Neuroscientists finally figured out why thinking can be so exhausting

The Pomodoro Technique, a time management strategy that centers on taking regular breaks from mind-intensive work, helps many people keep a steady pace throughout the workday without burning out.

Even so, a long day of thinking, even with little physical activity, can leave you feeling like you want to collapse onto the couch. Why do we get so tired from thinking? A group of neuroscientists might finally have the answer.

Neuroscientists in Paris devised an experiment in which two groups of people performed sets of tasks. One group completed more taxing work while the other could coast a little more. The researchers then used magnetic resonance spectroscopy (MRS) to examine the brain and also presented participants with instant or delayed gratification opportunities to track fatigue in behavior. Their findings help illuminate some of the neuroscience behind why one mentally hits the wall, justifying something like the Pomodoro Technique.

Published this week in the journal Cell Biology, the new study gets at the science behind why mental performance declines over time, and what we can do to care for our brains.

LONGEVITY HACKS is a regular series from Inverse on the science-backed strategies to live better, healthier, and longer without medicine. Get more in our Hacks index.

Science in action — While everyone is familiar with cognitive fatigue, the reason why spending brain energy can be so exhausting is still a mystery. Past theories have looked at resource depletion, suggesting that self-control and hard thinking may draw on energy sources such as blood glucose. These theories, as the researchers behind the current study point out, don’t cater specifically to issues of self-control. For example, if low blood sugar controlled mental prowess, wouldn’t other cognitive processes like vision also suffer? So they focused their attention on the brain.

Their hypothesis was that various metabolic processes in the brain enable hard thinking, which over time can be enervating. Specifically, the research team focused on the release of glutamate, a protein building block known as an amino acid. Glutamate also naturally occurs in foods — including tomatoes — and is best known as monosodium glutamate (MSG).

Glutamate is an excitatory neurotransmitter, which is how the brain communicates with nerve cells. Producing glutamate stimulates nerves, and in high quantities, it’s a potentially toxic byproduct. (But you don’t need to avoid heavy thinking for fear of death by glutamate.)

Researchers know that glutamate accumulates outside brain cells in stressful circumstances or during demanding tasks. High concentrations of the amino acid not only disrupt the balance of excitatory and inhibitory neurotransmitters, but also may hamper transmission of information.

For the experiment, the researchers employed a two-pronged experiment that used MRS to keep an eye on glutamate levels in participants while they completed either an easy or hard behavioral tests to clock correlations between brain power, fatigue, and glutamate levels. MRS can track brain chemical fluctuation without being invasive by detecting molecules' radio frequency electromagnetic signals. So, the researchers can observe how certain amino acids and metabolites surge or recede in a participant's brain in real-time.

One group of 24 people got the hard tests and the other group of 16 got the easy version. Both groups spent more than six hours performing tasks that required memory, focus, and attention to detail, such as identifying different letters by case, vowel or consonant, or color. These exercises were meant to tire participants out. Then, to see how fatigue grew over time, the participants periodically got to choose a reward. They would have to choose between a small-reward/low-cost option, such as 41.20 euros immediately, or a big-reward/high-cost option, such as 50 euros in one month. Sometimes one could earn a big reward with additional mental or physical exertion, or earn a small reward for doing nothing.

These economic choices helped reveal a participant’s fatigue. If you’re getting tired, doing extra work for another few euros isn’t worthwhile. Indeed, more participants in the difficult test group eventually opted for low-cost rewards more often.

As for glutamate levels, the group with harder work ended the experiment with higher levels than those in the easy-work group.

Why it’s a hack — Recognizing that issues such as burnout and even everyday tiredness are not moral failings can transform how we approach work.

“In our culture, we pride ourselves in the ability to overwork ourselves, and we think of that as a badge of honor,” neuroscientist Richard Harris tells Inverse. “So we're not really attuned to this phenomenon of cognitive fatigue.” Harris, a professor at the University of Michigan, was not involved in the research.

Perhaps we understand that it happens, but we believe it’s something that can be controlled and overcome. Isn’t a hack supposed to use biology to get around natural deficits? The hack behind cognitive fatigue is recognizing that it will happen inevitably — and what to do about it when it does.

Even the authors recognize that there’s no way to avoid cognitive fatigue. “I would employ good old recipes: rest and sleep!” co-author Mathias Pessiglione said in a press release. “There is good evidence that glutamate is eliminated from synapses during sleep.”

More specifically, sleep activates something called the glymphatic system, which clears waste from your mind. Harris says that research on the glymphatic system is still in its early stages, but it appears to be a promising agent for brain clarity.

How it affects longevity — One way to temper glutamate buildup and fatigue, Harris recommends, is meditation. Harris has practiced meditation for about 25 years. He and his colleague Kevin Boehnke teach a course for graduate students at the University of Michigan on meditation and mindfulness. In addition to attending class, students meditate five minutes a day throughout the semester.

“We find that that's enough to reduce anxiety and depression statistically and significantly,” he says, pointing to a 30 to 40 percent drop over a 14-week semester. “These are people that have never meditated before. If you get good instruction, you can get results quickly.”

He stipulates that the more one meditates, the better one gets, and the more effective meditation is. And, there still isn’t empirical evidence that meditation can actually alter metabolic processes in the brain.

There’s a panoply of meditation and relaxation apps out there, but Harris says you don’t need them necessarily. Simply doing something else or walking outside can do the trick.

Hack score — 🧠🧠🧠🧠🧠🧠🧠🧠 (8/10 adequately rested brains)