If it does, it's all in your head.
By Daniel Engber Posted 03.01.2013 at 12:00 pm 7 Comments

Thought Experiment
Thought Experiment George Marks/Retrofile/Getty Images

The brain makes up about 1⁄50 of our body weight but consumes about one fifth of the oxygen we breathe. It’s natural to assume that overtaxing the cerebrum would leave one feeling lethargic, but that’s not quite true. The brain uses most of its energy just to maintain its baseline state; one tenth of our energy at rest goes to pumping sodium and potassium ions across brain-cell membranes, a simple process that keeps each neuron charged and ready. Specific mental activity, whether chatting with a friend or doing a crossword, does not suck up much extra energy.

That said, studies show that people do slow down after performing taxing mental tasks. One experiment, conducted by Samuele Marcora of the University of Kent, split subjects into two groups. Members of the first played a mentally challenging computer game. Those in the second group watched a documentary about trains or sports cars. Then everyone took an endurance test on an exercise bike. Marcora found that people who were “mentally exhausted” gave up pedaling more quickly than the documentary-watching controls. It was as if the heavy thinking had worn them out.

At the same time, Marcora found no correlation between the mental task and measures of their cardiovascular response, such as blood pressure, oxygen consumption, or cardiac output. In other words, the mental workout didn’t seem to slow their bodies so much as it appeared to skew their perception of how hard a given physical task might be.

Have a burning science question you'd like to see answered in our FYI section? Email it to fyi@popsci.com.

7 Comments

Link to this comment
ppardee
03/01/13 at 1:05 pm

You mind has a lot to do with your ability to perform physical activity. Over-exercising can cause 'nervous system fatigue' where your muscles are capable of performing the required task, but your brain forces you to rest by restricting the number of muscle fibers recruited during a task. You feel like you're walking through mud. The same thing happens when exercising in extreme heat (like 115+ F)

I'd guess that this effect is similar. That or peoples' patience is gone. We all have a point where we're just plain done.

Link to this comment
gizmowiz
03/01/13 at 2:05 pm

Now I know I'm so worn out! Never would have thought it!

Link to this comment
fellowwonderer
03/01/13 at 4:51 pm

I believe this is one and the same phenomenon called Decision Fatigue by some researchers.

Here is an article and vid that talk more about it.

http://www.nytimes.com/2011/08/21/magazine/do-you-suffer-from-decision-fatigue.html?pagewanted=all

http://www.ted.com/talks/barry_schwartz_on_the_paradox_of_choice.html

Link to this comment
pixelstuff
03/02/13 at 6:44 pm

People with low serotonin levels, or other imbalances associated with depression, often feel tired or drained. Perhaps a similar but temporary exhaustion of chemicals occurs after extreme mental activity.

Link to this comment
African Rover
03/03/13 at 6:20 am

Caption to the picture: 3 + 2 ?? now how much could that possibly be?

Link to this comment
Auroria
03/03/13 at 12:00 pm

Our tool of thinking and problem solving is part of surviving and avoiding death.

As of yet, death is still winning, so yes the biology of human does wear out.

Now we know we are mortal, so please take time to feel the sand between your toes, see the blue sky, hug someone you care about and just feel life, before the experience leaves you.

Life, live it! :)

Link to this comment
uldissprogis
03/09/13 at 4:06 am

I do a lot of thinking and it doesn't wear me out if I take about 5 minute breaks every half or full hour.


140 years of Popular Science at your fingertips.



Popular Science+ For iPad

Each issue has been completely reimagined for your iPad. See our amazing new vision for magazines that goes far beyond the printed page



Download Our App

Stay up to date on the latest news of the future of science and technology from your iPhone or Android phone with full articles, images and offline viewing



Follow Us On Twitter

Featuring every article from the magazine and website, plus links from around the Web. Also see our PopSci DIY feed


April 2013: How It Works

For our annual How It Works issue, we break down everything from the massive Falcon Heavy rocket to a tiny DNA sequencer that connects to a USB port. We also take a look at an ambitious plan for faster-than-light travel and dive into the billion-dollar science of dog food.

Plus the latest Legos, Cadillac's plug-in hybrid, a tractor built for the apocalypse, and more.


Online Content Director: Suzanne LaBarre | Email
Senior Editor: Paul Adams | Email
Associate Editor: Dan Nosowitz | Email
Assistant Editor: Colin Lecher | Email
Assistant Editor:Rose Pastore | Email

Contributing Writers:
Rebecca Boyle | Email
Kelsey D. Atherton | Email
Francie Diep | Email
Shaunacy Ferro | Email

circ-top-header.gif
circ-cover.gif
bmxmag-ps