There's not much I can tell you about Morrel's short story. It's an engaging, psychological "horror" about one man's descent into madness as seen through the eyes of a doctor that specializes in sleep disorders. I put horror in quotation marks here, because it isn't horrific in the sense of axe murderers, homicidal ghosts, or supernatural monsters. It's more horrific in the sense of what happens to the man. The story is very Hitchcockian in style, actually.
Incidentally, this Hitchcock-esque style is also why I can't say much about it, as any real analysis of the story will completely ruin it for anyone that hasn't read it before. The story itself relies on the surprise of the ending to make it a good story, and revealing that ending would be doing too much of a disservice to readers. So all I can say is: find it for yourself, and give it a read. I'm sure you'll enjoy it.