A cluster headache is a rare cause of headache. It characteristically presents as severe unilateral orbital or temporal pain with a duration between 15 minutes and 3 hours. Cluster headaches are classically associated with autonomic features including conjunctival injection, lacrimation, rhinorrhea, facial swelling, miosis, and ptosis on the ipsilateral side. Cluster headaches are part of a group of primary headache disorders known as trigeminal autonomic cephalgias.

The "cluster" of cluster headache refers to periods where patients may experience a cycle of these types of headaches on a frequent basis, typically for between 2 weeks to 3 months, followed by spontaneous remission. During a cycle, frequency of attacks can range from 1 attack every other day up to 8 attacks per day, with an average of 2 attacks per day. Attacks most often occur overnight, with peak frequency occurring between midnight and 3 AM. The most common trigger for a cluster headache is alcohol consumption; however, other reported triggers include use of nitroglycerin, weather changes, strong odors, and bright or flashing lights.

Cluster headaches are rare, occurring in less than 1% of the population. They are most common in adult males, with a male-to-female ratio of about 3:1. Age of onset is typically 20-40 years. Smoking is a potential risk factor, with about 50% of cluster headache patients reporting current tobacco use. Depression, suicidality, and sleep apnea are more common in patients with cluster headaches compared with the general population.

About 25% of patients will only ever experience one cluster headache cycle during their life. Patients with a history of at least two cycles of cluster headaches lasting from 7 days to 1 year with a remission period of at least 3 months are classified as having episodic cluster headaches. A subset of patients with cluster headache do not experience a remission period and are classified as having chronic cluster headaches. About 10%-15% of patients with cluster headache have the chronic variant.

Cluster headaches can be disabling. About 8% of cluster headache patients are either out of work or on disability because of their headaches.