What is Idiopathic Hypersomnia?
Idiopathic Hypersomnia (IH) is a rare neurological sleep disorder. In 1976 Czech neurologist Bedrich Roth coined the term "Idiopathic Hypersomnia" and defined two forms of the disease, Monosymptomatic (previously referred as “without long sleep”) and Polysymptomatic (previously referred to as “with long sleep”). While the terms polysymptomatic and monosymptomatic are no longer used recent research  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..” 
For more information click here for our Fact Sheet and 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.
*If you are treating patients with Narcolepsy Type 2 (without cataplexy) or are a patient yourself you may also be interested in this review.
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.
Dr Anu Bhatia describes the 3 main symptoms of Polysymptomatic Idiopathic Hypersomnia
Is Idiopathic Hypersomnia/Hypersomnolence a Rare Disease?
We are often asked is Hypersomnia/Hypersomnolence a Rare Disease? and the answer is YES IT IS! Check out the Idiopathic Hypersomnolence page on Genetic and Rare Diseases Information Center (GARD).