drop off vs sleep in

drop off

verb
  • To fall asleep. 

  • To fall. 

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

  • To lessen or reduce. 

sleep in

verb
  • To sleep late; to go on sleeping past one's customary or planned hour. 

  • Used other than figuratively or idiomatically: see sleep, in. 

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