One of the worst features of nervous fatigue is that it acts as a sort of screen between a man and the outside world. Impressions reach him, as it were, muffled and muted; he no longer notices people except to be irritated by small tricks or mannerisms; he derives no pleasure from his meals or from the sunshine, but tends to become tensely concentrated upon a few objects and indifferent to all the rest. This state of affairs makes it impossible to rest, so that fatigue continually increases until it reaches a point where medical treatment is required.
Source: The Conquest of Happiness, 1930, chap.5: Fatigue
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