Predictive Analysis

James links to a piece that suggests that thinking hard wears you out because it produces high levels of toxic substances.

Their studies, reported in Current Biology on August 11, show that when intense cognitive work is prolonged for several hours, it causes potentially toxic byproducts to build up in the part of the brain known as the prefrontal cortex. This in turn alters your control over decisions, so you shift toward low-cost actions requiring no effort or waiting as cognitive fatigue sets in, the researchers explain.  

“Influential theories suggested that fatigue is a sort of illusion cooked up by the brain to make us stop whatever we are doing and turn to a more gratifying activity,” says Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France. “But our findings show that cognitive work results in a true functional alteration—accumulation of noxious substances—so fatigue would indeed be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning.”

I'll wager that further study eventually uncovers that low levels of alcohol consumption tend to dissolve and clear these toxic products, allowing the brain to continue hard work for longer. This explains why creative geniuses are often inclined to drinking at low levels but for long periods of the day; many of them, like Winston Churchill, prove to be quite heavy drinkers eventually. 

This feature of human nature is well enough known to have drawn satire.

Science will catch up.