day and night vs unrelentingly

day and night

adv
  • All the time; round the clock; unceasingly. 

noun
  • Synonym of chalk and cheese (“two things that are very different”) 

unrelentingly

adv
  • In an unrelenting manner; rigidly; cruelly. 

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