Motor behaviour during the sleep period has several possible causes. In this review paper, we discuss the typical features of slow wave sleep parasomnias and nocturnal dissociation to illustrate the difficulties when evaluating complex sleep behaviours. These two categories of disorders illustrate issues around criminal responsibility and social control measures, in particular. One such concern is that slow wave sleep parasomnias may be considered ‘not legally insane’ by medical sleep experts which may be contrary to the relevant law in the jurisdiction. Nocturnal dissociative behaviour, by contrast, clearly involves mental health issues. There are implications for the criminal justice system when the disorders that cause complex nocturnal behaviour entail a continuing risk of harm to others. This means that the insanity verdict or equivalent may be appropriate to ensure supervision and restrictions to protect the public. The options for disposal are discussed, alongside the implications for expert witnesses.