catnap vs go to sleep

catnap

verb
  • To take a catnap (short sleep or nap). 

  • To kidnap a cat. 

noun
  • A brief, light sleep, usually during the daytime. 

go to sleep

verb
  • To fall asleep. 

  • To be affected by paresthesia; to go numb. 

  • To die. 

How often have the words catnap and go to sleep occurred in a corpus of books? (source: Google Ngram Viewer )