call it a night vs drop off

call it a night

verb
  • To go to bed to sleep. 

  • To cease what one has been doing for the night. 

drop off

verb
  • To fall asleep. 

  • To fall. 

  • To deliver; to deposit or leave; to allow passengers to alight. 

  • To lessen or reduce. 

How often have the words call it a night and drop off occurred in a corpus of books? (source: Google Ngram Viewer )