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Why Thinking Hard Exhausts You, and What You Can Do About It

It's not an energy problem. It's a brain problem.

You spend the day in deep concentration. Maybe you’re working through a big problem, or you’re on deadline with a creative project.

By afternoon, you’re zonked. Drained. And you wonder: Should I keep pushing myself?

Now scientists have a good answer to that question — because they’ve found exactly what’s happening in your brain after a hard day of thinking.

The answer: It’s not pretty. And when our brains are worn out, we should take that seriously.

What’s going on in our brains

In the past, scientists believed that fatigue was all in your head: Your brain was faking you out, the theory went, because it wanted to do something more satisfying.

But in a new study in the journal Current Biology, a group of scientists in France have changed that story.

The scientists looked at two groups of people: One group worked on mentally challenging tasks all day, and the other did not. Both groups’ brains were monitored by magnetic resonance spectroscopy, so scientists could see how their brain chemistry changed.

There differences were notable: Junk literally built up inside the hard-thinking group’s brains.

The “junk” here was a common amino acid called glutamate, and it had collected in the synapses of their brains’ prefrontal cortexes. One of the scientists, Mathias Pessiglione of Pitié-Salpêtrière University in Paris, described this in Science Daily as the “accumulation of noxious substances.”

This accumulation had a clear impact on people in the hard-working group. They were fatigued, had reduced pupil dilation, and, by day’s end, really wanted to do easy tasks that had quick rewards.

That makes sense, Pessiglione says — because their brains were becoming harder to control. “Fatigue would indeed be a signal that makes us stop working ... to preserve the integrity of brain functioning," he said in Science Daily.

So, what can you do to save your brain?

Here’s a common scene at my home: After a hard day of work, my wife and I take care of our two little boys. Then when they’re finally asleep, my wife says she needs to “zone out” and watch some mindless television. Sometimes I join her, but sometimes I go back to work.

A few hours later, my wife is in bed telling me to wrap things up.

Who’s right? Probably my wife.

"I would employ good old recipes: rest and sleep!” Pessiglione said, when asked how to treat your brain after a hard day of thinking. “There is good evidence that glutamate is eliminated from synapses during sleep."

I keep working because I want to get things done, but in reality, I’m doing sub-par work on an addled brain. There is no “pushing through it,” because this isn’t simply a matter of summoning energy or focusing hard enough. It’s a brain chemistry thing. The only solution is a good rest; I’ll have more control of my brain, and therefore do much better work, in the morning.

It's funny — I even experienced this while writing this very newsletter! I first tried writing it on Saturday night, after the kids went to bed. I was tired but wanted to get it done — and yet I struggled to process the information. I wrote a few crummy paragraphs and then gave up.

Now it’s Sunday morning and I’m burning through this thing. It took me maybe 30 minutes to write.

The lesson here is simple, but still so difficult for hard-charging people to appreciate. We must listen to our brains! They are not computers, and they cannot do computations forever. Our minds need a rest.

If we want to do our best work, we cannot just work hard. We must work fresh.

Panic vs. Data

Why Thinking Hard Exhausts You, and What You Can Do About It

Illustration credit: Future

Why do we all panic over new things? And how can we overcome that? I write about this in my new book — and an excerpt of it just ran in Future, the awesome tech-focused publication from the VC firm Andreessen Horowitz. Check it out!

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Cover credit: Getty Images / PM Images