drop off vs nod off

drop off

verb
  • To fall asleep. 

  • To fall. 

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

  • To lessen or reduce. 

nod off

verb
  • To fall asleep, especially unintentionally. 

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