A new study shows that when heavy cognitive (认知的) work is lasted for several hours, it causes potentially poisonous materials to build up in the part of the brain known as the prefrontal cortex (前额叶). This in turn changes your control over decisions, so you shift toward low-cost actions requiring no effort or waiting as cognitive tiredness sets in.

Influential theories suggested that tiredness is a sort of illusion (错觉) cooked up by the brain to make us stop whatever we are doing and turn to a more pleasing activity but new findings show that cognitive work results in a true functional change — accumulation of poisonous materials — so tiredness would indeed be a signal that makes us stop working but for a different aim: to preserve the perfection of brain functioning.

To look for evidence of this, researchers monitored brain chemistry over the course of a workday. They looked at two groups of people: those who needed to think hard and those who had relatively easier cognitive tasks.

They saw signs of tiredness only in the group doing hard work. Those in that group also showed in their choices a shift toward options with rewards at short delay with little effort. Critically, they also had higher levels of poisonous materials in the brain. Together with earlier evidence, the researchers say it supports the concept that poisonous materials accumulation makes further activation of the prefrontal cortex more costly, such that cognitive control is more difficult after a mentally tough workday.

So, is there some way around this limitation of our brain's ability to think hard? The answer is negative. But there is good evidence that poisonous materials disappear in the brain during rest and sleep. The study result may have other practical uses. For example, the researchers say, monitoring of prefrontal metabolites (前额代谢物) could help to detect severe mental tiredness. Such an ability may help adjust work plans to avoid burnout. They also advise people to avoid making important decisions when they're tired.