Narcolepsy suffers fall asleep anywhere, at any time of day without warning.

At other times sufferers remain conscious, but their entire body goes limp and they are unable to move.

The condition impinges on every aspect of life, and can make it difficult for sufferers to hold down jobs, or personal relationships.

Now researchers have identified a segment of DNA they believe is responsible for narcolepsy in a breakthrough being hailed as the first significant step forward in understanding the condition.

The genes that have been discovered affect the regulation of the basic process of sleep.

Juliette Faraco, who worked on one of the two studies published in the journal Cell, said the research, although carried out on animals, would lead to the development of new drugs for human sufferers.

Study on dogs

Dr Faraco's team at Stanford University, California, have pinpointed defects that cause Doberman pinschers and Labrador retrievers suddenly to fall asleep.

The defects both appeared in the same stretch of DNA. Among the Dobermans, the defect is an extra piece of DNA that throws the rest of the gene out of alignment. Among the retrievers, a chunk of the DNA is missing.

In both instances, the body's cells can no longer use the DNA to build the protein that is supposed to sense the presence of a hormone, hypocretin2.

Hypocretins play a role in controlling appetite. However, the relationship between sleep and appetite remains a mystery.

Environmental factors

In the second study, researchers from the Howard Hughes Medical Institute in Dallas created mice that were incapable of making hypocretin.

They discovered that the animals could not stay awake. Some were videotaped falling asleep while running on their exercise wheel.

While the research is a significant advance, Dr Faraco warned that human narcolepsy may also involve as yet unidentified genes.