A new study investigates why hard thinking makes you feel tired

A large number of employees, managers, and leaders are engaged but exhausted, dedicated but depleted. When our brain is depleted, it is extremely difficult to become our best selves. But why is this the case?

The body uses 30% of the calories it consumes to maintain its current muscle mass, but the brain only accounts for 2% of total body weight. How much energy does the brain use each day? Every day, it consumes 20% of our available energy, significantly more than any other organ in our body.

So, if you are engaged but exhausted, dedicated but depleted, you will lose access to one very important thing: executive function. So, all of your brain's autonomic functions that control your immune system, digestive system, and fight mechanism continue to function, but the first thing you lose access to is executive function because the body is very judicious in its use of fuel.

Hard physical labour, of course, leaves you exhausted, but what about hard cerebral labour? It is exhausting to sit and contemplate for hours on end. Researchers now have new data to explain why this is the case, and their findings imply that it's not entirely in your brain when you experience mental exhaustion (as opposed to drowsiness) as a result of hard thinking.

According to studies published in Current Biology, the prefrontal cortex, a region of the brain, accumulates potentially harmful by-products after engaging in continuous, intense cognitive activity for several hours.

According to Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France, "influential theories indicated that exhaustion is a sort of illusion built up by the brain to make us stop whatever we are doing and turn to a more gratifying activity." The accumulation of noxious compounds brought on by cognitive activity, according to our research, causes a true functional alteration. As a result, fatigue would be a sign to cease working, but for a different reason: to protect the integrity of brain function.

Pessiglione and his associates, including the study's initial author, Antonius Wiehler, aimed to define mental tiredness precisely. The brain cannot continually calculate, whereas computers can. They were curious as to why. They hypothesised that the necessity to recycle potentially harmful by-products of brain activity was the cause.

They monitored the chemistry of the brain during a workday using magnetic resonance spectroscopy (MRS) to search for proof of this. They studied two groups of people: those who had challenging cognitive tasks to complete and those who had relatively simple cognitive tasks.

Only in the group engaged in heavy work did they notice indicators of fatigue, such as decreased pupillary dilation. Additionally, those in that group displayed a shift in their preferences toward options that promised benefits with minimal work and a short wait time. Importantly, they also had larger glutamate concentrations in the prefrontal cortex of the brain's synapses. The authors claim that data supports the theory that glutamate accumulation makes additional prefrontal cortex activation more demanding, making it harder to maintain cognitive control after a mentally taxing workday.

Is there a way to get beyond this restriction on how hard our brains can think?

I'm afraid not, Pessiglione stated. “I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep.”