Idiopathic Hypersomnia Fact Sheet

Are you finding it difficult to understand Idiopathic Hypersomnia or having trouble explaining it to others? This fact sheet is a brief overview of what Idiopathic Hypersomnia (IH) is, what the typical symptoms are, and where you can go for help and further information.

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What is Idiopathic Hypersomnia/Hypersomnolence?

Idiopathic Hypersomnia is a rare neurological sleep disorder characterised by excessive sleep. Traditionally it was described in two forms, Monosymptomatic and Polysymptomatic. While the terms polysymptomatic and monosymptomatic are no longer used recent research [1] supports the findings of previous studies [2,3,4,5] that show there are at least two forms of Idiopathic Hypersomnia. These studies show a subgroup of patients with “a complete form” of idiopathic hypersomnia with symptoms that are unique to this group ('complete' means they typically experience all the key symptoms of Idiopathic Hypersomnia). The research suggests that ‘complete’ Idiopathic Hypersomnia is, in fact, an independent sleep disorder of ‘incomplete’ Idiopathic Hypersomnia. It also supports the findings in other studies [6,7,8] that show the clinical features of ‘incomplete’ Idiopathic Hypersomnia (previously referred to as ‘without long sleep’) are more closely related to those found in Narcolepsy without cataplexy (Type 2 Narcolepsy).

  “(an) important result of this study is the confirmation of two forms of idiopathic hypersomnia: a complete one and an incomplete one..” [4]

Despite excessive amounts of good quality sleep patients with both sleep disorders or types of Idiopathic Hypersomnia are in an almost constant state of sleepiness.

What are the Symptoms?

'Complete' Idiopathic Hypersomnia  

Excessive deep and prolonged night time and daytime sleep with excessive daytime sleepiness. 

Sleep is at least 10 hours each day although it is often much longer. It is not uncommon to sleep as much as 14-16+ hours in a 24 hour period. Unlike in other sleep disorders (eg: Narcolepsy, Sleep Apnoea) sleep quality is normal, however, it is usually very deep. Sleep is typically not refreshing.

Long and unrefreshing naps

Daytime naps are usually longer than a normal daytime nap (more than 1 hour). Naps can be many hours in duration and are usually not refreshing. Regardless of how much sleep they get during the day people with Idiopathic Hypersomnia will still fall asleep without difficulty at night and will sleep solidly usually without arousal.

Difficulty awakening from sleep

People with the complete form of Idiopathic Hypersomnia find it extremely difficult to arouse from sleep, they can sleep through many alarms and even physical attempts to wake them by other people can sometimes fail despite normal quality and more than normal quantity of sleep.

Sleep inertia/sleep drunkenness

People with the complete form of Idiopathic Hypersomnia experience sleep drunkenness (i.e; severe sleep inertia): extreme and prolonged difficulty awakening from sleep, confusion, disorientation, irritability and poor coordination with an uncontrollable desire to go back to sleep. It can also be accompanied by automatic behaviour ie: performing tasks without consciously knowing it and not remembering you have done them eg: turning off alarm clocks or answering your phone.

Cognitive (Executive) Dysfunction

Problems with memory, automatic behaviour, concentration and attention. This is commonly referred to as 'brain fog'.

Unlike in other sleep disorders, the sleep in patients with the 'complete' form of Idiopathic Hypersomnia is normal; there are no disturbances that can account for the excessive daytime sleepiness and prolonged daytime naps.

Idiopathic Hypersomnia is a lifelong disorder. Symptoms typically begin in adolescence or young adulthood. It is debilitating often profoundly affecting work, education and relationships. The cause of Idiopathic Hypersomnia is unknown however it is thought to be caused by a dysfunction in the part of the brain that regulates sleep and wake. Some researchers also believe there is a genetic link. Further research is necessary.

'Incomplete' Idiopathic Hypersomnia 

The main symptom of the 'incomplete' form of Idiopathic Hypersomnia is excessive daytime sleepiness. People with this form of Idiopathic Hypersomnia may experience some difficulty awakening in the morning however they generally do not experience sleep drunkenness. They may or may not sleep longer than normal during the night however irresistible naps during the day are common making their overall sleep time over 24 hrs longer than normal (ie: in excess of 10hrs). Unlike the unrefreshing sleep in the 'complete' form of Idiopathic Hypersomnia, the naps in the incomplete form can sometimes be refreshing.

Research [1] suggests that the incomplete form of Idiopathic Hypersomnia is a separate disorder to the complete form and that its clinical features are more similar to Narcolepsy without Cataplexy (Type 2 Narcolepsy) [6,7,8]. It is thought that they are either a single entity or more likely made up of a number of other conditions, some of which may not yet be known. Further research is required.

How is Idiopathic Hypersomnia Diagnosed?

Diagnosing Idiopathic Hypersomnia can be difficult as excessive daytime sleepiness can be caused by various disorders and/or conditions as well as numerous medications. A physical examination, medical tests and a comprehensive medical history are necessary to rule out all other possible causes, including insufficient sleep. Sleep studies involving a Polysomnography (PSG) followed immediately by a Multiple Sleep Latency Test (MSLT) are also carried out to exclude other sleep disorders such as sleep apnoea.

