Why Does Thinking Hard Make Me So Tired?

People were put in either the radish group or the sweets group, and told to eat only the food that had been assigned to them, for the purposes of a “taste perception study.” Then an experimenter left the room, and watched the participants behind a one-way mirror. 

The idea behind this cruel test was to see what happened to the people in the radish group, who would have to exert mental effort in order to resist eating the cookies. (“None of the participants in the radish condition violated the rule against eating chocolates,” the researchers wrote later. “Several of them did exhibit clear interest in the chocolates, to the point of looking longingly at the chocolate display and in a few cases even picking up the cookies to sniff at them.”)

After the tasting, the radish and cookie groups, along with a control group who didn’t eat anything, were given a difficult puzzle: they were asked to trace a geometric figure without lifting their pen from paper, or crossing over any lines twice. Unbeknownst to them, the puzzles were unsolvable, and the researchers measured how long it would take the groups to give up. The radish group gave up more quickly than the chocolate group—potentially, the scientists argued, because they had already depleted mental energy in forgoing the temptation to eat cookies. 

This was seen as a demonstration of the “ego depletion hypothesis,” and it captures a lived experience we’re all familiar with. When we mentally exert ourselves, even if it doesn’t involve physical activity, we feel exhausted in a way that sometimes can affect our later choices and behaviors. Doing mentally taxing work for hours on end can make us perform worse on later tasks, or reach for quicker and easier rewards.

That cognitive effort produces cognitive fatigue seems straightforward, but it’s still somewhat of a mystery why this is the case. Why do mental tasks drain us so much? There are other mental processes like vision, which we use the entire day, that don’t fatigue the brain in the same way. From what singular mental “resource” are we pulling from that drives both avoiding eating dessert and doing a difficult brain puzzle? 

“Wanting chocolate but eating radishes instead, especially under circumstances in which it would seemingly be easy and safe to snitch some chocolates, seems to have consumed some resource and therefore left people less able to persist at the puzzles,” they wrote.

There have been a century’s worth of explanations suggested throughout behavioral economics and social psychology around what exactly drives cognitive fatigue, and what mechanisms are at play when we run out of mental steam. In a new paper published on Thursday in Current Biology, researchers have proposed another: that cognitive fatigue is a result not of a depletion of something, but a buildup of a neurochemical called glutamate, which might be detrimental at concentrations above a certain threshold. They proposed that cognitive exertion can lead to too much glutamate in parts of the brain involved in cognitive control, and give way to brain down-regulations and making less effortful choices and decisions later on. 

The study itself was correlational, it didn’t definitively show that an excess of glutamate causes mental fatigue. But it is the latest attempt to explain the mechanisms of a phenomenon that has important implications in our daily lives: how and why we become mentally exhausted, and how much that might be affecting our decisions, even on an unconscious level. 

“We have this situation we all share, that this is demanding and strenuous,” said Antonius Wiehler, a psychologist and researcher in computational psychiatry at Le Groupe Hospitalier Universitaire Paris, and first author of the paper. “And at the same time, we understand so little about why this is the case, why some activities feel so different from others.” 

Ego depletion is a dated way of referring to this conundrum, and in the many years since it has been studied, many of the early experiments have failed to replicate—especially in studies where huge effects on choices or problem solving were shown after brief mental exertion tasks. Alexander Soutschek, a researcher at Ludwig-Maximilians-University of Munich who studies decision making and self-control, said that the original account of ego depletion claimed that doing a hard task for only five minutes could meaningfully impair performance on later tasks—which is likely too extreme.

But Soutschek said that most agree that there is some finite limit to performing well on a task that requires a lot of effort, and eventually performance will decline after a period of time. Today it’s sometimes called the resource depletion model, though there are several differing theories—from functional to biological—about why, exactly, cognitive effort can run out. 

This is reflected in other conceptions of fatigue too, like in “spoon theory,” coined by Christine Miserandino. In disability communities, the theory describes the concept of limited energy, using each spoon as a symbol of energy—and once the spoons are given away or used up, there’s no more energy to give to any task, no matter if the task is urgent or easy. 

