That neural buzz adds tension to the demands of getting something done. Selecting one sharp focus requires inhibiting a multitude of others. The mind has to fight off the pull of everything else, sorting out what’s important from what’s irrelevant. That takes cognitive effort. Tightly focused attention gets fatigued—much like an overworked muscle—when we push to the point of cognitive exhaustion. The signs of mental fatigue, such as a drop in effectiveness and a rise in distractedness and irritability, signify that the mental effort needed to sustain focus has depleted the glucose that feeds
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