Hypersomnia Foundation

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Function: noun
1: sleep of excessive depth or duration.
2: the condition of sleeping for excessive periods at intervals with intervening periods of normal duration of sleeping and waking.
– compare to Narcolepsy. Source: Merriam-Webster Medical Dictionary


Hypersomnia and excessive daytime sleepiness are complex traits whose formal study is in its infancy and whose socioeconomic burden is enormous.  The term hypersomnolence is used to refer to excessive sleepiness during the day and hypersomnia as prolonged sleep periods (i.e., more than 10 hours per night). There are several different classification systems for the hypersomnia syndromes, but, broadly speaking, hypersomnia can be classified as primary or secondary, and the primary hypersomnias can be classified based on clusters of symptoms and test results or based on underlying cause.


Hypersomnia is judged to be “secondary” if it is caused by a problem with night time sleep, inability to get enough sleep, or other medical problems that result in sleepiness.  For example, patients with obstructive sleep apnea, who have multiple interruptions in breathing throughout the night, are frequently sleepy during the day, but their hypersomnia is judged to be secondary to their sleep apnea.  Secondary hypersomnia can be triggered by infections, depression, kidney failure, chronic fatigue syndrome, and neurodegenerative diseases such as Parkinson’s disease and myotonic dystrophy.  The primary hypersomnias, in contrast, occur in the absence of such medical problems and despite normal quality and quantity of night time sleep (and sometimes despite exceptionally long periods of night time sleep).  The primary hypersomnias are thought to arise from problems with the brain’s systems that regulate sleep and wake.


One common classification of primary hypersomnia syndromes is that of the International Classification of Sleep Disorders, which divides hypersomnia syndromes primarily between narcolepsy, idiopathic hypersomnia, and the recurrent hypersomnias.  Narcolepsy is further divided between that with cataplexy (brief episodes of muscle weakness triggered by strong emotion) and that without cataplexy.  Idiopathic hypersomnia is characterized as having either long sleep time (> 10 hours/night) or normal sleep time.  The recurrent hypersomnias involve severe periods of sleepiness lasting days, interspersed with periods of more normal alertness, and include such entities as Kleine-Levin syndrome.


As the above discussion shows, different primary hypersomnia syndromes sometimes have overlapping clinical features because they were defined before the underlying causes were known.  In practice, it can be helpful to cluster primary hypersomnia syndromes by underlying cause when it is known.  Cerebrospinal fluid analysis is helpful in this regard, because it allows distinction between hypocretin-deficient narcolepsy (i.e., narcolepsy with cataplexy) and the primary hypersomnias related to abnormal functioning of the GABA system (which researchers at Emory have recently shown to be the case in many patients with IHS,  narcolepsy without cataplexy, and others requiring extraordinary amounts of sleep greater than 70 hours per week but with normal MSLT results).  These observations support the consideration of these latter syndromes as a single entity; the proposed revisions to the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-V) create a new category termed hypersomnolence disorder that encompasses many patients with these hypersomnia subtypes, although it is not specifically based on underlying cause.