Sleep-Related Violence in REM and NREM Parasomnias: Neurobiology, Mechanisms, and Forensic Implications. A narrative Review

Sleep-related violence is an uncommon but clinically relevant phenomenon that may occur in the context of specific parasomnias. The disorders most consistently associated with potentially injurious behaviours are non-rapid eye movement (NREM) disorders of arousal and REM sleep behaviour disorder (RBD). In both conditions, complex motor acts may emerge during sleep and occasionally result in self-injury or harm to bed partners and cohabitants. This narrative review examines current evidence on the neurobiological mechanisms underlying violent behaviours in REM and NREM parasomnias, with particular attention to the concept of state dissociation and its forensic relevance. NREM parasomnias are generally understood as incomplete arousals from slow-wave sleep, during which motor and limbic systems may become activated while associative frontoparietal networks remain partially sleep-like. High-density electroencephalography and functional imaging studies suggest that local sleep instability, together with reduced executive control, may facilitate automatic and sometimes defensive behaviours. By contrast, RBD is characterized by the loss of normal REM-related muscle atonia, allowing dream enactment behaviours to occur. These episodes often reflect emotionally intense or threat-related dream mentation. Dysfunction of pontomedullary circuits regulating REM atonia, combined with altered modulation of limbic activity, is thought to permit disinhibited motor output during REM sleep. Across both disorders, converging evidence supports a model in which preserved subcortical emotional activation coexists with reduced prefrontal inhibitory control and impaired integration of behavioural state boundaries. This may create the neurophysiological conditions for complex but non-volitional behaviours. Although severe violence remains rare, the possibility of significant injury gives these disorders clear clinical and medico-legal relevance. Retrospective determination of whether a violent act occurred during a parasomnia episode remains inherently probabilistic and must rely on convergent clinical history, polysomnographic findings, and contextual evidence. Future progress will depend on the development of objective biomarkers of state instability and improved multimodal characterization of parasomnia-related neural dysfunction.