Why Thinking Makes You Tired

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Daily brief research updates from the cognitive sciences

We all know that feeling of having engaged in hard cognitive, thinking, tasks, and feeling tired. If it goes on long enough, we may feel mentally fatigued, drained, or even exhausted.

But why?

We tend to know that physical activity uses a lot of energy and so will tire you out, but mental activity less so. So why does this happen? And researchers, though they know the feelings themselves as much as the average person in the street, haven’t been able to give a clear explanation.

This latest piece of research by Wieler et al. has managed to find out the precise cause of this.

To do this the researchers measured participants over a day and used brain scanning to elucidate changes in brain functioning. This is a pretty cool research protocol because it represents real-life functioning of real-life people. There were two groups: those with cognitively demanding jobs and those with less cognitively demanding jobs. At the end of the day they were given various cognitive tasks in a brain scanner.

What did they find?

They found that a part of the brain called the prefrontal cortex exhibited signs of fatigue. The prefrontal cortex is considered our “executive centre” or thinking part of the brain that is involved in control, intentional thought, calculations, and decision-making. This was in comparison to other regions of the brain.

This fatigue led to altered processing and also altered decision making with a preference for easier decisions or pathways. So when tired, you go for the easy solution or draw on a habit.

We already knew that but more than that they were able to identify the mechanisms. The results of this showed that the fatigue led to a build up of waste products that can become potentially toxic in the region. The brain uses chemicals to generate the electrical impulses and if these can’t be cleared away they lead to build up between the neurons. This alters processing but also increases the cost of processing.

They specifically were able to measure increased glutamate in the prefrontal cortices of those in the cognitive effort group. Glutamate is common chemical in the brain used for processing signals.

So, this shows that this fatigue is not just “psychological” but based on biological mechanisms with the build up of substances which impede and alter processing. This feeling of fatigue is therefore your brain trying to rest and to stop this build up and allow cleaning out of toxic substances. Fatigue is after all a sign of needing a rest – but the downside is that this will also lead your brain to aovid thinking as much and take easy options – which could be worse decisions, or alternatively, succumbing to an unhealthy snack.

It will be hard to avoid this totally, but this shows why the good old advice of having regular breaks really is very, very important. And also why you should have breaks before you get tired.

It also shows that you shouldn’t make important decisions at the end of the day!

Andy Habermacher

Andy is author of leading brains Review, Neuroleadership, and multiple other books. He has been intensively involved in writing and research into neuroleadership and is considered one of Europe’s leading experts. He is also a well-known public speaker, speaking on the brain and human behaviour.

Andy is also a masters athlete (middle distance running) and competes regularly at international competitions (and holds a few national records in his age category).

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Reference

Antonius Wiehler, Francesca Branzoli, Isaac Adanyeguh, Fanny Mochel, Mathias Pessiglione. 
A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions. 
Current Biology, 2022
DOI: 10.1016/j.cub.2022.07.010

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