Ferdinand Hérold's 1827 ballet-pantomime La Somnambule, written for the Paris Opéra, is generally remembered today only as a source for Bellini's 1831 opera La sonnambula. However, Hérold's work – which also inspired a series of popular vaudevilles on the same theme – illustrates the strong, voyeuristic appeal of the trance phenomenon at the end of the Bourbon Restoration. It can be viewed as encapsulating wide-ranging contemporary ideas about the relationship between sleepwalking, mesmerism, madness and the supernatural. The aims of this article are twofold. First, it seeks to introduce important nuances into the received and often generalised claims usually made about the containing nature of trance scenes in nineteenth-century theatre, positing an alternative model to that of the unhinged heroine of Italian opera familiar from recent feminist writing on opera. Second, it illuminates the musical practices specific to late Restoration Paris that were so crucial to the aesthetic – and the success – of these sleepwalking heroines. A web of visual and musical allusions conjured up an entranced figure who, although related to the Italian operatic madwoman, has a personality and social implications all her own.