Genetic research could shed light on what is happening in people with the mysterious sleep disorder narcolepsy.
The condition causes extreme daytime sleepiness, and sudden muscle weakness.
Japanese researchers found a genetic variant linked to a much higher risk of narcolepsy, publishing their results in the journal Nature Genetics.
It is linked to genes involved in regulating sleep, and the scientists say their finding could help unravel narcolepsy's causes.
The condition is an uncommon and distressing one - people with it can suffer "sleep attacks" without any warning during any normal activity.
In addition, some people can experience "cataplexy", where strong emotions such as anger, surprise, or laughter can trigger an instant loss of muscle strength, which, in some cases, can cause collapse.
The causes are still not completely clear, although some scientists believe they revolve around a shortage of a chemical called hypocretin which sends signals to the brain about sleeping and waking up.
There is strong evidence that the condition can run in families, so the University of Tokyo team are looking for the genetic differences which may be involved.
They looked at the genetic code of hundreds of volunteers, some with narcolepsy, some without, to look for differences.
The variant they found was linked to an 79% higher chance of narcolepsy in Japanese people, and a 40% increased chance in other ethnic groups.
It is found close to two genes, CPT1B, and CHKB, which have already been singled out as candidates for involvement in the disorder - as they both have a role in regulating sleep.
The researchers said they believed that while current treatments focused on dealing with the symptoms of narcolepsy, their genetic find could help point the way to understanding the underlying mechanisms responsible - in particular what was causing the shortage of hypocretin.
"We hope that the discovery of the present susceptibility gene(s) might contribute to the establishment of novel therapeutic approaches," they wrote.
UK sleep specialist Dr Renata Riha, from Edinburgh's Royal Infirmary, said the variant was unlikely to provide the sole explanation for narcolepsy in patients.
She said: "The findings are very interesting but preliminary - the exact function of the genes has been largely determined in mice, which may not necessarily translate to human biology.
"This suggests that they as individuals may be more susceptible to the development of narcolepsy, but in no way explains why others develop the disease and why some of those carrying the abnormal allele (variant) don't."