.., all through working hours, and still more in the time spent between work and home, the urban worker is exposed to noise, most of which, it is true, he learns not to hear consciously, but which none the less wears him out, all the more owing to the subconscious effort involved in not hearing it. Another thing which causes fatigue without our being aware of it is the constant presence of strangers.
Source: The Conquest of Happiness, 1930, chap.5: Fatigue
More info.:http://russell-j.com/beginner/HA15-010.HTM