The exact cause of cluster headaches has not been identified. However, researchers believe the condition may result from several factors, including:

• Greater sensitivity of nerve pathways. The trigeminal nerve (the main nerve of the face responsible for pain and other sensations) serves the areas behind and around the eyes.
  
  
  
  
  Stimulation of the trigeminal nerve can trigger the arteries that supply blood to the head to dilate, causing eye pain. Stimulation of the trigeminal nerve also triggers activation of the autonomic nerves (which regulate involuntary bodily functions), resulting in teary eyes, drooping eyelids and congestion. Some experts believe that inflammation or abnormal activity of blood vessels in this area may also contribute to the development of cluster headaches.
  
  
• Abnormal functioning of the hypothalamus. The hypothalamus is an area of the brain responsible for controlling internal rhythms, such as hunger, thirst and the sleep-wake cycle. Many researchers believe abnormalities of the hypothalamus may explain the cyclical pattern and regularity associated with cluster headaches. A number of studies have shown an increased level of activity in the hypothalamus during a cluster headache. Studies have also discovered that people who suffer from cluster headaches have larger hypothalamuses than people who do not experience the headache. The presence of abnormal levels of certain hormones (e.g., testosterone, melatonin) during cluster periods has also been attributed to abnormalities in the hypothalamus.
  
  

• Histamine. Researchers have determined that the level of histamine in a person’s blood and urine increases during a cluster headache. As a result, many experts believe that histamines, which trigger the blood vessels to dilate, are related to the development of cluster headaches.

A number of factors may trigger a cluster headache, including:

• Smoking
• Use of alcohol
• Glare
• Changes in sleep patterns or inadequate sleep
• Certain medications that cause blood vessels to swell or widen (vasodilators), such as nitroglycerin, other nitrates and many antihypertensives (drugs for high blood pressure)

In addition, a number of factors appear to make a person more likely to develop cluster headaches. The two biggest risk factors linked to cluster headaches are age and sex. According to the American Council for Headache Education (ACHE), about 85 percent of people affected by the condition are men.