While hard physical labour can wear you out, researchers have found out why sitting around thinking hard for hours also makes one feel worn out.
The findings, reported in Current Biology, showed that when intense cognitive work is prolonged for several hours, it causes potentially toxic by-products 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, explained the researchers from Pitie-Salpetriere University in Paris, France.
"Influential theories suggest 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," said Mathias Pessiglione from the university.
"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," he added.
The team wanted to understand what mental fatigue really is. They suspected the reason had to do with the need to recycle potentially toxic substances that arise from neural activity.
To find out, they used magnetic resonance spectroscopy (MRS) to monitor 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 fatigue, including reduced pupil dilation, only in the group doing hard work. Those in that group also showed in their choices a shift toward options proposing rewards at short delay with little effort. Critically, they also had higher levels of glutamate in synapses of the brain's prefrontal cortex.
Together with earlier evidence, the team noted it supports the notion that glutamate 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?
"Not really, I'm afraid," Pessiglione said.
"I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep."
Further, monitoring of prefrontal metabolites could also help to detect severe mental fatigue. Such an ability may help adjust work agendas to avoid burnout. Pessiglione also advises people to avoid making important decisions when they're tired.