A study asks first-, second-, and third-degree relatives if they experience sleep paralysis, cataplexy, and other symptoms.

It’s known that narcolepsy is a genetic condition, but some sleep researchers have wondered if specific symptoms of this sleep disorder are more commonly shared between non-narcoleptic relatives than others. A better understanding of how sleep problems manifest within families could help physicians keep an eye on people who might otherwise not get the care they need.

This is one reason why researcher Maurice Ohayon, MD, DSc, PhD, set out to investigate narcoleptic symptoms within families, looking at the rates of hypersomnia, sleep paralysis, and hypnogogic hallucinations.

“This is something that has been interesting to me for many years; I suppose a lot of people in the world who specialize in narcolepsy were interested in this question,” says Ohayon, a professor of psychiatry and behavioral sciences at Stanford University.

After more than a decade of research, Ohayon and his team published a study in July in the journal Sleep Medicine that looked at narcolepsy symptoms in first, second, and third-degree relatives in multiplex and simplex families. Multiplex families are defined as those in which more than one person has narcolepsy; simplex families are those in which only one person has narcolepsy.

To conduct this research, the team worked with 4,045 family members (who were interviewed at least once) and 362 people with narcolepsy. The researchers interviewed 3,255 family members twice, 5 to 7 years apart, using the Sleep-EVAL system, a computerized tool designed for the assessment of sleep disorders.

The results showed that some relatives of narcoleptic patients are at greater risk of a broad range of symptoms, including a higher prevalence of sleep paralysis in first-and second-degree relatives in multiplex families. Prevalence of hypnagogic hallucinations was comparable between multiplex and simplex families, but all were at least 4 times higher than the control group, according to the study “Longitudinal Study of Narcolepsy Symptoms in First, Second, and Third-Degree Relatives of Simplex and Multiplex Narcolepsy Families.” These relatives may benefit from close follow-up.

The biggest surprise, says Ohayon, was the finding that in relatives of people with narcolepsy, rates of cataplexy-like symptoms were generally low. Cataplexy was only reported in family members who had a diagnosis of narcolepsy.

The research also found that family members of people with narcolepsy had higher incidence of hypersomnia, the symptom most commonly shared among non-narcoleptic family members. This causes people who sleep normal or excessive hours to still feel sleepy during the day.

It’s a symptom that could be easily missed by primary care providers or written off as a mental health issue, says Ohayon. “We must treat these people. We must treat the family member when they have hypersomnia,” he says.

By looking more closely at sleepiness in the family members of people with narcolepsy, physicians will have more of an opportunity to address sleep issues in non-narcoleptic people who are also suffering, says Ohayon. “Hypersomnia is neglected by the majority of people. It is a phenomenon that impacts a large part of the American population,” he says. “Sleepiness is a big problem in America and probably needs more attention from physicians.”

Lisa Spear is associate editor of Sleep Review.