A study with cyclists showed that brain activity was lower at 104°F vs 100.4 (38°C) and motivation decreased as the body temperature rose. Often I think while running in the heat that I just need to suck it up and get moving. I now understand there is science behind my decreased performance.
Gabrielle Andersen, staggering and swaying through the Olympic stadium in Los Angeles, typifying the human response observed in the research subjects at the point of exhaustion during exercise in the heat - paralysis on one side of the body, confusion, loss of co-ordination and balance. -The Science of Sport