Pope Benedict XVI Revealed the Reason for his Resignation in a Letter Written Before his Death to Peter Seewald

Nine weeks before his death, the Pope Emeritus Benedict XVI wrote a last letter to his official biographer Peter Seewald. In it he named his insomnia as the main reason for his health exhaustion and thus for his resignation.

 The news magazine "Focus" reported this last letter; as Peter Seewald has just released a book called "Benedict's Legacy", which is about to be published by the Hamburg publishing house Hoffmann und Campe. In Benedict's letter, which "Focus" (with the exception of a small, blackened paragraph) published in its entirety as a photo document and which Seewald confirmed as authentic, the Pope Emeritus wrote a few weeks before his death that since World Youth Day in Cologne (2005) "the insomnia" "continuously accompanied" him. 

Initially, my doctor had no qualms about using strong remedies that initially guaranteed full availability the next day," said the emeritus. However, these soon “reached their limits”. In 2012, on his trip to Latin America, his handkerchief was "totally soaked with blood" in the morning, which a surgeon got under control in such a way that the injuries from an alleged fall in the bathroom" were not visible. 
Accordingly, a new personal physician then pleaded for the reduction of sleeping pills and for the reduction of the public program for trips abroad. “It was clear that under these circumstances I would no longer be able to cope” with the World Youth Day in Rio de Janeiro in July 2013, and that a new pope would have to take on the task. That meant the need for my resignation before Easter 2013.” 
Benedict XVI During his lifetime he did not want to make a fuss about the circumstances of his resignation. Unfortunately, there is still speculation about possible other reasons for his resignation, especially after his death. 
After the death of Benedict XVI Seewald felt obliged to publish the crucial detail entrusted to me from the medical history of the German Pope. By this publication he hopes that the conspiracy theories and speculations will finally end. Benedict's resignation was for health reasons, just as he expressed it in his resignation statement. Incidentally, he had already unequivocally announced in 2010 in our interview book “Light of the World” that he would make use of the option to resign as soon as his strength no longer permitted him to exercise the office of Peter. 
Sources; Focus.de and Kath.net