doss down vs fall asleep

doss down

verb
  • To sleep on someone's sofa or floor because there is no bed spare. 

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 doss down and fall asleep occurred in a corpus of books? (source: Google Ngram Viewer )