Maybe Sybil just needed a good night's sleep.
Multiple personality disorder is a rare and extreme form of what psychiatrists call "dissociative disorder," and it was popularized by the publication in the early 1970s of the novel Sybil. Psychiatrists have long thought that dissociative disorder might be a person's natural response to extreme trauma, such as child sexual abuse, during which a victim might psychologically protect him or herself by "going away." A patient experiencing dissociation might describe feeling outside or separate from himself or from reality. Taking on an alter personality -- or several -- was thought to be a variant of that psychological strategy of self-protection.