There is no cure for narcolepsy
Scientists have uncovered genetic evidence suggesting the sleep disorder narcolepsy is linked to a fault in the immune system's "foot soldier" cells.
It suggests these T-cells may cause the condition by attacking cells in the sleep centres of the brain.
Narcolepsy, which causes extreme daytime sleepiness and sudden muscle weakness, has previously been linked to a malfunctioning immune system.
The Stanford University research appears in the journal Nature Genetics.
Narcolepsy is a mysterious, uncommon condition that can be very distressing for those who have it.
It can trigger "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.
There is currently no cure for narcolepsy, only ways to minimise symptoms such as taking frequent, brief naps evenly spaced throughout the day.
The condition has previously been linked to depletion of cells deep in the regulatory regions of the brain.
But lead researcher Dr Emmanuel Mignot said while previous research had only suggested a link with a fault in the immune system, the latest study provided firm evidence.
The Stanford team carried out an extensive genetic analysis to identify specific areas of the genome which appeared to be linked to the condition.
They pinpointed three specific genetic variants in the same gene in people with European and Asian ancestry that appeared to be associated with an increased susceptibility for narcolepsy.
The gene in question plays a key role in the functioning of an important receptor used by T-cells to recognise foreign proteins in the body.
The only previous genetic variant linked to narcolepsy was in a gene which also plays a role in T-cell receptors.
Dr Mignot said: "Narcolepsy is probably the result of a series of unfortunate events, starting with genetic predisposition, involvement of an environmental trigger such as an infection, then T-cell activation, then effects on many other arms of the immune system."
He said the latest study raised the possibility of developing a therapy for narcolepsy which worked by blocking activity of the specific T-cell receptors.
Identifying the genetic variants may also provide a diagnostic tool to pick up narcolepsy at an early stage.
Quality of life
Dr Mignot added: "I believe that narcolepsy could be a very interesting model for studying autoimmune diseases that affect the brain.
"Very few such diseases are known, and I believe that we will find many more brain autoimmune diseases in the future that cause neuropsychiatric disorders for example."
UK sleep specialist Dr Renata Riha, from Edinburgh's Royal Infirmary, said the idea that narcolepsy was linked to an immune system attack on brain cells was gaining ground, with evidence that many patients shared the same genetic variants controlling immune system function.
She said: "There is a strong immunological link to the development of narcolepsy."
But she added: "Disease expression is rarely the result of one single factor.
"The condition is disabling and if severe can be most distressing.
"Much research has been done to show that patients' with narcolepsy have lower self-perceived quality of life comparatively speaking and increased incidence of depression."