sleep in vs slumber

sleep in

verb
  • To sleep late; to go on sleeping past one's customary or planned hour. 

  • Used other than figuratively or idiomatically: see sleep, in. 

slumber

verb
  • To be in a very light state of sleep, almost awake. 

  • To be inactive or negligent. 

noun
  • A state of ignorance or inaction. 

  • The snooze button on an alarm clock. 

  • A very light state of sleep, almost awake. 

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