Background: The circadian rhythm regulates various physiological processes, including sleep-wake cycle, cell division and cancer development. Aims: This study aimed to investigate circadian rhythm patterns in cancer patients. Methods and results: In this cross-sectional study, 150 cancer patients admitted to the internal ward of Tehran’s Imam Khomeini Hospital Complex in the fall of 2022 were evaluated. The demographic characteristics of patients were collected using a checklist. Patients also completed a morningness-eveningness questionnaire (MEQ). Utilizing Analysis of Variance (ANOVA) and Fisher’s exact test, circadian rhythm types with continuous and categorical variables were compared. The mean age of the study’s participants was 49.83 ± 14.53 years. A total of 82.7% (n=124) had non-hematological cancers, and breast cancer was the most prevalent type of cancer among patients (23.3%). The MEQ of the patients studied ranged from 41 to 74, with a mean score of 56.6 ± 6.34. Conclusion: According to the findings of this study, the circadian rhythm is distributed normally among the participants.