Somnambulism or sleepwalking (SW) is a sleep-related behavioural disorder characterized by complex movements or behaviours occurring whilst asleep. It typically occurs during non-rapid eye movement (NREM) sleep and is thought to be an issue with the regulation of arousal states during slow-wave sleep. Up to twenty-nive drugs have been associated with triggering or exacerbating SW in predisposed individuals1. SW is also a rare and often overlooked antipsychotic side effect considered to occur in ~1% of those treated with quetiapine2, olanzapine3 and asenapine4. However, there are no cases reported with other atypical antipsychotics such as clozapine, despite being a drug with no alternative for those treated with it, and with SW posing a significant risk of injury and impact on treatment adherence. Here, we examined the electronic records embedded in an ethically approved database (NRES 13/EE/0121) of a well-characterized cohort of clozapine-treated patients with a psychotic disorder in order to describe the prevalence and clinical characteristics. Among the 224 subjects in the database, we found two cases of SW onset or exacerbation, accounting for ~1% described below. Patients gave permission for describing their cases.