Forgotten Diseases Research Foundation

Idiopathic Hypersomnia (IHS)

Idiopathic hypersomnia (IHS) is a chronic sleep disorder and is one of the central disorders of hypersomnolence (CDHs). This term refers to conditions that cause excessive daytime sleepiness, meaning that patients have trouble staying awake during the day (1). CDH-type sleepiness isn't caused by temporary loss of sleep or by misaligned circadian rhythms (a preferred sleeping schedule that's shifted from times that are typical of the rest of the population --- jet lag is an example). Rather, the problem in the CDHs is internal, and it happens over an extended period of time (generally, for at least three months).

Some people with CDHs may feel increasingly drowsy as the day goes along, and need to take a nap. They may or may not feel refreshed when they wake up, and they may feel drowsy again later. Others may have sleep attacks, which cause them to fall asleep suddenly, without feeling drowsy first. Others may become sleepy when doing something that isn't mentally stimulating, such as sitting in the passenger seat of a car. Some may be able to put naps off by doing something active. Many sleep for at least hours per night, yet are still tired during the day.

Daytime sleepiness can be assessed with an easy-to-use quiz called the Epworth Sleepiness Scale (2). The quiz has 8 simple questions and a basic scoring system. Higher scores (10 or more points) indicate the possibility of excessive sleepiness and suggest that seeing a doctor may be a good idea; scores of 16 or more are strongly suggestive of excessive daytime sleepiness and the need to see a doctor. In addition to this scale, physicians can study sleep with specialized tests, such as an MSLT (Multiple Sleep Latency Test) and polysomnography. The MSLT measures sleep latency, which is how quickly a person falls asleep. Polysomnography measures a variety of physiological states while a person is sleeping. Both are used for studying sleep and for diagnosing sleep disorders.

Other central disorders of hypersomnolence include Kleine-Levin syndrome, narcolepsy, insufficient sleep syndrome, and hypersomnia due to medical/psychiatric disorders or to medications or other substances. Overall these conditions are rare to very rare, with narcolepsy being the most common.

IHS is a very rare disease; a recent study estimated its prevalence at roughly 2-5 cases per 100,000 (3). Unlike many rare disorders, IHS may be overdiagnosed, primarily because of lack of knowledge and a tendency to label hypersomnias that aren't narcolepsy or sleep apnea as IHS (3). IHS has been reported in people of a variety of ethnicities and locations.

Clinical information

A defining feature of IHS is daytime sleepiness. Some patients may always feel drowsy and in need of a nap/ While sleepiness to this extreme does not occur in every person with IHS, some daytime sleepiness is present in all patients (4). In IHS, sleepiness is not caused by a poor night's sleep the previous night, by infection, head injury, or any other identifiable factors. The problem is also constant in that it isn't marked by weeks or months of normal sleep-wake patterns. These facts distinguish IHS from other conditions (see Differential Diagnosis, below).

Signs and symptoms

The most common clinical features of IHS are listed below.

    Common problems in IHS

  • Excessive sleep in a 24-hour period (usually >11 hours)
  • Falls asleep very quickly (sleep latency <8 minutes)
  • Hypersomnia that is a consistent daily problem
  • A consistent need for daytime naps
  • Excessive daytime sleepiness
  • Naps are not refreshing
  • Difficulty waking up
  • Difficulty concentrating
  • Memory loss
  • Depression


The term idiopathic means "without a known cause." Thus, as its name implies, the cause of IHS is currently unknown. There seems to be a genetic cause in at least a subset of patients, as roughly 40-60% IHS patients have an affected family member. (1, 5, 6). However, to date, researchers have not found a gene that is associated with IHS.

Some IHS patients have a gene allele called HLA DQB1/06:02 (1, 7, 8). This term HLA refers to a gene called Human Leukocyte Antigen, and the rest of the term specifies a certain sequence of the gene. Most people with narcolepsy have this allele, and an unknown percentage of IHS patients have it as well. However, roughly 15-25% of the normal population also has it as well (9-11). Practically speaking, this means that having the HLA DQB1/06:02 allele may increase the risk for IHS, and testing for it may be somewhat useful for someone with the symptoms of IHS, though the usefulness of testing has not yet been determined. Either way, because so many people without a sleep disorder have this allele, having it does not cause IHS.

Other potential causes of IHS or factors that have been associated with it are discussed in reference 1.

Diagnosis and Testing

Because the causes of IHS are unknown, there are no definitive laboratory tests that can diagnose it. This means that diagnosis is based on clinical observation and on tests that exclude other conditions (see Differential Diagnosis, below). However, diagnostic criteria have been established and are refined periodically. The list below lists the most current diagnostic (4):

  • A daily irrepressible need to sleep. This problem must exist for at least 3 months.
  • Episodes usually occur at least once per year but at least once every 18 months
  • No cataplexy
  • An MSLT must be performed. It must show 1) no more than 1 occurence of sleep-onset rapid eye movement (SOREMP). This term refers to the onset of REM sleep when a person falls asleep (REM sleep does not usually begin until 90 minutes after a person falls asleep) or 2) no SOREMPs if REM latency on a previous preceding polysomnogram was ≤15 minutes.
  • Either the MSLT shows a sleep latency of ≤8 minutes OR total sleep in a 24 hours period is ≥660 minutes (it is typically 12-14 hours) when using polysomnographic monitoring. Correction for sleep deprivation should is essential. Aternatively, 24-hour sleep my be determined with a log of hours slept or by use of wristwatch actigraphy.
  • Clinicians have ruled out insufficient sleep syndrome.
  • The symptoms are not better explained by drugs, medications or another disorder (e.g. depression, bipolar disorder, neurologic, metabolic, or mental disorders).

