dullness vs lethargy

dullness

noun
  • Lack of interest or excitement. 

  • The quality of not perceiving or kenning things distinctly. 

  • The quality of being uninteresting; boring; humorless or irksome. 

  • The quality of being slow of understanding things; stupidity. 

  • The lack of visual brilliance; want of sheen. 

  • bluntness. 

lethargy

noun
  • A state of extreme torpor or apathy, especially with lack of emotion, energy or enthusiasm; (loosely) sluggishness, laziness. 

  • A condition characterized by extreme fatigue or drowsiness, deep unresponsiveness, or prolonged sleep patterns. 

How often have the words dullness and lethargy occurred in a corpus of books? (source: Google Ngram Viewer )