How is Idiopathic Hypersomnia Treated?

While the complete form of Idiopathic Hypersomnia is thought to be caused by a dysfunction in the part of the brain that regulates sleep and wake an exact cause is not known. Therefore treatment involves targeting the symptoms, not the underlying cause. There are no approved medications in Australia specifically for Idiopathic Hypersomnia. Medications used to treat Narcolepsy including stimulants and wake-promoting medications are prescribed to counter daytime sleepiness, however, there are no medications that assist with the extreme difficulty waking up or the sleep drunkenness that people with this form of Idiopathic Hypersomnia find so difficult to manage. Stimulant and wake-promoting medications can be helpful to relieve sleepiness for some patients however they are rarely effective long term.

Incomplete Idiopathic Hypersomnia and Narcolepsy without Cataplexy (Type 2 Narcolepsy) are thought to be either a single entity of which a single cause is not known or are more likely made up of a number of other conditions, some of which may not yet be known. Therefore treating incomplete Idiopathic Hypersomnia also involves treating the symptom/s. Stimulants and wake-promoting medications can be effective in treating excessive daytime sleepiness/sleep however it should be noted that some people with incomplete Idiopathic Hypersomnia are known to have refreshing naps so it is important to take this into consideration. Sometimes taking a brief nap can be beneficial particularly for those who find naps refreshing.

Click here to read a comprehensive review of how we have gone from the identification of Idiopathic Hypersomnia to where we are now. Drawn from nearly 60 references, including at least 40 peer-reviewed studies on Idiopathic Hypersomnia and Narcolepsy that span more than 6 decades as well as numerous personal conversations with the world's leading Idiopathic Hypersomnia researchers. 

Who is Hypersomnolence Australia?

Hypersomnolence Australia is Australia's only Health Promotion Charity registered with the Australian Charities and Not for Profit Commission (ACNC) specifically dedicated to being a strong advocate, to raising awareness and educating others about Idiopathic Hypersomnia.

You can sign up to our newsletter and complete the survey for our Patient Registry. You can “Like” our Facebook page to keep up to date with relevant information, and you can also Follow Us on TwitterInstagram and Pinterest.

We hope you have found this Fact Sheet informative. Advocacy and raising awareness of Idiopathic Hypersomnia is important. Just as people with Idiopathic Hypersomnia need a reliable source of information and support, we need funding to continue to provide this information and to hopefully fund vital research. Hypersomnolence Australia currently receives no funding and charges no membership.

We need your help!
Donations can be made via cheque or through our ANZ bank account in the name of:

Hypersomnolence Australia

BSB: 014286

A/C 210329584

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Hypersomnolence Australia is a registered not for profit health promotion charity. All donations are tax deductible.


1. Sonka, K., Susta, M., and Billiard, M. Narcolepsy with and without cataplexy, idiopathic hypersomnia with and without long sleep time: a cluster analysis. Sleep Medicine. 2015; 16: 225–231.

2. Aldrich MS, The clinical spectrum of narcolepsy and idiopathic hypersomnia, Neurology 1996, 46, pp.393–401.

3. Bassetti, C., and Aldrich, M.S. Idiopathic hypersomnia. A series of 42 patients. Brain. 1997; 120: 1423–1435.

4. Billiard, M. Idiopathic hypersomnia. Neurol. Clin. 1996; 14: 573–582.

5. Billiard, M., Merle, C., Carlander, B., Ondze, B., Alvarez, D., and Besset, A. Idiopathic hypersomnia. Psychiatr Clin Neurosci. 1998; 52: 125–129.

6. Sasai-Sakuma, T., and Inoue, Y. Differences in electroencephalographic findings among categories of narcolepsy-spectrum disorders. Sleep Med 2015

7. Sasai T, Inoue Y, Komada Y, Sugiura T, Matsushima E, Comparison of clinical characteristics among narcolepsy with and without cataplexy and idiopathic hypersomnia without long sleep time. J Clin Sleep Med. 2008; 5: pp.572–578. 

8. Ozaki, A., Inoue, Y., Nakajima, T., Hayashida, K., Honda, M., Komada, Y., and Takahashi, K. Health related quality of life among drug-naïve patients with narcolepsy with cataplexy, narcolepsy without cataplexy, and idiopathic hypersomnia without long sleep time. J Clin Sleep Med. 2008; 4: pp.572–578. 

Billiard M, Dauvilliers Y Idiopathic Hypersomnia, Sleep Medicine Reviews 2001, Vol. 5, No. 5, pp 351–360.

Dauvilliers Y, Bassetti CL, Principles and Practice of Sleep Medicine (Sixth Edition) 2017, e.4, c.91 Idiopathic Hypersomnia, pp.883–891. 

Roth B, Nevsimalová S, Rechtschaffen A. Hypersomnia with "sleep drunkenness". Arch Gen Psychiatry. May 1972;26(5):456-62.

Roth B. Narcolepsy and hypersomnia: review and classification of 642 personally observed cases. Schweiz Arch Neurol NeurochirPsychiatr. 1976;119(1):31-4. 



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