Preliminary research has found evidence that exposure to sunlight reduces self-regulated exercise intensity outdoors. The study was published in Physiology & Behavior.

“Everybody knows that there are ‘good days’ and ‘bad days’ when exercise performance is above or below what is expected. This is mainly associated with self-selected exercise intensity which is largely influenced by environmental conditions,” explained Hidenori Otani of Himeji Dokkyo University.

“However, no research has investigated the effect of variations in solar radiation on self-regulated exercise intensity. We therefore examined this effect in the heat outdoors when an individual’s perception of effort is somewhat hard.”

The study of 10 healthy, college men found that power output on an exercise bicycle decreased as solar radiation increased during a 45 minute exercise.

“The results of this research showed that self-selected exercise intensity was lowered with increasing solar radiation, which resulted from a higher skin temperature and thermal sensation and a greater body heat gain from the sun as sunlight increased, but core temperature was not different at any given solar radiation conditions,” Otani told PsyPost.

“These observations suggest that self-regulated exercise intensity during daily exercise in hot outdoor environments is lowered with increasing solar radiation. Also, this reduced self-regulated exercise intensity during exposure to high solar radiation in the heat outdoors might prevent excessive core temperature rise and lower the risk of exertional heat-related illness.”

“The results of this research also demonstrate that use of a sunshade to reduce solar radiation below 600 W/m2 might be of benefit to maintain endurance exercise performance in hot outdoor environment,” Otani added

The study — like all research — includes some limitations.

“Since this research simulated daily exercise intensity, further research needs to carry out the same experiments when an individual’s perception of effort is hard to very hard to simulate competitive exercise intensity,” Otani explained.

The study, “Exposure to high solar radiation reduces self-regulated exercise intensity in the heat outdoors“, was authored by Hidenori Otani, Mitsuharu Kaya, Akira Tamaki, Heita Goto, and Ronald J. Maughan.