Editing Cluster headache (section)

===Recurrence===
Cluster headaches may occasionally be referred to as "alarm clock headache" because of the regularity of their recurrence. Cluster headaches often awaken individuals from sleep.  Both individual attacks and the cluster grouping can have a metronomic regularity; attacks typically strike at a precise time of day each morning or night. The recurrence of headache cluster grouping may occur more often around [[solstice]]s, or seasonal changes, sometimes showing circannual periodicity. Conversely, attack frequency may be highly unpredictable, showing no periodicity at all. These observations have prompted researchers to speculate an involvement or dysfunction of the hypothalamus. The hypothalamus controls the body's "biological clock" and [[circadian rhythm]].<ref>{{cite journal |doi=10.1017/S0317167100001694 |pmid=11858532 |title=Cluster Headache: Evidence for a Disorder of Circadian Rhythm and Hypothalamic Function |journal=The Canadian Journal of Neurological Sciences |volume=29 |issue=1 |pages=33–40 |year=2014 |last1=Pringsheim |first1=Tamara |doi-access=free }}</ref><ref>{{cite journal |doi=10.1046/j.1526-4610.2003.03055.x |pmid=12603650 |title=Clinical, Anatomical, and Physiologic Relationship Between Sleep and Headache |journal=Headache: The Journal of Head and Face Pain |volume=43 |issue=3 |pages=282–92 |year=2003 |last1=Dodick |first1=David W. |last2=Eross |first2=Eric J. |last3=Parish |first3=James M. |s2cid=6029272 }}</ref> In episodic cluster headache, attacks occur once or more daily, often at the same time each day for a period of several weeks, followed by a headache-free period lasting weeks, months, or years. Approximately 10–15% of cluster headaches are [[Chronic (medicine)|chronic]], with multiple headaches occurring every day for years, sometimes without any remission.<ref name="NHS">{{cite web |url=https://www.nhs.uk/conditions/cluster-headaches/ |title=Cluster headaches:Pattern of attacks |author=<!--Not stated--> |date=22 May 2017 |website=NHS |publisher=Gov.UK |access-date=13 December 2018 |archive-date=20 June 2019 |archive-url=https://web.archive.org/web/20190620122856/https://www.nhs.uk/conditions/cluster-headaches/ |url-status=live }}</ref>

In accordance with the International Headache Society (IHS) diagnostic criteria, cluster headaches occurring in two or more cluster periods, lasting from 7 to 365 days with a pain-free [[Remission (medicine)|remission]] of one month or longer between the headache attacks may be classified as episodic. If headache attacks occur for more than a year without pain-free remission of at least three months, the condition is classified as chronic.<ref name=IHS>{{cite web |url= https://ichd-3.org/3-trigeminal-autonomic-cephalalgias/3-1-cluster-headache/3-1-2-chronic-cluster-headache/ |title= IHS Classification ICHD-3 3.1.2 Cluster headache |publisher= The International Headache Society |access-date= 2024-02-08 |archive-date= 8 February 2024 |archive-url= https://web.archive.org/web/20240208085008/https://ichd-3.org/3-trigeminal-autonomic-cephalalgias/3-1-cluster-headache/3-1-2-chronic-cluster-headache/ |url-status= live }}</ref>
Chronic cluster headaches both occur and recur without any remission periods between cycles; there may be variation in cycles, meaning the frequency and severity of attacks may change without predictability for a period of time. The frequency, severity, and duration of headache attacks experienced by people during these cycles varies between individuals and does not demonstrate complete remission of the episodic form. The condition may change unpredictably from chronic to episodic and from episodic to chronic.<ref>{{cite journal |doi=10.1007/s11916-002-0026-5 |pmid=11749880 |title=What predicts evolution from episodic to chronic cluster headache? |journal=Current Pain and Headache Reports |volume=6 |issue=1 |pages=65–70 |year=2002 |last1=Torelli |first1=Paola |last2=Manzoni |first2=Gian Camillo |s2cid=37173661 }}</ref>