nightmare vs woolgathering

nightmare

noun
  • A very bad or frightening dream. 

  • Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure. 

  • A feeling of extreme anxiety or suffocation experienced during sleep; Sleep paralysis. 

verb
  • To have a nightmare. 

woolgathering

noun
  • Indulgence in idle fancies or daydreams. 

  • The gathering of fragments of wool torn from sheep by bushes, etc. 

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