By Marc Sorenson, EdD, Sunlight Institute..
Just when one thinks that there is nothing new that sunlight can do, new research belies that idea. It has now been shown that among children who growth-hormone deficient, and are being treated for that deficiency, growth is more rapid during summer months. In a one-year study using 118 children from 14 countries as subjects, growth was measured and compared to the amount of sunlight received by the children. Those who were exposed to more sunlight had faster growth. The investigators also implicated a role for circadian-clock pathways in influencing growth (see my previous blogs on the importance of sunlight in correctly setting the circadian clock).
Although this research was claimed to be the first to demonstrate an influence of sunlight on accelerated growth among children being treated with growth hormone, another investigation from 2013 came to the same conclusion.  Others have also observed that children seem to grow more rapidly in summer.   
We want our children to have reasonable rates of growth, and the vitamin D produced by sunlight may produce larger and stronger bones. Or, it may be another factor such as nitric oxide, serotonin, endorphins or other less studied photoproducts. Whatever the mechanism, we now know that sunlight has one more critically important effect on human health, this time for our children.
 De Leonibus C, Chatelain P, Knight C, Clayton P, Stevens A. Effect of summer daylight exposure and genetic background on growth in growth hormone-deficient children. Pharmacogenomics J. 2015 Oct 27. [Epub ahead of print].
 Dorothy I Shulman, James Frane, and Barbara Lippe. Is there “seasonal” variation in height velocity in children treated with growth hormone? Data from the National Cooperative Growth Study. Int J Pediatr Endocrinol. 2013; 2013(1): 2.
 Marshall WA. Evaluation of growth rate in height over periods of less than one year. Arch Dis Child. 1971;46:414–420.
 Lee PA. Independence of seasonal variation of growth from temperature change. Growth. 1980;44:54–57.