It’s no surprise that hard physical labor wears you out, but what about hard mental labor? Sitting around thinking hard for hours makes one feel worn out, too.
Now, researchers have new evidence to explain why this is …
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.
Fatigue would indeed be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning.
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 authors say 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.
Well that’s a relief, it’s not just me.