low sleep quality and delirium in hospitalized patients is common and occurs at the same time but their relationship is unclear. The aim of this study is "to determine the relationship between sleep and the prevalence of delirium in patients undergoing cardiac surgery". This analytical-descriptive study was conducted on patients undergoing cardiac surgery in educational hospital affiliated to Medical Sciences University of Tehran in 2014. 44 patients were studied by available method. The incidence of delirium in patients was assessed twice daily on the second to fifth day after surgery through the CAM-ICU scale (Confusion Assessment Method -ICU).Also sleep of patients on the second day after surgery was assessed using Pittsburgh Sleep Quality scale. Data were analyzed using statistical tests of Chi-square, Mann-Whitney and Fisher's exact test under16 SPSS software. There is a significant relationship between demographic characteristics and the prevalence of delirium. (52.5%) of patients had good sleep quality. The results of Fisher's exact test showed there was a significant difference between the sleep disorder and the prevalence of delirium (P=0.007). Due to the significant relationship between sleep quality and prevalence of delirium, the need to use a program of prevention from sleep deprivation in the clinical nursing instructions in hospitals is essential particularly critical and surgery wards.