interregnum vs rest period

interregnum

noun
  • The period of time between the end of a sovereign's reign and the accession of another sovereign. 

  • A period of time during which normal executive leadership is suspended or interrupted. 

  • An intermission in any order of succession; any breach of continuity in action or influence. 

rest period

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