“The puzzling thing is why some mental activities are perceived as effortful at all and lead to fatigue, while others are virtually automatic and do not lead to fatigue (like vision),” Soutschek said, who wasn’t involved in the new study. The new paper is in line with others that are exploring whether the experience of cognitive fatigue is related to neurobiology—similar to how chemicals like lactic acid prevent or reduce performance and induce fatigue in muscles, he said. 

The researchers used imaging called magnetic resonance spectroscopy, which monitors brain metabolites throughout a simulated “work day” that participants did in the lab. In a previous study, the same group found that after a day of difficult cognitive tasks, and changed decision making, people had less activity in a part of the brain called the left lateral prefrontal cortex—so they focused attention there, and also to the visual cortex of the brain, as a control. 

The people in the study did either high-demand or low-demand cognitive control tasks, and then made economic decisions that resulted in them receiving real monetary rewards. People in the high-demand task group started to apply less self control to the decision making, and became skewed more towards immediate rewards. And it was only the people in the high-demand group who shifted their decision making in this way; they also exhibited a reduction in pupil dilation (a biomarker of cognitive effort).

The high-demand group also had higher glutamate concentration in the parts of their brains associated with cognitive control, compared to the low-demand group, and there was no difference in the amount of glutamate in the visual parts of the brain. 

Wiehler said that rather than the idea of a resource depletion leading to different economic choices being made, the hypothesis stemming from their results is that something in the brain is accumulating—the glutamate. 

“The brain is monitoring this, and to avoid the accumulation of glutamate, the brain is reducing its activity, and participants are applying less control during their choices, and therefore their choices change,” Wiehler said. 

Glutamate is the main excitatory neurotransmitter in the brain, and is present nearly everywhere. Though they’re proposing that glutamate build up could play a role in cognitive fatigue, Wiehler said that it’s unlikely that fatigue is due only to glutamate. Their study was correlational, meaning they weren’t able to show that glutamate build up definitively causes fatigue. 

Martin Hagger, a professor of health psychology at University of California Merced, said he found the proposal somewhat convincing, with caveats. This study had small sample sizes, and reported correlations between changes in decision-making and glutamate build-up, but doesn’t have the ability to claim that decision-making was causally related to the glutamate accumulation.  

“These effects do dovetail well with theoretical explanations of cognitive fatigue, that is, a cost-effort model in which exertion enables adaptive responding, but comes at a cost,” he said. 

Some past theories have suggested that it might more be a matter of motivation or attention that leads to downstream effects in decision making from mental fatigue. But Soutschek said that those explanations alone don’t fully explain the effects of cognitive fatigue, and the search for underlying reasons why some mental processes are effortful, while others are not, is still worthwhile. 

“Scientists are still searching for the root cause of cognitive fatigue, hence the search for these endogenous chemicals that show up when people engage in these demanding tasks and experience fatigue,” Hagger said. “I think the key is to unify the research work in these areas to come up with converging evidence for explanations of how phenomena work.” 

Soutschek added, “I think it is too early to draw practical conclusions from these findings. As stated also by the authors, it would first be necessary to show that glutamate is causally involved in creating the feeling of fatigue.”

How this aligns with lived experience is also subtle. In the study, they asked people to report how tired they felt, and found that everyone reported an increase in fatigue. But only the participants who did the difficult version of the task showed a shift in their decision-making behavior towards the more immediate rewards, or the glutamate accumulation.  

“It’s not so clear that we are aware of these changes in choice behavior,” Wiehler said. “It’s not like participants told us in the end, ‘I’m so tired, and therefore I’ve been choosing the immediate rewards.’ It’s more subtle than this.”

Even if we don’t fully understand the mechanism for why mental work makes us tired, Wiehler said their study suggests we should still probably be wary about making important choices when we are tired. This has, perhaps, even larger ramifications for daily life: when we make decisions, and are tired, we may not fully realize that our choices are altered in small ways. 

It could also influence how we think about rest, and how much we really need in order to be mentally rejuvenated. In the experiment, they gave people two 10 minute breaks, which didn’t make much of an impact on the change in choices later on; a 10 minute break was too short to counter the effects of cognitive fatigue. Another research question they hope to explore, along with follow-up work on the glutamate hypothesis, Wiehler said, is how to restore the brain after it is tired. 

“We don't know how long breaks need to be, but they need to be longer than 10 minutes probably,” he said. 

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