asleep vs fast asleep

asleep

adj
  • In a state of sleep; also, broadly, resting. 

  • Inattentive. 

  • Having a numb or prickling sensation accompanied by a degree of unresponsiveness. 

  • Dead. 

fast asleep

adj
  • Sleeping, in a deep sleep. 

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