Differential Diagnosis

The differential diagnosis for IHS includes drugs and prescribed medications (e.g. cannabis, illicit drugs, alcohol, benzodiazepines). Urine tests may confirm or exclude use of these agents. Other conditions in the differential diagnosis of KLS are described below; for more information on other conditions that are related to IHS, see references 1, 3, and 4.

Hypersomnia associated with a psychiatric disorder. This condition is often associated with depression or other psychiatric symptoms. The daytime sleeping pattern of people in this group may resemble that of IHS patients, but members of this group often complain of poor sleep at night. This problem is less common in IHS, though it does occur.

Note that some people with IHS may also experience depression as a result of having a debilitating condition, and that depression itself does not rule out IHS.

Chronic fatigue syndrome (CFS) is a disorder that causes fatigue that may be persistent or that may relapse and remit. CFS does not get better with sleep or rest. People with this condition complain about feeling fatigued, and many also have pain in their joints and muscles. Joint pain may move from joint to joint, and there is no swelling or redness. This type of pain is a key distinction, as it does not appear to be a feature of IHS (IHS patients may, however, complain of headaches and/or migraines, as do CFS patients). In addition, the mean MSLT sleep latency is normal in CFS.

Narcolepsy is another CDH that makes patients feel very sleepy during the daytime, regardless of the amount of sleep the previous night. However, in contrast to IHS, sleep episodes in narcolepsy often last for no more than a few minutes (though they may last for over an hour), and may feel refreshing (though the person may feel drowsy again later). In addition, and unlike IHS patients, most narcolepsy patients experience cataplexy, which is a sudden loss of muscle tone while awake. This problem provokes a feeling of weakness and/or loss of muscle control; patients may collapse or have trouble speaking. Patients are typically awake and aware of the problem. Cataplexy may be very mild (such as affecting the eyelids only) or severe (affecting all muscles). For more information on narcolepsy, see reference 12.

At least one study has found that length of naps may help distinguish IHS from narcolepsy (Anderson). This study compared napping and other factors in 77 people with IHS and 63 people with narcolepsy. It found that naps of at least one hour were helpful in distinguishing the two conditions (sensitivity of 87%; specificity of 87%).

Kleine-Levin syndrome. KLS, like IHS, is a condition in which people sleep excessively. However, unlike IHS and narcolepsy, excessive sleepiness in KLS is episodic. This term means that it happens from time to time, in episodes. A sleepiness episode typically lasts for days to weeks, but can last for months in extreme cases. During an episode, a person may sleep for 18 hours a day or more, may eat voraciously, and may feel disconnected from reality. Some patients (usualy males) may have hypersexual feelings. In between episodes, patients sleep and act normally, and may not have good memories of the sleepy periods. An important point about KLS is that it with time, it becomes more mild and appears to eventually disappear. The course of disease may last for several years.


  1. 1. Khan Z & Trotti LM (2015) Central disorders of hypersomnolence: focus on the narcolepsies and idiopathic hypersomnia. Chest 148(1):262-273. Abstract on PubMed.
  2. 2. Johns MW (1991) A new method for measuring daytime sleepiness: the Epworth Sleepiness Scale. Sleep 14(6):540-545. Abstract on PubMed.     Full text from publisher.
  3. 3. Bassetti C & Aldrich MS (1997) Idiopathic hypersomnia: a series of 42 patients. Brain 120(Pt 8):1423-1435. Full text from publisher.
  4. 4. American Academy of Sleep Medicine (2014) The International Classification of Sleep Disorders, 3rd edition. Westchester, IL: American Academy of Sleep Medicine. ISBN: 978-0991543410. Publisher website.
  5. 5. Billiard M & Dauvilliers Y (2001) Idiopathic hypersomnia. Sleep Med Rev 5(5):351-360. Abstract on PubMed.
  6. 6. Janáčková S et al (2011) Idiopathic hypersomnia: a report of three adolescent-onset cases in a two-generation family. J Child Neurol 26(4):522-525 Abstract on PubMed.
  7. 7. Coelho FM et al. (2009) Prevalence of the HLA-DQB1*0602 allele in narcolepsy and idiopathic hypersomnia patients seen at a sleep disorders outpatient unit in São Paulo. Rev Bras Psiquiatr 31(1):10-14. Full text from Scielo.
  8. 8. Miyagawa T et al. (2009) Polymorphism located between CPT1B and CHKB, and HLA-DRB1*1501-DQB1*0602 haplotype confer susceptibility to CNS hypersomnias (essential hypersomnia). PLoS ONE 4(4):e5394. doi: 10.1371/journal.pone.0005394 Full text on PubMed.
  9. 9. Akintomide GS & Rickards H (2011) Narcolepsy: a review. Neuropsych Dis Treat 7:507-518.   doi: 10.2147/NDT.S23624. Full text on PubMed.
  10. 10. Hor H et al (2010) Genome-wide association study identifies new HLA class II haplotypes strongly protective against narcolepsy. Nat Genet 42(9):786-789. Abstract on PubMed.
  11. 11. Mignot E (1998) Genetic and familial aspects of narcolepsy. Neurology 50(2 Suppl 1):S16-S22. Abstract on PubMed.
  12. 12. National Institute of Neurological Disorders and Stroke (2015) Narcolepsy Fact Sheet. Full text.
  13. 13. Anderson KN et al. (2007) Idiopathic hypersomnia: a study of 77 cases. Sleep 30(10):1274-1281. Full text on PubMed.

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Page last modified on 6 March 2021.