Mental Energy

Everyone knows, in a general way, that thinking is hard work. Our brains can only do so much of this work in a day before they get groggy and stupid. Freud understood this, and called the wearing down of the mind by overwork "ego depletion." In recent decades some psychologists have found ways to measure this effect, and they have shown that doing mental work tires us out and leads to sloppy thinking and poor decisions. The first really convincing experiments involved self control, and they showed that resisting temptation is a grueling mental task that eventually wears out our willpower:

there is a finite store of mental energy for exerting self-control. When people fended off the temptation to scarf down M&M’s or freshly baked chocolate-chip cookies, they were then less able to resist other temptations. When they forced themselves to remain stoic during a tearjerker movie, afterward they gave up more quickly on lab tasks requiring self-discipline, like working on a geometry puzzle or squeezing a hand-grip exerciser. Willpower turned out to be more than a folk concept or a metaphor. It really was a form of mental energy that could be exhausted. The experiments confirmed the 19th-century notion of willpower being like a muscle that was fatigued with use, a force that could be conserved by avoiding temptation.

Now there is good evidence of what is being called "decision fatigue":

No matter how rational and high-minded you try to be, you can’t make decision after decision without paying a biological price. It’s different from ordinary physical fatigue — you’re not consciously aware of being tired — but you’re low on mental energy. The more choices you make throughout the day, the harder each one becomes for your brain, and eventually it looks for shortcuts, usually in either of two very different ways. One shortcut is to become reckless: to act impulsively instead of expending the energy to first think through the consequences. (Sure, tweet that photo! What could go wrong?) The other shortcut is the ultimate energy saver: do nothing. Instead of agonizing over decisions, avoid any choice. Ducking a decision often creates bigger problems in the long run, but for the moment, it eases the mental strain.

So only make important decisions when your mind is fresh. And if there are temptations you need to resist -- donuts, dubious women -- try to stay far away from them instead of relying on your conscience to always be strong for you, because it gets tired, too.