beat a dead horse vs overwork

beat a dead horse

verb
  • To persist or continue far beyond any purpose, interest or reason. 

overwork

verb
  • To make (someone or something) work too hard. 

  • To decorate all over. 

  • To fill too full of work; to crowd with labour. 

  • To work too hard. 

noun
  • excessive work 

  • a superstructure 

How often have the words beat a dead horse and overwork occurred in a corpus of books? (source: Google Ngram Viewer )