According to the International Classification of Headache Disorders, there are two groups of cluster headaches, based on the lengths of the cluster periods and remission periods:

• Episodic. This term may be used to describe cluster headaches occurring daily in periods of one week to one year, followed by remission lasting at least one month before the development of another cluster period.
  
  
• Chronic. This term may be used to describe cluster headaches occurring daily for more than a year, with no remission or with pain-free phases lasting less than one month.
  
  

According to the National Headache Foundation, about 10 to 20 percent of people who experience cluster headaches have the chronic form. Although chronic cluster headache may develop after a phase of episodic attacks, it may also develop in people without a history of headaches. Rarely, patients may also experience alternating phases of chronic and episodic headache.