Idiopathic hypersomnia is a long-term sleep disorder in which you are excessively sleepy by day, even after a full night's sleep. "Idiopathic" means the cause is unknown. The NHS describes people sleeping for long periods and waking confused, irritable and unrefreshed. That groggy, hard-to-wake state is called sleep inertia.
It is diagnosed in a sleep clinic, after other causes are ruled out. Tests usually include an overnight sleep study and a daytime nap test (the MSLT). There is no cure, but symptoms can often be improved. In the US, one medicine is approved for it. In Ireland, the UK and the rest of Europe, no medicine is licensed for it, so treatment is prescribed "off-label" by specialists.
This guide covers how it differs from narcolepsy, how it is diagnosed, the treatment evidence, and availability in Ireland and the UK.
The core symptom is daily sleepiness that has lasted at least three months. Many people also describe:
A 2024 systematic review found that people can wait up to 9 years for a diagnosis. Part of the reason is that it overlaps heavily with other conditions, especially narcolepsy type 2.
Narcolepsy type 2 sits in between. A Swiss study followed 179 people with these conditions. Their diagnosis changed in 17%, most often among those with narcolepsy type 2. Five people with idiopathic hypersomnia went into remission. For more on narcolepsy, see why narcolepsy takes years to diagnose.
Sleep specialists use the International Classification of Sleep Disorders (ICSD). Its current version is the 2023 text revision (ICSD-3-TR). A 2025 review in the Journal of Sleep Research summarises the criteria. All of these must apply:
The 11-hour measure can come from a 24-hour sleep recording. It can also come from a wrist activity monitor plus a sleep diary, averaged over at least seven days of unrestricted sleep.
The usual route is an overnight sleep study (polysomnography), then a multiple sleep latency test (MSLT) the next day. Our guide to sleep studies and the MSLT explains what happens, and why medicines and short sleep can upset the result.
The MSLT has real limits in this condition. The Swiss study confirmed that it gives poor repeat results in idiopathic hypersomnia. A 2022 review notes that waking people for scheduled naps stops the test capturing long sleep. That is one reason the 24-hour sleep route exists.
The NHS lists other causes of excessive sleepiness to check first. These include sleep apnoea, restless legs, depression, bipolar disorder and ME/CFS. Some medicines, alcohol and drugs can also cause it.
The commonest cause of all is simply too little sleep. See how much sleep adults need, and our guide to feeling tired all the time for other medical causes.
The NHS says changing sleep habits may not cure hypersomnia, but can help. It suggests a regular bedtime, avoiding alcohol and caffeine, and avoiding medicines that cause drowsiness. Ask your specialist before changing any prescribed medicine.
Modafinil. A 2021 Cochrane review found two placebo-controlled trials with 102 people. Modafinil cut sleepiness questionnaire scores by about 5 points more than placebo, on a 24-point scale (high-certainty evidence). It also improved time awake on a stay-awake test by about 5 minutes. Naps per week did not change. Nearly all participants did not have the long-sleep form, so the result may not apply to everyone.
Lower-sodium oxybate (Xywav). It is the only medicine approved for idiopathic hypersomnia by the US Food and Drug Administration (FDA), since August 2021. Its main trial used a "withdrawal" design with 154 adults. Everyone took it openly first. Then 115 were randomly switched to placebo or kept on it for two weeks. Sleepiness scores worsened on placebo and stayed stable on Xywav, a difference of 6.5 points.
The open-label fall in scores, from 15.7 to 6.1, had no placebo comparison. So it cannot be read as the effect of the drug alone.
Sodium oxybate (Xyrem). A French trial randomised 45 adults with idiopathic hypersomnia to sodium oxybate or placebo for 8 weeks. Sleepiness scores were 6.9 points lower than placebo. Sodium oxybate carries serious warnings, including slowed breathing and abuse potential.
Pitolisant. A phase 3 withdrawal trial in 214 adults did not meet its main goal. After four weeks, the difference in sleepiness scores against placebo was not statistically significant.
Other options. The 2021 American Academy of Sleep Medicine (AASM) guideline strongly recommends modafinil. It makes weaker ("conditional") suggestions for methylphenidate, clarithromycin, pitolisant and sodium oxybate. It came out just before the Xywav results were published. The Cochrane review found too little evidence on clarithromycin.
A 2025 review states that no medicine is approved for idiopathic hypersomnia in Europe.
"Off-label" means a medicine is used outside the uses listed on its licence. Your specialist can explain why they are suggesting it.
We found no National Centre for Pharmacoeconomics (NCPE) or HSE assessment of any medicine for idiopathic hypersomnia. Modafinil is on the HSE reimbursable list. Ask your consultant or pharmacist how your medicine will be covered.
We found no NICE technology appraisal or Scottish Medicines Consortium (SMC) advice for any medicine in idiopathic hypersomnia (checked 3 November 2026).
We searched ClinicalTrials.gov, the EU clinical trials register and ISRCTN on 3 November 2026. Several trials are recruiting, including orexin-receptor agonists (TAK-360, ALKS 2680, ORX750). We found none listing a site in Ireland or the UK. Trials are research, not treatment.
Never drive if you feel sleepy.
No. It is excessive sleepiness, not just low energy. The NHS says hypersomnia is different from feeling tired all the time.
No. NICE describes home respiratory polygraphy as a test for suspected sleep apnoea. It records breathing and oxygen, not the brain activity needed to measure sleep. Diagnosis needs a full sleep study, usually with an MSLT or a 24-hour sleep measure.
Often it is long-term, but not always. In one Swiss clinic study, 5 of 55 people with idiopathic hypersomnia went into remission.
It is not authorised in the EU or the UK. Your specialist can explain the options that are available here.
This article is general information, not medical advice. It cannot diagnose a sleep disorder. Talk to your GP or sleep specialist about your own symptoms and treatment.
Reviewed by: Proco Team
Last updated: November 2026