Most patients with breast cancer are assigned to radiotherapy, which may cause fears leading to sleep disorders. Very few data are available regarding the prevalence of sleep disorders and corresponding risk factors. Data of 175 patients with breast cancer presenting for adjuvant radiotherapy were retrospectively analyzed. Twenty-three patient and tumor characteristics were investigated for associations with pre-radiotherapy sleep disorders. Seventy-eight patients (44.6%) stated sleep disorders prior to radiotherapy. These were significantly associated with higher distress score (p<0.0001); greater number of emotional (p<0.0001), physical (p<0.0001) or practical problems (p<0.001); and request for psycho-oncological support (p<0.001). Trends were found for worse performance status (p=0.062) and higher comorbidity index (p=0.059). Sleep disorders prior to radiotherapy for breast cancer are common. This applies particularly to patients with risk factors including distress due to emotional, physical or practical problems. These patients should be offered psycho-oncological support as soon as possible.