Similar to physical strength, there is variation in the attention reservoir between any two people and within one person over time. Some have more muscle; some have more endurance; some have less; some are weaker. Mental capacity is probably more complicated, and the rate of ego depletion or exhaustion may depend on the activity and effort. Some people may have more to begin with. A small minority of the population need substantially less sleep than the rest of us. Some people mentally lift more for longer than others.
Obviously, a person's strength peaks in youth and decline in the
middle age. I suspect the same curves can be observed in the brain.
Neuronal loss and brain mass shrinkage are natural aging process. Older
adults' brain compensate for the loss to an extent by increasing
processing efficiency. But the
decline is inevitable.
We already know that watching TV is less strenuous than reading, and reading is less strenuous than writing. Lying is more strenuous than telling the truth or omitting the truth, except for psychopaths. The energy expended by the brain in any task can be quantified by measuring the radioactively labeled glucose consumption and blood flow in the brain.
I suspect this is why, most of the time, we prefer reading/writing emails to writing a long, complicated article (like what I'm doing now). Organizing a collection of thoughts and ideas and sort out the logic and connections between them and put it all down into words is heavier lifting than responding to one question, one thought, on a single matter, one email at a time. I am feeling this difference right at the moment. I can spend hours reading and answering emails without fatigue, but writing an article or technical document exhausts me after a couple of hours.
So, is it any wonder that we would rather doodle on the Web and check and re-check emails every 15 minutes rather than do some real work for an hour? The same reason humans would rather watch TV than reading a book, even though neither has a very long history in evolution. The same reason most people love to hear stories but only a select few can tell stories, and making up stories is an even rarer talent.
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