The English language was influenced greatly by Shakespeare. He invented over 1700 of our common words by changing nouns into verbs, changing verbs into adjectives, connecting words never before used together, adding prefixes and suffixes, and creating words new words.
He invented lots of new idioms and this is an example of one. A night owl is someone who likes staying up late and is more productive late at night. The opposite of this type of person could be described as an early bird.
Are you an early bird or a night owl? There is a Part 1 topic on sleep so these words could be used in that topic among others.