Aim: The aim of this study was to investigate the frequency, quantity and quality of subjective sleep problems, and their correlation with accompanying factors in cancer patients undergoing systemic therapy. Materials and methods: The study included 100 patients – 50 females and 50 males – undergoing oncological treatment who filled out an anonymous questionnaire specifically designed for this research. Results: 88% of the respondents reported the duration of sleep in the range of 5 to 9 hours. 48% of the respondents declared a deterioration in the quality of sleep after the diagnosis of cancer. Following the onset of sleep problems, 66% of respondents did not pursue any treatment. Among the examined patients, symptoms of insomnia were reported by 18%, and symptoms of hypersomnia – by 6%. Mixed sleep disorders were reported by 11% of respondents. The patients undergoing surgical treatment reported a statistically significantly lower quality of sleep compared to the patients not undergoing surgery (median score in the quantitative scale 6 vs. 7 on a scale of 1–10, where 1 indicated the worst quality of sleep and 10 indicated the best quality of sleep; p = 0.04). A very small percentage of patients sought pharmacotherapy or specialist help for sleep disorders, which precludes drawing statistically significant conclusions about the impact of these interventions on sleep quality. Conclusions: Sleep problems are often reported by patients during oncological treatment (48%). The main sleep-related complaints were advanced sleep phase disorders. Due to the frequent occurrence of sleep disorders among patients, it is important to conduct a thorough medical interview to diagnose them and implement appropriate treatment.