The contribution of cognition to the sleep-aggression relationship is explored via three connected studies, involving adult male forensic patients detained in a high secure hospital. Study 1 included 31 patients, interviewed to examine their experiences of specific sleep problems. In Study 2, 42 patients completed a series of measures examining sleep dysfunction, aggression, and cognition, while Study 3 was designed to impact on sleep via a cognitive approach. In the latter, 48 patients were randomly assigned as part of a feasibility trial to one of three conditions: mindfulness (cognitive approach), sleep education, and treatment as usual. Collectively, the studies demonstrated the multifaceted nature of cognition in the sleep-aggression relationship, with a need to account fully for cognitive factors. A preliminary conceptual model is outlined - the Cognitive Sleep Model for Aggression and Self Harm (CoSMASH), as a direction for future research to consider.