Narcolepsy, a chronic neurological disorder caused by the brain's inability to regulate sleep-wake cycles normally, is described by the National Institutes of Health thusly: "At various times throughout the day, people with narcolepsy experience fleeting urges to sleep."

There is no cure. But now there's a hint as to the cause.

Researchers have tied it to two immune system genes, suggesting that it's an autoimmune disease. That could lead to treatments. "This study doesn't conclusively prove narcolepsy is autoimmune, but it's a major step in that direction," said Michael Silber, a Mayo Clinic neuroscientist.

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