New genetic research may help explain the unusual and distressing sleep disorder narcolepsy.
Narcolepsy is a rare condition which causes extreme daytime sleepiness, impaired vision and sudden muscle weakness where sufferers suddenly fall asleep without any warning during a normal activity.
Some narcolepsy sufferers also 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 condition occurs in 1 in every 2,500 people in the United States and Europe, but is at least four times more common in Japan.
Japanese researchers from the University of Tokyo have now found a genetic variant linked to a much higher risk of narcolepsy which is associated with the genes involved in regulating sleep, and the scientists say their finding could help explain the causes of narcolepsy.
Though the causes of narcolepsy remain unclear, some scientists believe they involve around a shortage of a chemical called hypocretin which sends signals to the brain about sleeping and waking up and there is strong evidence that the condition can run in families.
In their search for the genetic differences which may be involved the research team analyzed the genomes, or DNA, of 222 narcoleptic Japanese and 389 others who did not have that condition.
The scientists also examined the gene variant in 424 Koreans, 785 people of European descent and 184 African Americans.
They found one gene variant occurred with significant frequency among those with narcolepsy and it was linked to an 79% higher chance of narcolepsy in Japanese people, and a 40% increased chance in other ethnic groups - it occurred with significant frequency among narcoleptic Koreans, but the association was not evident in the Europeans and African Americans.
The culprit gene is found close to two genes, CPT1B, and CHKB, which have already been identified as candidates for involvement in the disorder as they both have a role in regulating sleep.
The researchers believe that while current treatments focus 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.
They say they hope that their discovery might contribute to the establishment of novel therapeutic approaches.
Professor Katsushi Tokunaga of the department of human genetics at the University, says the identification of this gene variant could help experts in their search for a treatment for the disorder.
Professor Tokunaga says why this gene variant occurs with such high frequency among narcoleptic Japanese and Koreans is a mystery - it may be selection, or simply chance.
The researchers hope that the discovery of the present susceptibility gene(s) might contribute to the establishment of novel therapeutic approaches.
The research is published in the journal Nature Genetics.