Overheating is known to have a detrimental effect on physical performance, but the extent to which heat affects an athlete is dependent on a number of factors including fitness level, ambient temperature, hydration status and type of exercise . This means essentially that no two cases are the same and as a result blanket advice is difficult to prescribe.
We know with certainty that acclimatising to higher temperatures allows for a significant improvement in physical performance when exercising in the heat. Research suggests that a period of 1-2 weeks is sufficient  although longer is more desirable if possible. What is also apparent is that training in the heat has a positive impact on performance when you return to a lower temperature environment – an effect which has echoes of the altitude training model, where training at altitude makes for more efficient oxygen usage at sea level .
But to what extent does heat impact performance? What does the data show?
In a study involving elite footballers , two games were played in different temperatures, 21 degrees (ambient) and 43 degrees (hot). The study showed that in the hot conditions total distance covered was 7% lower and the high intensity sprints were 26% shorter, suggesting time to exhaustion was reduced, especially at higher intensities.
In a larger scale study involving statistical analysis of Stockholm marathon runners over a 28 year period (1980-2008), there was a clear link between the heat and finishing times . In warmer conditions the finishing times were slower, the average completion times were slower and the number of non-finishers were higher. The data analysed thousands of runners and the results were statistically significant, proving that endurance performance is impacted by the heat.