increased heart efficiency
There is conclusive evidence that exposure to sunlight produces a metabolic effect in the body that is very similar to that produced by physical training, and one of these effects is increased heart efficiency. In one study, the output of blood from the heart was increased by an average of 39% in a group of patients. The effect continued for five or six days following a single UV exposure.

increased energy and endurance
It has been shown that UV light actually increases the oxygen in the blood and its oxygen carrying ability. It also increases the use of oxygen in the tissues, which in turn can produce more available energy.

increased muscular strength
In one study of college men at the University of Illinois, UV light treatments were given to half the members of a PE class. The experiment ran for ten weeks, and at the end of that time, the group that was receiving UV treatment had increased their performance on the fitness test by almost 20%, while the other group had improved only 1%. No one is sure as yet exactly what factor in sunlight produces the increased muscular strength.