Why Lupin Was Sleeping in the Hogwarts express in Harry Potter

You know why In Harry Potter and the Prisoner of Azkaban. Professor Lupin is sleeping on the Hogwarts Express. This detail is intentional because the story takes place on September 1st, 1993, and there was a full moon the night before. As a werewolf, lupine would have transformed and been exhausted from the experience, hence his fatigue on the train.