Due to uncertainties in diagnostic criteria, the exact prevalence of IH is unknown. Based on evidence collected by sleep disorder centers, the prevalence appears to be between one case in 10,000 individuals to one case in 50,000 individuals. IH seems to affects women slightly more than men.

The cause of IH remains unknown. There is some evidence suggesting a few different processes may be involved. Since IH or other similar sleep disorders have been observed to run in families, genetic factors may contribute to this condition. Some studies indicate an autosomal recessive mode of inheritance pattern, which means both parents must carry the gene and pass on the gene to their child. However, an associated gene has not been found. 

There is also evidence showing that brain chemistry may be altered in affected individuals. Gamma-Aminobutyric acid (GABA) is produced by all healthy individuals and reduces the activity of brain cells. GABA activates its receptors in the brain to exert its effects. The activation of GABA receptors induces sleep. In some IH cases, the activity of GABA receptors is enhanced which induces the sleep function of GABA. Studies have shown that in the presence of GABA, the cerebrospinal fluid (CSF) of affected individuals activate GABA receptors more than healthy individuals. CSF is a protective fluid that surrounds the brain in the skull. This suggests that a molecule may be present in the CSF of affected individuals which interacts with GABA and enhances its ability to activate its receptor. 

Affected individuals also have differences in their immune system compared to healthy individuals. For example, IgG is an antibody produced by all humans. Antibodies are proteins made by the immune system that attack foreign particles in the body. Individuals affected by IH have different amounts and types of IgG in their blood. In addition, some affected individuals have increased levels of immune-related conditions such as allergies, inflammatory conditions, or viral illness that may trigger the onset of IH which further suggests the immune system may also play a role. 

There is also evidence suggesting that affected individuals experience different sleep patterns throughout the night compared to healthy individuals. Normally, when healthy individuals fall asleep, they go through different stages of sleep which repeats as a cycle throughout the night. Each stage of sleep is characterized by specific brain waves that are repetitive patterns of neuronal activity. Affected individuals exhibit abnormalities in brain waves as they go through different stages of sleep. For example, these individuals experience a lower intensity of certain waves which suggests they may need prolonged sleep to compensate for this lower intensity. 

The most common symptoms of IH is excessive daytime sleepiness. Longer sleep time at night does not improve sleepiness during the day and naps are unrefreshing. Affected individuals often sleep for longer hours at night and yet experience sleep drunkenness. Sleep drunkenness is difficulty waking up and experiencing an uncontrollable need to go back to sleep. Although healthy individuals may also want to return to sleep after waking up, this transition is much more challenging to individuals affected by IH. 

Many affected individuals also experience a brain fog which is difficulty performing cognitive tasks that require memory, attention, and concentration. Short sleep latency is another symptom of IH which is when it takes a shorter than expected time for the individual to fall asleep. Other symptoms include headaches, hard time regulating body temperature, excessive sweating, cold extremities, fainting or low blood pressure after standing up (orthostatic hypertension). Others may experience hallucinations at the onset or offset of sleep and sleep paralysis or a state between sleep and wakefulness when the individual is awake but cannot move. The onset of symptoms is usually between early teens to young adulthood and the severity of symptoms may fluctuate over time or remain stable.

IH should be suspected in any teenager or young adult that experiences excessive daytime sleepiness despite prolonged night-time sleep or unrefreshing daytime naps and difficulty waking up. Other causes for excessive daytime sleepiness such as medications, insufficient sleep, or other sleep disorders must be ruled out. Sleep tests can be done to look for changes in sleep stages and brain waves and measure the time required for the individuals to fall asleep.

Polysomnography is a test  that is done during nighttime sleep. It is done at a sleep center using sensors that are placed on the scalp, chest, and legs. The sensors monitor heart rate, blood oxygen levels, brainwaves, breathing rate, and eye and leg movement throughout the night. In IH, polysomnography may find increased total sleep time, changes in specific brain waves, and changes in sleep stages. 

The day after the polysomnography was performed, another test called multiple sleep latency test (MSLT) follows. MSLT tests for excessive daytime sleepiness by measuring an individual’s sleep latency or how quickly they fall asleep. This test involves five scheduled fifteen-minute naps separated by two-hour breaks. In individuals affected by IH, MSLT usually finds a short sleep latency.

There are currently no FDA-approved medications for IH but some medications approved for other sleep disorders such as narcolepsy have been used to improve excessive daytime sleepiness associated with IH. Most commonly, wakefulness-promoting agents such as modafinil are used which are drugs that directly target sleep pathways in the brain to promote wakefulness. These drugs work by affecting brain chemistry and influencing the amount of certain chemicals such as dopamine and histamine. Dopamine and histamine are chemicals that are involved in the sleep-wake cycle. Other drugs such as sodium oxybate influence the activity of GABA receptors to promote wakefulness. Another class of drugs that are used in rare cases is psychostimulants. Psychostimulants such as amphetamines increase the activity of the nervous system and one of their effects is increased alertness and wakefulness.

IH can have significant impacts on the social, professional, and personal life of affected individuals. In some cases, symptoms improve spontaneously. However, in most cases, symptoms fluctuate in severity or remain stagnant. In many individuals, medication can cause significant relief from symptoms. However, some individuals may not respond well to treatment. 

Affected individuals can benefit from safety counseling as driving or other safety-critical activities may be dangerous for these individuals. 

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