Insomnia and cognitive impairment are both highly prevalent in breast cancer patients. This study, conducted among women treated for non-metastatic breast cancer, aimed at assessing the relationship between insomnia and cognitive functioning, measured objectively and subjectively, and evaluating the moderating role of age, educational level and intellectual potential. Sixty-three women completed a neuropsychological battery, self-report scales of cognitive functioning, 2 weeks of a daily sleep diary, and a demographic and medical questionnaire. Significant differences between women with insomnia and good sleepers were found on verbal episodic memory and executive functioning, assessed objectively, and some aspects of subjective cognitive functioning. Moreover, older age and a higher education were associated with more severe cognitive impairments related to insomnia.