TRENDING SCIENCE: Don’t think too hard, it can wear you out

Had a demanding day at the (virtual) office? Thinking hard and long can really make you tired, just like your body after hard physical labour. We usually call it mental exhaustion. Science still doesn’t know why our deep thoughts cause feelings of tiredness.

Findings published in the journal ‘Current Biology’ claim the culprit is a neurotransmitter, or chemical messenger, called glutamate. It sends messages between nerve cells in our brain and plays a major role in learning and memory.

Using magnetic resonance spectroscopy, researchers from Pitié-Salpêtrière University Hospital in Paris analysed the chemical composition of the brains of 40 people while they completed repetitive tasks on a computer for over 6 hours. One group was assigned easy tasks, while the other was given more challenging versions of the same activities. The group that carried out the more complex tasks showed signs of fatigue.The more demanding tasks resulted in the collection of glutamate in the brain, specifically in the prefrontal cortex area. To manage this build-up, the brain slows down its activity. This is why we feel tired. “Even when you resist scratching an itch, for example, your brain is exerting cognitive control,” first author Antonius Wiehler of the Pitié-Salpêtrière University’s Paris Brain Institute told ‘The Guardian’. Cognitive control is the process by which goals or plans influence behaviour. The prefrontal cortex is the region of cognitive control.

“We found that glutamate was accumulating in the region of the brain which controls the tasks we set participants,” the cognitive neuroscientist explained. “Our understanding is that the brain has some kind of clearance mechanism to counteract this, which may slow down activity.” Too much glutamate can do harm because it overexcites neural cells. “The accumulated glutamate needs to be cleared away, which we think is likely happening during sleep,” Dr Wiehler added.

The research team also found that dealing with this excess makes other prefrontal cortex activity, such as planning and decision-making, more difficult. As result, the participants preferred low-effort, high-reward actions when they became fatigued.“Influential theories suggested that fatigue is a sort of illusion cooked up by the brain to make us stop whatever we are doing and turn to a more gratifying activity,” co-author Mathias Pessiglione, also from the Paris Brain Institute, commented in a news release. “But our findings show that cognitive work results in a true functional alteration—accumulation of noxious substances—so fatigue would indeed be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning.”

Can we limit our brain’s ability to think hard? “Not really, I’m afraid,” stated Dr Pessiglione. “I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep.”

The science has spoken. A night of rest can help you to feel mentally refreshed the next day. That’s an effective way to avoid burnout at work or from home.