probably stretched by adding "晒太阳促使体内分泌更多多巴胺")

(Update 2024-09) Is it possible that the AC (alternating current) of the indoor light plays a role? In the US and Canada, the AC has a frequency of 60 Hz, and in most other areas of the world, it is 50 Hz. Regardless the actual frequency, as long as the electricity current fluctuates, it may cause frustration to eyes. This hypothesis can explain why reading inside a quilt as I mentioned above did not lead to near-sightedness for that boy. I personally feel that outdoor light gives me less eye strain, and interestingly, even the outdoor light that comes indoors through glass windows does so, too. To test the hypothesis that AC light causes more eye strain, all we need is a light source powered by DC (direct current). Cell phone light is fine but may be too narrow or too focused.