Cluster headache is a type of primary headache that occurs in cyclical patterns or clusters. It is characterized by severe, intense pain, usually around one eye or one side of the head. Attacks can last from 15 minutes to three hours and may occur multiple times a day during a cluster period, which can last weeks to months. The exact cause of cluster headaches is not fully understood, but they are believed to involve the hypothalamus, a part of the brain that regulates biological rhythms. Symptoms may include redness in the eye, nasal congestion, and restlessness during an attack. Treatment options include medications and lifestyle changes to help manage the pain.