Cluster headaches are a type of severe headache that occur in cyclical patterns or clusters. They are characterized by intense, burning pain, usually around one eye or one side of the head. These headaches can last from 15 minutes to three hours and often occur at the same time each day during a cluster period, which can last weeks to months. The exact cause of cluster headaches is not fully understood, but they may be related to changes in the body's biological clock or hypothalamus. Symptoms can include nasal congestion, eye watering, and restlessness. Treatment options include medications and oxygen therapy to help alleviate the pain.