call it a night vs fall asleep

call it a night

verb
  • To go to bed to sleep. 

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

fall asleep

verb
  • To pass from a state of wakefulness into sleep. 

  • To be affected by paresthesia; to go numb. 

  • To die (often seen on gravestones). 

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