How Mental Fatigue Corrupts Decision Making

This piece, by Onno Berkan, was published on 01/21/25. The original text, by Wiehler et al., was published by Current Biology on 08/22/22.

Mental fatigue is very real and affects us all. This Paris Brain Institute study explores why our brain gets tired after extended periods of mental work and how it affects our decision-making.

The researchers conducted an experiment where participants performed cognitive tasks for over six hours. They divided people into two groups - one doing challenging mental tasks and another doing easier versions of the same tasks. Throughout the day, participants had to make various economic decisions, testing whether they would choose smaller immediate rewards or larger delayed rewards.

The study revealed several important findings:

1. Mental Fatigue and Decision-Making:

   1. People who performed challenging tasks showed a clear shift toward choosing easier, quicker options as the day progressed. (Think about the multitude of ways this affects you, from your food choices to your tendency to binge on Reels.)

   2. This shift wasn't just about being bored or time passing– it was specifically related to how demanding the mental work was

2. Brain Chemistry:

   1. The researchers discovered that demanding mental work led to higher levels of the neurotransmitter glutamate in the lateral prefrontal cortex, the brain region responsible for cognitive control

   2. This chemical buildup appears to make it more "costly" for the brain to maintain focused control over our decisions

3. Subjective vs. Objective Fatigue:

   1. Interestingly, both groups reported feeling equally tired, but only the group doing harder tasks showed actual changes in their decision-making

   2. This suggests that our subjective feeling of fatigue might not always match the actual fatigue in our brain's control systems. One potential theory here may be that we’ve programmed ourselves to feel tired after spending a certain amount of time working on a task.

4. Recovery and Rest:

   1. The study suggests that sleep might be crucial for clearing out accumulated glutamate, explaining why rest is essential for mental recovery

   2. This finding shows why cognitive fatigue can be a serious issue in conditions like burnout and depression. It also shows why naps and power naps in particular can be so powerful, clearing out glutamate and allowing you to make decisions more easily

5. Practical Implications:

   1. The research helps explain why we tend to make less thoughtful decisions after a long day of mental work. 

   2. It suggests that we should make our most important decisions in the morning or when our cognitive fatigue is lower.

When we engage in demanding mental tasks, our brain accumulates certain substances that make it more difficult to maintain cognitive control. This is different from simply running out of energy– it's more like the brain's way of preventing excessive accumulation of potentially harmful substances.

This study provides a scientific explanation for why mental work can be so exhausting, even though we're not physically moving much. It suggests that our brain, the selfish organ, has built-in mechanisms that push us toward easier options when we've been engaging in intense mental activity for too long, similar to how physical fatigue makes us stop exercising.

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