Don’t leave important decisions to the end of the day when your brain is tired

Our single focus on performance, productivity, and results is harming us. Even though we may not feel fatigued, a day of hard cognitive work can hamper creativity and insight, and can even lead to making decisions that may be easier but that can thwart our goals. When you make a decision that is easy but not ideal, it will negatively affect your business and the organization you work for in ways you had not intended.

Here's a study from the Paris Brain Institute that looked at how 6 hrs of demanding cognitive tasks affected subsequent decision-making.

After 6 hours of difficult cognitive work, our decision-making suffers.

Methodology and Results

The participants performed either hard cognitive tasks (24 participants) or easy cognitive tasks (16 participants). These tasks alternated with economic choice tasks. The economic choice tasks offered either a small short-term reward for low effort or a large, longer-term reward requiring high effort. And they performed these tasks for 6.25 hours which simulated a work day. 

Although the participants in the easy condition performed better on the cognitive tasks (greater accuracy and faster reaction time), both groups reported similar levels of subjective fatigue. The participants in the hard condition did not feel more fatigued than the participants in the easy condition.

However, as the day wore on, the participants in the hard condition showed a preference for the smaller short-term reward that required less effort than the larger, longer-term reward requiring high effort. 

The researchers found that, "at the end of the day, high-demand cognitive work resulted in higher glutamate concentration and glutamate/glutamine diffusion in a cognitive control brain region (lateral prefrontal cortex [lPFC]), relative to low-demand cognitive work." "The group performing hard cognitive tasks ended the experiment with more glutamate in the lateral prefrontal cortex (lPFC) than the control group." The participants in the easy condition showed a decrease in glutamate throughout the day.


"Daylong cognitive work leads to glutamate accumulation in the lateral prefrontal cortex, which reduces control over decisions and thereby favors effortless behaviors with immediate gratifications."

Even though we may not feel fatigued after a hard day, we are more likely to choose the easier decision. And this can be counter to our goals.

Glutamate is the major excitatory neurotransmitter in the brain. "Since the majority of the brain relies on glutamate as the main neurotransmitter for communication between neural cells, it is essential that glutamate is reabsorbed and disposed of to prevent blockage."

Rest and sleep can clear the concentration of extracellular glutamate. Glutamate may be one of the potentially toxic substances that are cleared during sleep

Exercise "has the potential to increase the use of glutamate in the brain and may help reduce the toxicity caused by glutamate build-up."


Herbst, E.A.F. and Holloway, G.P. Exercise increases mitochondrial glutamate oxidation in the mouse cerebral cortex. Applied Physiology, Nutrition, and Metabolism, 41(7): 799-801. 

Wiehler, A., Branzoli, F., Adanyeguh, I., Mochel, F., & Pessiglione, M. (2022). A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions. Current Biology, 32(16), 3564-3575.e5.

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