lethargy vs lifelessness

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. 

lifelessness

noun
  • The property of being lifeless, the lack of apparent animation. 

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