Cluster headaches are typically very short in duration, excruciating headaches, usually felt on one side of the head behind the eyes. Cluster headaches affect about 1 million people in the United States and, unlike migraines, are much more common in men. This is the only type of headache that tends to occur at night. The reason that they are called ‘cluster’ headaches is that they tend to occur one to four times per day over a period of several days. After one cluster of headaches is over, it may be months or even years, before they occur again. Like migraines, cluster headaches are likely to be related to a dilation of the blood vessels in the brain, causing a localized increase in pressure.