Harry Potter fans have finally "worked out" why Professor Lupin slept through the entire journey on the train to Hogwarts. It has been 10 years since the final film was released but die-hard Potter fans are still trying to put all the pieces together. .
One scene, that fans should remember well, is Harry's first encounter with a Dementor. In Harry Potter and the Prisoner of Azkaban, Harry, Ron and Hermione are making their way back to school on the Hogwarts express. Sat in the same carriage is Professor Lupin, though for the entire journey he is asleep with his coat covering him.
It isn't until Dementors attack the train that Lupin wakes up and uses the Patronus Charm to deter them. Dementors suck a person's soul out through their mouths, and they drain feelings of happiness and joy. As a relief for Harry, Lupin offers him a piece of chocolate as it has chemicals in that release endorphins.
But it isn't really explained why Lupin is asleep for so long and why he's covering himself with his jacket. In a Reddit thread however, one eagle-eyed fan has worked out a possible explanation. Later on in the book and film we learn that the Professor is actually a werewolf and transforms every month.
He is sent away and confined to the Whomping Willow until he transforms back. The date of the train ride to Hogwarts is September 1st 1993 and according to the Reddit user, there really was a full moon the previous night. This meant that Lupin would have transformed the night before and woul.