brain fag vs overtiredness

brain fag

noun
  • A condition to which male high school and college students are prone consisting of difficulty in concentrating, mild forgetfulness, and difficulty in maintaining a train of thought, arising from "too much thinking". Physical symptoms can manifest as well. 

  • cerebropathy 

overtiredness

noun
  • The state of being overtired; exhaustion. 

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