doze vs z's

doze

noun
  • A light, short sleep or nap. 

verb
  • To bulldoze. 

  • To sleep lightly or briefly; to nap, snooze. 

  • To make dull; to stupefy. 

z's

noun
  • Sleep. 

How often have the words doze and z's occurred in a corpus of books? (source: Google Ngram Viewer )