fast asleep vs soporific

fast asleep

adj

  • Sleeping, in a deep sleep. 

soporific

adj

  • Tending to induce sleep. 

  • Boring, dull. 

noun

  • Something boring or dull. 

  • Something inducing sleep, especially a drug. 

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