at night vs nocturnal

at night

adv

  • During the night. 

nocturnal

adj

  • Taking place at night, nightly. 

  • Primarily active during the night. 

noun

  • A device for telling the time at night, rather like a sundial but read according to the stars. 

  • A person or creature that is active at night. 

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