According to some theories of emotion regulation, dreams could modify negative emotions and ultimately reduce their intensity. We introduce here the idea of cathartic dream, a specific and separate type of emotional dream, which is characterized by a dynamic plot with emotional twists, and where negative emotions are expressed and ultimately decreased. This process would reflect psychological relief (catharsis according to the Aristotelian definition) and fulfil an emotion regulation function. We developed and validated a tool using a large language model to emotionally categorize the different dreams from dream diaries. Based on this tool, we were able to detect the prevalence of cathartic dreams in datasets of both healthy participants and patients with nightmares. Additionally, we observed the increase of cathartic dreams during 2 weeks of imagery rehearsal therapy and targeted memory reactivation during rapid eye movement sleep. We also demonstrate how the increase of cathartic dreams correlates significantly with the decrease of depression scores in patients with nightmares under therapy, thus supporting their likely functional role in well-being and their distinct nature among other emotional dreams.