lethargy vs torpor

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. 

torpor

noun
  • A state of apathy or lethargy. 

  • A state similar to hibernation characterised by energy-conserving, very deep sleep. 

  • A state of being inactive or stuporous. 

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