Why is Thinking Hard So Exhausting?

Prolonged intense thinking floods your brain with a toxic substance that acts like a circuit breaker for your mind

Robert Roy Britt
Aha! Science
Published in
3 min readAug 11, 2022

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You might want to think long and hard about thinking long and hard about things. It can leave you exhausted mentally and physically. But why, exactly? One idea — a rather flimsy one, it seems —is that the brain fabricates a feeling of fatigue to encourage us to shift from thinking hard to doing something more gratifying.

But research suggests a more concrete mechanism is at work, and that such fatigue is your brain’s way of crying out for a serious break.

Several hours of intense thinking generates a buildup of a toxic chemical messenger in the prefrontal cortex, the brain area responsible for, among other vital functions, making decisions and solving problems, scientists explained in the journal Current Biology. The buildup can alter how effectively we think, and it apparently pushes us to stop thinking so much, to protect the mind from overwork.

Think of it as a circuit breaker that prevents an electronic device from exploding when it overheats.

“Our findings show that cognitive work results in a true functional alteration — accumulation of noxious substances,” says study team member Mathias Pessiglione, PhD, a biologist and psychologist at Pitié-Salpêtrière University in Paris. “So fatigue would indeed be a signal that makes us stop working but for a…

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Robert Roy Britt
Aha! Science

Editor of Aha! and Wise & Well on Medium + the Writer's Guide at writersguide.substack.com. Author of Make Sleep Your Superpower: amazon.com/dp/B0BJBYFQCB