Story at a glance
- Doing a lot of thinking can lead to fatigue that is different from physical fatigue.
- Scientists are curious about why and how this might happen.
- A study suggests how thinking too much over a long period of time may lead to changes in the brain that make you feel tired.
After a long day of work, you may feel like you want a nap. Although your body might not be tired, your brain could use some rest. Neuroscientists are curious about why thinking a lot or doing difficult cognitive tasks can make you tired and how that all plays out in the brain.
In a study published in Current Biology, researchers report results of an experiment where they measured specific signaling molecules in the brain in two groups of people. One group was given harder tasks that would lead to more cognitive work (high demand cognitive control tasks), and another was given easier tasks (low demand cognitive control tasks). While the groups were working on their tasks, the scientists measured glutamate levels. Glutamate is an amino acid that acts as a neurotransmitter in the brain.
In total, there were 39 paid participants in the study who were split into the two groups. Each group had to complete tasks over a six-hour period, with multiple breaks. This experimental setup builds off a previous study by some of the same researchers where they examined how cognitive work may affect choice impulsivity.
In this study, one group had to look at letters or numbers that flashed onto a screen and determine whether they were red or green, uppercase or lowercase and other similar characteristics. The other group was given similar but easier tasks. The researchers monitored any metabolites, which are products of cell activity, in the brain using magnetic resonance spectroscopy.
The team found a correlation between glutamate levels and cognitive fatigue among study participants. They believe that fatigue comes from exerting additional cognitive effort, and underlying that is the metabolism in the cells in the brain that leads to a build up of glutamate.
This study doesn’t prove this hypothesis, but it does provide some suggestion that glutamate could be related to cognitive fatigue. The why and how it’s related would need further more in-depth studies. This is partly also because the sample size of the study is small and the correlation found in the data may not indicate causation. “We’re still far from the point where we can say that working hard mentally causes a toxic buildup of glutamate in the brain,” said Antonius Weihler, who is a computational psychiatrist at the GHU Paris Psychiatry and Neurosciences and first author on the study, to Science.
Glutamate may be one clue to figuring out how and why thinking harder makes us tired, and it’s not yet clear why it’s related to cognitive fatigue.
Sebastian Musslick, a neuroscientist at Brown University who was not involved with the study and spoke to Science, thinks that it’s not likely that glutamate or other metabolic waste products are key factors responsible for cognitive fatigue. Instead, he thinks that the uptick in glutamate could serve a purpose such as providing a status update regarding physical needs like for food, water and sleep, according to Science.
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