A new study shows that insomnia also has a genetic background.
Millions of people in the United States struggle with the disorder, and approximately 10 percent of the country's population are estimated to have chronic insomnia.
A team of researchers - led by professors Danielle Posthuma, of Vrije Universiteit (VU) in Amsterdam in the Netherlands, and Eus Van Someren, of the Netherlands Institute for Neuroscience - set out to identify the genetic factors for reported insomnia. To this end, they carried out a genome-wide association study (GWAS), together with a genome-wide gene-based association study (GWGAS).
The findings were published in the journal Nature Genetics.
During a GWAS, researchers quickly scan for genetic markers in complete sets of human DNA in an attempt to find genetic variations typical of specific illnesses.
A GWGAS focuses more narrowly on genes and examines the correlations between genetic variations within a single gene. For this reason, a GWGAS may be a more powerful testing method in cases where the number of genetic variations increases, or when only "the cumulative effect" of several genetic variations correlates with a disease.
Insomnia shares genetic background with other illnesses
In this new study, the scientists performed these genetic analyses in 113,006 people. The researchers found three genetic loci and seven genes for insomnia.
Additionally, they found that some of these genes have previously been associated with other conditions. The strongest genetic association for insomnia was also involved in restless legs syndrome (RLS).
Other conditions whose genetic underpinnings overlapped with those of insomnia included anxiety disorders, depression, neuroticism, and poor overall well-being, as experienced subjectively and self-reportedly.
"This is an interesting finding, because these characteristics tend to go hand in hand with insomnia. We now know that this is partly due to the shared genetic basis," says first author Anke Hammerschlag, a neuroscientist and Ph.D. student at VU.
More specifically, the MEIS1 gene had been associated with RLS and another sleep disorder called periodic limb movements of sleep (PLMS).
RLS is a condition affecting up to 10 percent of the U.S. population and is characterized by the irresistible urge to move the leg, which becomes worse during the night and causes sleepiness and exhaustion during the following day.
The researchers found that PLMS, RLS, and insomnia all share a variant in the same MEIS1 gene. Interestingly, the authors point out, PLMS and RLS are both characterized by physical restlessness, whereas insomnia is defined by restlessness of the mind.
Prof. Eus Van Someren comments on the significance of the study:
"As compared to the severity, prevalence, and risks of insomnia, only [a] few studies targeted its causes. Insomnia is all too often dismissed as being 'all in your head.' Our research brings a new perspective. Insomnia is also in the genes."
The study also identified genetic differences between sexes. Prof. Posthuma explains, "We found a difference between men and women in terms of prevalence: in the sample we studied, including mainly people older than 50 years, 33 percent of the women reported to suffer from insomnia. For men this was 24 percent."
"This suggests that, for some part, different biological mechanisms may lead to insomnia in men and women," Prof. Posthuma concludes.