Cluster headaches - Causes 

The cause of cluster headaches is unknown and they usually occur unexpectedly.

Research suggests that when a cluster headache happens, there's a lot more activity in an area of the brain called the hypothalamus. It's thought that the hypothalamus releases chemicals that trigger the cluster headaches, possibly by causing blood vessels to widen, causing an increase in bloodflow to the brain.

It's not known what causes the hypothalamus to act in this way. However, in some people, cluster headaches are triggered by certain things, such as drinking alcohol or an extreme increase in temperature (such as from exercising in hot weather).