What is the last name of the person who suffers from a sleep disorder?  Answer the above question based on the context below:  College student Sarah Foster is found by the police, as she is sleepwalking in her nightgown on the road. Since the suicide of her husband Jonathon, who worked as a novelist, she is suffering from sleep disorder. A few days later, she talks to Dr Cooper, whose student she was, about the sleepwalking and a recurring nightmare, in which she is attacked by an unknown man. Cooper sends her to a therapy in a sleep laboratory. During a walk on a cemetery, Sarah talks about it with her room mate Dawn, who shows a personal interest in her professor Owen. Then an attractive man gets out of a black car and Sarah imagines him being a single. At the evening in the sleep laboratory, Dr. Koslov explains to her that her neuronal activity will be observed during the night. He also introduces her to Dr. Scott White, the director of the lab. It is the man whom Sarah has seen at the cemetery. He tells her, that a student was buried and he was there with a colleague. Sarah confides to him that she loved her husband, but not his work as a novelist. The next morning she wakes up in a different room after a silent, dreamless night. White takes her case. He reports about irregularities in the theta waves and asks her to spend some more nights in the lab. Sarah recognizes that something is wrong. In the lecture hall she questions the statement of her teacher, who thinks that love stories are just a dopamine kick or a bipolar disorder. But she is even more irritated when he addresses her as Miss Wells and a student repeats this name. Also Dawn, her driver's license, her diary and a dedication in her husband's book affirm this surname. Sarah is rejected by Cooper's assistant. In the sleep laboratory Dr Koslov shows her a protocol about her dream in which she is pursued. She denies having dreamed anything, but sees her signature on the form.
Answer:
Foster