lethargy vs somnolence

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. 

somnolence

noun
  • A state of drowsiness or sleepiness. 

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