About Idiopathic Hypersomnia

Vetted by Our Medical Advisory Board


Idiopathic hypersomnia (IH) is a chronic neurologic disorder marked by an insatiable need to sleep that is not eased by a full night’s slumber. People with idiopathic hypersomnia sleep normal or long amounts of time each night but still feel excessively sleepy during the day. They may take long naps, but wake up feeling no better than when they fell asleep. Check out our IH Summary: Characteristics & Diagnostic Criteria and Idiopathic Hypersomnia by LM Trotti—A free full text 2017 Sleep Medicine Clinics journal article summarizing IH. Also read how people with IH describe living with IH in a nutshell.

What Are the Symptoms of Idiopathic Hypersomnia?

The main symptom of IH is excessive daytime sleepiness despite adequate, or more typically, long sleep amounts (e.g., more than 10-11 hours per night). Additional symptoms commonly include unrefreshing or non-restorative sleep, and severe sleep inertia/sleep drunkenness, which is extreme difficulty awakening from sleep, accompanied by feelings of grogginess and disorientation upon awakening. While people without sleep disorders may wake up and briefly want to return to sleep, in people with idiopathic hypersomnia, this sleep-to-wake transition is much more difficult and prolonged. Sleep seems to leave a mental fogginess, which can remain throughout the few hours that people with IH can remain awake. Thinking clearly and carrying out even basic tasks can be difficult.

Many people with IH sleep more than 11 hours out of every 24. The disorder is chronic, and the symptoms can be relentless. If an effective medication to control symptoms cannot be found, it can be extremely difficult for people with IH to hold down jobs, remain in school, maintain marriages, and fully engage with their family and friends. Even with medication, patients may struggle with these activities.

Symptoms often first appear in the mid-to-late teens or early twenties, although they can begin in childhood or at a later age. Symptom intensity often varies (between weeks, months, or years) and can worsen just prior to menses in women. Symptoms may spontaneously remit in 10-15% of patients (read more HERE). 

Sleep is usually described as “deep,” and arousal from sleep is usually difficult, often requiring multiple alarm clocks and morning rituals to ensure that patients arise for school or work. In contrast to the short and generally refreshing daytime naps observed in narcolepsy type 1, those naps in IH patients can be very long—on the scale of hours—and are usually unrefreshing.

IH symptoms and the medications used to treat them may lead to difficulties with anesthesia and hospital care. To create a care plan for emergencies or upcoming planned anesthesia or hospitalization, see our Anesthesia, Hospitalization and Emergency Planning page.

Diagnosing Idiopathic Hypersomnia

Proper diagnosis of idiopathic hypersomnia is key to establishing beneficial treatment strategies and includes:

  • Presence of excessive daytime sleepiness for at least 3 months
  • Excluding prescribed medications as a cause
  • A comprehensive medical history, physical examination, and medical tests to rule out other conditions as a cause
  • Excluding disturbances in sleep rhythms (e.g., circadian sleep disorders) and insufficient sleep (usually assessed via a ‘sleep diary’ in combination with a wearable sensor called an actigraph)
  • An overnight sleep test or polysomnography (PSG), followed immediately by a daytime Multiple Sleep Latency Test (MSLT), ideally performed in an accredited sleep laboratory

Additional Diagnosis Resources:

What Causes Idiopathic Hypersomnia?

IH is a disorder of the brain and central nervous system whose cause is not known. It is possible that there may be multiple causes, each of which applies to a subset of the IH population.

One current hypothesis is that in a few cases, there appears to be overproduction of a small molecule present in cerebrospinal fluid that acts like a sleeping pill or anesthetic. Although the exact composition of this small molecule is yet to be determined, it is known to interact with γ-aminobutyric acid (GABA), a principal player in the brain mechanisms that promote sleep. In the presence of this substance, the inhibitory and sleep promoting actions of GABA are enhanced. Research is currently ongoing to identify this small molecule and determine what proportion of the IH population may be affected by it.

Researchers continue to search for other potential biomarkers or indicators of the cause of IH. While studies have found that some people with IH have altered physiological patterns (such as circadian rhythms, autonomic responses, and brain activity during sleep), the underlying causes of these changes and their link to IH symptoms are still unknown. 

Genetic studies have been limited. The genetics of sleep and wake are complex, and the likelihood that subsets of the IH population may have different causes for their symptoms makes it even more challenging to conduct genetic research.

Additional Resources

Idiopathic Hypersomnia Resources:


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