ObjectiveThis case–control psychological autopsy study aimed to explore a relationship between sleep disturbances and suicide among Japanese, as well as determine the importance and usability of screening for sleep disturbances in suicide prevention. MethodsA semi-structured interview was conducted with the close family members of 49 adult suicide completers and 145 gender-, age-, and residential municipality-matched living controls. The survey included sections on demographics, sleep disturbances, and mental disorders. Conditional logistic regression analyses were performed to compare sleep disturbance prevalence between the two groups. ResultsA significantly higher prevalence of sleep disturbances was found among the suicide group (75.5%) compared to the controls (11.0%) (odds ratio [OR]=21.6, p<0.001). The association remained significant after adjusting for mental disorders (OR=12.7, p<0.001). The population attributable risk percent of suicide associated with sleep disturbances and mental disorders was estimated to be 56.4% and 35.3%, respectively. ConclusionsThe study confirmed that sleep disturbances are an important risk factor of suicide, independent of mental disorders. Sleep disturbances accounted for a greater proportion of suicide cases than did mental disorders in the Japanese population given the higher prevalence, and could thus be considered an important target in suicide prevention in Japan.