Almost no one realizes the dramatic improvement that sun exposure can make on athletic performance. Years ago I helped Dr. John Cannell obtain translations of many esoteric and decades-old studies that had been forgotten, probably due to the fact that sun lamps were used to create some of the improvements in athletics, and have fallen out of favor due to the sunscare movement. I co-authored a paper with Cannell, called Athletic Performance and Vitamin D. That paper is the source of much of the material covered here, and it demonstrates the remarkable, positive effect of sun or other ultraviolet (UV) exposure on human performance. I would also strongly suggest that the readers avail themselves of Dr. Cannell’s book on the subject, called The Athlete’s Edge, which discusses in far greater detail the materials introduced here.
One of the salient studies on UV exposure took place in 1957 and assessed the influence of sun exposure on strength and performance over a two-year period. During that time six subjects were able to increase athletic performance and muscle trainability through systematic UV exposure. But when vitamin D3 was used, it not only did not work, it inhibited the performance-enhancing effect of the UV. I sometimes fear the public is beginning to believe that if sun exposure is proven to enhance human health, one needs only to take a vitamin D pill. Don’t get pulled into that idea. Sun exposure will always be more important than any of the photoproducts whose production it stimulates.
Here are some of the other salient studies on sun exposure and performance. In 1938, Russian researchers demonstrated that a series of four UV treatments improved speed in the 100-meter dash compared to four non-irradiated students, when both groups were undergoing daily physical training. The times improved from 13.51 seconds to 13.28 seconds in the non-irradiated group and from 13.63 to 12.62 seconds in the irradiated group. In other words, the UV-treated group improved by three-fourths of a second more than the non-UV group. That may seem like a relatively small improvement, but three-fourths of a second better time in a 100-meter dash could be the difference between first and last place!
German research from 1944 showed that the exposure of 32 medical students to UV, twice weekly during for six weeks, associated with a 13% improvement in endurance, whereas performance of a control group was unchanged.
Other German research shows that the ability of a muscle to gain strength (trainability) is much better in summer than winter, and peaks in September. In fact, the trainability in September was 2½ times higher than the average monthly trainability for the entire year.
So you who love athletics, don’t avoid the sun. It may make you a winner. But don’t burn.
Reaction time has also been shown to improve significantly in the sunnier months. 
When we consider reaction time, muscle and bone strength, speed and endurance, we should realize that these measurements are not only important for athletes; they are important for all aspects of living for all people. Everyone wants to be stronger, quicker, and faster, as well as have more endurance in daily activities. So embrace the sun, but do it safely and don’t burn.
 Cannell JJ, Hollis BW, Sorenson MB, Taft TN, Anderson JJ. Athletic performance and vitamin D. Med Sci Sports Exerc. 2009 May;41(5):1102-10.
 E. Seidl and Th. Hettinger. The Effect of Vitamin D3 on the Strength and Performance of a Healthy Adult. International Journal Physiology, including Industrial Physiology, Vol. 16, Pages 365-372 (1957).
 Gorkin Z, Gorkin MJ, Teslenko NE. [The effect of ultraviolet irradiation upon training for 100m sprint.] Fiziol Zh USSR. 1938;25:695-701.
 Lehmann G, Mueller EA. [Ultraviolet irradiation and altitude fitness.] Luftfahrtmedizin. 1944;9:37-43. [Article in German].
 Hettinger T, Muller EA. Seasonal course of trainability of musculature. Int Z Angew Physiol. 1956;16(2):90-4.
 Sigmund, R. The effect of ultra-violet rays on the human reaction time. Strahlentherapie. 1956;101(4):623-9.
 Seidl E. [The effect of ultraviolet irradiation on reaction time.] Int Z Angew Physiol. 1958;17(4):333-40.