Two men visited Mrs. Althouse Thursday evening, and it is said one of them, while in the sick room, was seen to handle a cup of coffee and milk from which the patient drank occasionally.... The men were unknown to the family, but Mrs. Althouse seemed to know who they were, although she declined to talk about them. Vague talk about money and Mrs. Althouse's husband adds to the mystery. The men are said to be from Syracuse...."An article published in the NYT on June 24, 1888. If it was mysterious at the time, it's even more mysterious now. How does one become "well-known" for being a "mysterious... sleeper"? Presumably — given that the men were said to be from Syracuse — Attica is Attica, New York... which you probably associate with the prison, the one with the riot and the Al Pacino... but the prison was not built until 1930.
ADDED: Several commenters are piecing together the story. Apparently, the woman's first name was not Helen but Emma. Edutcher found this old newspaper article, which includes some efforts at understanding the woman's condition (falling asleep for 25 days or more). One doctor said: "It is simply a condition of hysteria. The subjects are almost invariably women and of a particular temperament. There must be this temperament. I don to wish to be understood as ascribing it to outside hypnotic influences." (The doctor was interested in hypnosis as a treatment for hysteria.)
Quaestor suggests Kleine-Levin syndrome — AKA "Sleeping Beauty Syndrome" — which has a Wikipedia page here. There doesn't seem to be much understanding of the cause or the treatment even today. But it's not true that the subjects are mostly women. They are 3 males to 1 female.
It's such an odd thing that we fall asleep and then wake up. We assume that will just happen. We've all had trouble sleeping, I assume, and when that happens we may observe the mystery of sleep. We seem to know how to do it but not how to do it. We don't so much worry that we won't know how to wake up. Outside of the unique problem of dying in one's sleep, we take it for granted that we will wake up. We have the knack. Who knows why?