Harry Potter fans are just finding out incredible reason Professor Lupin sleeps for entire Hogwarts train journey

Harry Potter fans have had their minds blown after discovering the reason why Professor Remus Lupin is sleeping on the Hogwarts Express during The Prisoner of Azkaban.

It's been well over two decades since the Harry Potter franchise changed the lives of children across the world and spawned a generation of people who are still convinced their Hogwarts acceptance letters were simply lost in the post.

The first books were released during the latter part of the 90's and The Philosopher's Stone film premiered all the way back in 2001, however, fans are still learning new bits of trivia about the franchise all these years later.

The latest little Easter egg to do the rounds on the internet is from the third instalment Harry Potter and The Prisoner of Azkaban, which premiered in cinemas back in 2004.

In the early part of the film, Harry, Ron and Hermione share a cabin on the Hogwarts Express with a man who later turns out to be their new Defence Against the Dark Arts teacher, Professor Remus Lupin.

The teenagers are baffled to see an adult on the train, as the train is only ever seen transporting school students to and from Hogwarts, however, Lupin's presence quickly comes in handy when he wakes up and defends the trio against a dementor attack.

And the reason behind JK Rowling's decision to have Lupin boarding the train has long been a discussion point amongst fans.

Lupin's presence on the train saves Harry from the dementor attack. (Warner Bros)

The in-universe explanation is because he's unable to afford any other form of travel due to prejudice over his condition as a werewolf, however, the reason to have him on the train does feel a lot like a plot device, considering that wizards can essentially teleport.

Debates around his presence on the train aside, one eagle-eyed fan took to Reddit to explain that the reason they believe Lupin spends most of the journey fast asleep is because he was exhausted from transforming into a werewolf the previous day.

"In Harry Potter and the Prisoner of Azkaban, Professor Lupin sleeps through most of the train ride to Hogwarts. The date is the 1st of September, 1993. There really was a full moon the previous night; he was exhausted because he'd transformed the night before," the original poster wrote.

The observation had been missed by the majority of fans, with one person replying: "I'm genuinely impressed by this movie detail. Good stuff!"

"I remember seeing that scene and was wondering why! Omg mystery solved," a second person added.

However, it seems that more people were surprised to learn that Harry Potter was set in the early 1990's, with several responses highlighting their surprise that Harry and co were actually born in 1980.

"I didn’t realise the Harry Potter series took place in the early 90s. I just assumed it was early 2000s," one user admitted, while a second added: "Woah I didn’t know they were set in the nineties."