Sentence:  Most men who are of sufficient intelligence and moral character to make thoroughly satisfactory block signalmen will naturally seek a respite occasionally from such a confining routine, and if they cannot accomplish this without securing a substitute at their own expense they are under constant temptation to “change off”—the day man working for the <WSD>nightman</WSD> and the <WSD>nightman</WSD> for the day. 
Meaning of the word marked with <WSD> tags:  A male night shift worker.