Escape from the Sandman
Ernst Theodor Amadeus Hoffmann may be the only one who saw the Sandman as a sinister figure. He's a popular character in European folklore, who brings good dreams to heavy-eyed children by sprinkling sand into their eyes. The German witnessed too many conflicts in the continent. It made him weary. He became isolated from the rest of society. It took a toll on his health. Hoffmann died at 46, which was a relatively young age. But these won't point to the Sandman.
Hoffmann's father abandoned him (and the family) at a young age. It would have a grave effect on the writer's emotional life. "The Sandman", published in 1816, could be a Freudian Personality Type Test.
"No doubt you are all very worried, because I have not written for such a long, long time."
Some readers see an opening line of a Gothic tale. Not exactly. One must know the background of E.T.A. Hoffmann, in able to know what to expect from the short story. If you're ready, then close your eyes. Imagine a flock of sheep coming to you. Better yet, think of the Sandman.
Here's the case
Nathanael had a terrifying vision of the Sandman, as the old woman (who looked after his older sister) described a wicked man who searched for children who won't go to bed. He would throw handfuls of sand in their eyes, so they popped right out of their heads. And then he threw the eyes, covered in blood, into a sack. He carried it to the half moon, where his hungry children would feed on it.
The story had a profound effect on the young Nathanael, such that he became wary of any visitor coming to their home. He suspected Coppelius, who had private meetings with his father, to be the Sandman in flesh and blood. A brutal incident took place, where his old man's eyes were plucked out of his head. The incident would haunt Nathanael, threatening his engagement to Klara. He insisted that Coppelius, who vanished after his father's death, came back. And peace of mind would become ever more elusive.
How readers see it
Hoffmann described a fascinating case, where readers won't agree on a single thing. How they saw Nathanael's tale would depend on their perception of the events.
Some would see a conflict between Romanticism and Enlightenment. It won't be hard to figure it out after going through the conversations between Nathanael and Klara. Hoffmann intended it, as this issue could be seen in many literary works of the 19th century. Other readers chose to focus on Nathanael, wondering if the incidents were products of a deranged mind. No need to study psychology, as the answer isn't hard to figure out.
Nathanael had a vivid imagination. A tragic incident blurred the line between what was reel and real. Hoffmann concluded each chapter with an open ending. He didn't intend to tease his readers, as no one knew where the Sandman came from.

