Idiopathic Hypersomnia Explained

A disorder of excessive sleep in which adequate sleep and long naps still leave a person unrefreshed.

Idiopathic hypersomnia (IH) is a chronic sleep disorder marked by excessive daytime sleepiness despite normal or long nighttime sleep, with no identifiable cause.¹ Many people with it sleep 11 hours or more in a day, wake with great difficulty, and find naps unrefreshing.^1,2 It is diagnosed with an overnight sleep study and a daytime nap test, sometimes with a longer recording, once other causes have been ruled out.¹ One medication is FDA-approved for IH, and others are used off-label.^3,4

What It Is

“Idiopathic” means the cause is unknown. IH belongs to the same family as narcolepsy, the central disorders of hypersomnolence, in which the problem lies in the brain’s regulation of sleep and wakefulness rather than in poor or interrupted sleep. There is no biomarker. The diagnosis is reached by testing and by excluding everything else that causes sleepiness.¹

The Signs

Sleepiness through the day. Present in everyone with IH, regardless of how long they slept.¹

Long sleep. In a controlled series of 75 patients, those with IH averaged roughly 11 to 12 hours of sleep across 24 hours of recording, against about 8 to 9 hours in healthy controls.²

Severe difficulty waking. Often called sleep drunkenness: confusion, slowness and repeated returns to sleep that can last long after the alarm. It was reported by 36% of patients in the same series.²

Naps that do not help. Long naps that leave a person no more alert, reported by 46%.²

Sleep paralysis and vivid images at sleep onset also occur in IH, which is one reason it can be mistaken for narcolepsy.²

How It Differs From Narcolepsy

The two share daytime sleepiness and sometimes look alike, but the pattern differs. People with narcolepsy struggle to stay awake and usually wake refreshed from a short nap. People with IH struggle to wake up, and naps rarely help.^1,2 Cataplexy, the emotion-triggered muscle weakness of narcolepsy type 1, does not occur in IH.¹ On testing, IH shows fewer than two episodes of early REM sleep, where narcolepsy shows two or more.¹

How It Is Diagnosed

Diagnosis requires at least three months of daily sleepiness and objective testing, usually an overnight sleep study followed by a daytime nap test. IH is supported when sleep onset on the nap test is fast, or when total sleep across 24 hours reaches 11 hours or more, and early REM sleep is largely absent.¹

A normal nap test does not rule it out. In the 75-patient series, the nap test was normal in 71% of patients with long sleep time, which is why a 24-hour recording or extended monitoring is sometimes needed.² Insufficient sleep, sleep apnea, depression and sedating medications must all be excluded first.¹

How It Is Treated

There is no cure. Treatment aims to reduce sleepiness and make mornings manageable, and it is usually directed by a sleep specialist.

An approved treatment. In August 2021 the FDA approved a low-sodium oxybate (Xywav), taken at night, as the first medicine for IH in adults.³

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Finding the Right Specialist

Because IH is diagnosed by exclusion and confirmed in a sleep lab, the usual route is a sleep medicine specialist who can arrange both the testing and the work-up for other causes. Bring a two-week sleep diary that includes weekends, and a list of current medications. When tiredness needs a doctor covers what else helps that first conversation.