Non-24-hour sleep-wake disorder

Ahmer Ali MD (Dr. Ali of Northwestern University has no relevant financial relationships to disclose.)
Michael J Howell MD, editor. (Dr. Howell of the University of Minnesota received grant support from Apnex and GE and honorariums from Inspire as a panel member.)
Originally released November 22, 1993; last updated May 12, 2017; expires May 12, 2020

This article includes discussion of circadian rhythm sleep disorder: non-24-hour sleep-wake disorder, circadian rhythm sleep disorder: nonentrained type, hypernychthemeral syndrome, non-24-hour sleep-wake rhythm disorder. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.


The author describes the pathophysiology and treatment of this relatively rare but potentially debilitating circadian rhythm sleep disorder. Although found primarily in blind individuals, it can also be seen in individuals with normal sight, and is characterized by alternating periods of insomnia and excessive daytime sleepiness. This treatable disorder is thought to be caused by an impaired ability to respond and/or decreased exposure to daily entraining signals, including light, resulting in a “free-running” sleep-wake cycle that is alternately in and out of phase with the desired sleep period. New treatment options are being developed, such as sustained-release melatonin agonists, to provide additional treatment options for this disorder. This review highlights basic circadian biology in the context of this unique medical disorder.

Key points


• Non-24-hour sleep-wake disorder consists of progressive, relentless cyclic delay of sleep and waking times as an individual's circadian pacemaker does not entrain to a 24-hour cycle.


• This disorder should be suspected in any blind individual with sleep complaints.


• It is rare but increasingly recognized in sighted individuals, in whom it may be secondary to premorbid psychopathology or evolve from delayed sleep phase disorder.


• In blind individuals, this disorder is frequently responsive to melatonin or melatonin receptor agonist therapy.


• In sighted individuals, treatment is more challenging and often depends on targeting multiple zeitgebers.

Historical note and terminology

The non-24-hour sleep-wake disorder (N24SWD), formerly labeled as free-running circadian rhythm sleep disorder and non-24-hour sleep-wake syndrome prior to that, is also known as the "hypernychthemeral syndrome." Nychthemeron is the Greek term for "a night and a day," ie, a single 24-hour cycle of light and darkness. Therefore, "hypernychthemeral" means greater than 1 night and 1 day and refers here to the longer-than-24-hour sleep-wake cycles of patients with the non-24-hour sleep-wake syndrome. The first report of a person with this disturbance was published in 1971 (Eliott et al 1971). The second report, of a blind man living in community, was reported in 1973 (Miles et al 1973). A sighted person with non-24-hour sleep-wake syndrome was described in 1978, and the authors coined the term hypernychthemeral rhythm (Kokkoris et al 1978).

Throughout this topic, the current formal name established in the 3rd edition of the International Classification of Sleep Disorders, which is non-24-hour sleep-wake rhythm disorder, will be used; other common names are nonentrained or free-running disorder (American Academy of Sleep Medicine 2014).

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