For a small segment of the population, cluster headaches can cause severe pain with little warning. Cluster headaches attack at seemingly random intervals, for which they gain their name. Patients can go years without a single cluster headache, and suddenly suffer them for days or weeks at a time. The exact cause of cluster headaches is not known, although it is thought that stress, alcohol, and drugs are all prompters of the condition. Even without these factors, however, a cluster headache can strike a patient at any time. A cluster headache can be easily identified by the pain level.