Until recently, scientists thought that, much like muscles, the brain becomes fatigued when it uses up energy.

The problem with that theory is that the energy needed to think is pretty minimal.

So if it's not a lack of energy that causes cognitive fatigue, what does?

In 2022, a team of scientists asked participants to perform six hours of various assignments that involved thinking.

One group was asked to do easy tasks and another, hard tasks.

Throughout the study, the scientists measured the biochemical changes in the participants' brains.

When the volunteers were performing hard tasks, the scientists found higher concentrations of glutamate in the lateral prefrontal cortex, the part of the brain associated with cognitive control.

Glutamate is an excitatory neurotransmitter, a chemical signal which triggers neurons to fire.

Scientists think that the build-up of glutamate might make it more difficult for the prefrontal cortex to perform activities, such as planning, decision-making, and learning.

When you rest or sleep, the glutamate levels fall.

Scientists speculate that when thinking makes you feel tired, it's your brain signaling that you need a break to preserve its function.

Future research into glutamate levels in the brain could help scientists devise a way to help avoid burnout.