Have you found yourself physically exhausted after a long day at work, even if you were just sitting behind a computer desk? Scientists have discovered a reason for this, as a toxic chemical begins to accumulate in the brain after being active for long periods. Consequently, the brain shifts towards actions that do not require as much effort to avoid further circulating this chemical, glutamate. This manifests as a lack of desire to work or cognitive fatigue, according to neuroscientists from Pitié-Salpêtrière University in Paris, France.
Dr. Matthias Pessiglione, who led the study, stated, "Influential theories suggest that fatigue is a sort of illusion created by the brain to make us stop what we're doing and shift to a more rewarding activity. However, our findings show that cognitive work leads to real functional changes — the accumulation of harmful substances — so fatigue will actually be a signal that makes us stop working but for a different purpose: to maintain the integrity of brain functions."
Physical fatigue is a direct result of hard manual labor, but intense thinking for long periods causes mental exhaustion instead, leading to symptoms such as lack of motivation and difficulty concentrating. In the study published in Current Biology, researchers set out to understand what mental fatigue actually is and why it manifests.
To do this, they used magnetic resonance spectroscopy (MRS) to monitor brain chemistry over the course of a workday for two groups of study participants. They divided the group into easy and hard computational memory tasks that involved remembering and matching a sequence of differently colored letters for nearly six and a half hours. The results showed that those with harder tasks tended to choose options that offered smaller rewards for less effort or those that yielded quicker results.
Afterward, the scientists examined their glutamate levels at synapses in the prefrontal cortex of the brain and found that they were higher for the group engaged in mentally taxing tasks. The researchers say this supports their hypothesis that the accumulation of glutamate causes the brain to shift towards less strenuous actions.
This is its way of avoiding exacerbating the accumulation of this toxic chemical, which can impair brain functions. The researchers hope that this finding will provide a new way to detect severe mental fatigue and inform work agendas to help prevent burnout. In future studies, they hope to increase understanding of why the prefrontal cortex is susceptible to glutamate accumulation and fatigue after neuronal activity. They are also curious to see if the same signs of fatigue in the brain might predict recovery from health conditions such as depression or cancer.
