Why Thinking Too Much Can Leave You Mentally Exhausted

08/31/2022

With information from New Scientist

The cause of mental fatigue

Thinking too much for several hours can make us mentally tired.

The question that scientists have always tried to answer is: How do we mentally tire if the brain is completely different from the muscles that tire after exercise?

Now we have the answer to why we are tired after a heavy mental load.

The answer is that prolonged mental focus generates a buildup of a chemical called glutamate in areas of the front of the brain.

This extra load on glutamate, the greater it is, makes mental work more difficult, which eventually leads to the generation of nervous exhaustion.

This explains why we eventually give up or avoid challenging tasks when we are mentally exhausted: that excess glutamate can be harmful. “The brain wants to avoid this, so it tries to reduce activity,” explains Professor Antonius Wehler, from the Brain Institute in Paris (France), who led the research.

Why do we get mentally tired?

The brain doesn’t seem to run out of energy after hard work, and even when we’re not intentionally thinking about anything specific, some areas of the brain, called the “default mode network,” are constantly active. But this does not generate mental fatigue.

To unravel this mystery, Professor Wehler and his team used a technique called magnetic resonance spectroscopy (MRS), which harmlessly measures levels of various chemicals in living tissue.

The team asked 40 volunteers to do memory tasks for six hours while lying on an ERM scanner—26 of the participants did a more difficult version of the task, while the other 14 received an easier task.

Meanwhile, the team monitored levels of eight different chemicals in the brain, including glutamate, the main chemical for signaling between neurons.

Among the participants who performed the most difficult task, the increase in the level of glutamate corresponded to the dilation of the pupil, another well-known measure of fatigue. The volunteers who performed the simplest task reported feeling tired but no increased glutamate or pupil dilation.

According to the researchers, doctors can use this information to assess people with conditions that can make it difficult to focus, such as so-called “brain fog” after COVID-19 and ADHD in children.