So you have a tough job, huh? While it may seem like you’re making excuses for poor après-work performance, one study backs up your observation. Published in the Journal of applied physiology in 2009, the study is titled “Mental fatigue impairs physical performance in humans,” and it has received a bit of internet buzz lately, popping up on sports performance blog Conditioning Research, LifeHacker, and PopSci.
In the study, researchers found that people who were mentally exhausted from playing a challenging computer game for 90 minutes gave up cycling more quickly than people who were not mentally fatigued. More importantly, the mentally exhausted group did not differ physiologically from the non-fatigued group. Their heart rates, oxygen consumption, and cardiac output were all similar.
In other words, the mentally fatigued group gave up faster because they perceived they were working harder than they were. As the researchers put it, “fatigue limits exercise tolerance in humans through higher perception of effort,” rather than through any physiological changes.
As for whether your brain burns more calories when it’s thinking hard, researchers say it doesn’t—at least not enough that you’d notice. A study published in 2012 approximates the brain uses up 56 grams of glucose per day at rest. That’s 224 calories.
“At this point we do not have clear answer to how much extra energy will be needed in a ‘working’ brain (note: our brain works all the time even when it seems ‘resting’),” wrote lead researcher Dr. Xiao-Hong Zhu in an email.
For certain actions, however, specific parts of the brain will burn extra glucose. The visual cortex, for example, will take in about 50 percent more glucose during visual stimulation. But it will only burn about five percent of that for energy, according to Dr. Zhu. And that’s just one region of the brain. If your entire brain burned five percent more energy than usual in order to fuel itself during a strenuous mental task, it would only use 11 extra calories—hardly enough to sap your glycogen stores before a workout.
THE BOTTOM LINE: Yes, performing a strenuous mental task can negatively affect your workout because it can increase your perception of effort. Working your brain should not significantly affect your heart rate, oxygen consumption, or cardiac output. And it definitely shouldn’t deplete your body’s glycogen stores.