Well, getting hotter than you can handle is (at least sometimes) called heatstroke. And some of the symptoms include "Altered mental state or behavior. Confusion, agitation, slurred speech, irritability, delirium, seizures and coma can all result from heatstroke." At least some of those sound roughly like "getting stupider," though it should be a surprise that a complex system doesn't respond in a simple manner to going outside of its "design limits."

http://www.mayoclinic.org/diseases-conditions/heat-stroke/ba...