Aim: There is a longstanding, popular belief that the lunar cycle has an impact on human behavior. The purpose of this study was to evaluate changes in behavior amongst psychiatric patients at a mental health facility over a 7-year period in comparison to the lunar cycle. Method: A retrospective analysis was conducted using behavioral occurrence reports from a hospital database for a 252-bed adult psychiatric hospital in Canada. There were 7,153 behavioral occurrences during the study period across 627,763 patient days. Results: Occurrences varied greatly by lunar day, with peaks above the average for some days and dips below for others, while maintaining the average for the rest of the lunar days; however, no specific pattern was observed and the variation was not statistically significant. The popular notion that a full moon arouses behavioral disturbances was not supported in this analysis of psychiatric inpatients. Clinical Implications: The belief that the full moon causes escalating behaviors should be critically evaluated by healthcare providers as it may contribute to further stigmatization of individuals with mental illness.