lassitude vs lethargy

lassitude

noun
  • Lethargy or lack of energy; fatigue. 

  • Listlessness or languor. 

lethargy

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

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

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