Researchers have shown a malfunction in the immune system is likely to be a factor in some cases of narcolepsy.
Narcolepsy is a sleep disorder
People with narcolepsy fall asleep anywhere, at any time without warning. It has been linked to hormones and nervous system activity.
Researchers from Australia's Flinders Medical Centre found antibodies from narcolepsy patients triggered the disorder in mice.
It is hoped The Lancet study may lead to better tests and treatments.
The researchers induced narcolepsy-like symptoms in the muscles of mice by injecting them with antibodies from the blood of nine people with the disorder.
Mice injected with antibodies from nine people who did not have narcolepsy did not develop such symptoms.
Antibodies are produced by the immune system to attack infections and other foreign invaders in the body.
The research suggests that in some narcolepsy patients the immune system produces antibodies which inflict damage on the brain tissues, and trigger symptoms of narcolepsy.
Lead researcher Professor Tom Gordon said: "Our findings provide the first direct evidence that autoimmunity plays a role in this debilitating yet fascinating neurological disorder.
"At present there is no simple diagnostic test for narcolepsy and the pharmacological management is often unsatisfactory.
"Identification of the precise neural target of the autoantibody will lead to improved diagnostic testing, and development of methods to neutralise the autoantibody are likely to open up new possibilities for management."
Also writing in The Lancet, Dr Merrill Wise, of Baylor College of Medicine, Houston, said the findings, if replicated in larger studies, could be significant.
He said narcolepsy was linked to a loss of a particular type of brain cell called a hypocretin neuron. The hypocretin system is known to play a key role in controlling sleep and wakefulness.
The new study suggested that the antibody produced by the malfunctioning immune system might trigger a reaction which led to the destruction of these cells.
It could pave the way for a new test to identify people at risk, so action could be taken to prevent the loss of hypocretin neurons before they fell to a level low enough to trigger narcolepsy symptoms.
Dr Andrew Cummin, of the Sleep Laboratory at London's Charing Cross Hospital, said: "This is potentially very exciting work but will need confirmation.
"The finding of this antibody may be the missing link which explains the cause of narcolepsy and may lead to a simple blood test for the condition.
"It also raises the possibility of new treatments directed at the underlying cause. In the future, it might even be possible to prevent narcolepsy developing."