Objectives: This study examined the regularity of the month and time of death in ICU patients. Methods: In this retrospective study, we surveyed time-related phenomena regarding the month and time of death in patients with fatal outcome during the ICU stay. The survey items included the year, date, and time of death, as well as the direct cause of death. We analyzed diurnal changes regarding the time of death using the least squares method employing a triangular function, and evaluated the 24-hour rhythm. Results: It was demonstrated that patient deaths peaked in the summer and winter months. The peak hour was around 15:00. Conclusion: Consideration should be given to the rhythmic regularity shown in the month and time of death in patients to improve the ICU shift work and management system.