The terms 'hypersomnia of unknown origin' and 'excessive daytime sleepiness' are not synonymous with idiopathic hypersomnia (they do not mean the same thing).
Idiopathic Hypersomnia is an unfortunate name. This is because the word “idiopathic” means ‘unknown cause’ and this can lead people to assume it is merely a symptom – the symptom of being ‘sleepy’ and doctors don’t know why. This is not correct.
Idiopathic hypersomnia was identified and described as a separate disease entity after years of extensive and meticulous study of a large number of patients from the mid 1950’s until the end of the 70’s. This led to the naming of Idiopathic hypersomnia and its acceptance as an independent clinical entity. It was included in the first ICSD, Diagnostic Classification of Sleep and Arousal Disorders in 1979.
Hypersomnia of unknown origin means the cause or the ‘origin’ of the hypersomnia is unknown, however unlike Idiopathic Hypersomnia, hypersomnia of unknown origin is not a disease or disorder. The difference between them is that hypersomnia of unknown origin does not include the other symptoms and features that are typical of idiopathic hypersomnia.
The term “excessive daytime sleepiness” (EDS) is often used interchangeably with hypersomnia and idiopathic hypersomnia however these two terms do not mean the same thing. Like 'hypersomnia of unknown origin', EDS is not a disease or disorder. Understanding the meaning of hypersomnia is important. ‘Sleepy terms’, such as tired, sleepy, sleepiness, fatigue and excessive daytime sleepiness do not have the same meaning as hypersomnia. Read more here
“hypersomnia is a broader symptom including extended nocturnal sleep, unplanned daytime sleep and an inability to remain awake or alert in situations where it is required (excessive sleepiness). There is also a growing trend in labeling excessive sleepiness as a disease or a disorder. So far, there is no data supporting this claim. Excessive daytime sleepiness is not a disease or a disorder; it is a symptom of a sleep disorder or of another disease.”- Maurice M Ohayon, Wakefulness to excessive sleepiness: what we know and still need to know" Sleep Med Review 2008 UPDATE: New paper published Feb 2020 with more supporting evidence: Diagnosis of central disorders of hypersomnolence: A reappraisal by European experts
Published in Sleep Medicine Reviews Volume 52, August 2020