Scientists at the Stanford University School of Medicine studying the mysteries of sleep have discovered a new model for their research - and it's swimming around their laboratory.

Emmanuel Mignot, the Stanford scientist who discovered the cause of narcolepsy first in dogs and later in humans, has successfully shown that common zebrafish can develop insomnia.

The finding paves the way for future study of humans, who have many of the same sleep-producing chemicals, he said Monday.

"Why we have to sleep is still a big mystery," Mignot said. And scientists have yet to figure out cures for many of the sleep problems that ail an enormous segment of the population.

Insomnia alone disturbs between 10 and 15 percent of adults, while half the population in general has problems sleeping, according to the Palo Alto Medical Foundation's Web site.

But zebrafish may provide some answers. Because they are transparent, researchers can easily observe their internal functioning. And as vertebrates, zebrafish are more similar to humans than fruit flies, a common lab model. Also, because zebrafish are cheap and easy to breed, scientists can pick from thousands of fish to find useful specimens with key genetic mutations, said Mignot and senior research associate Philippe Mourrain.

To determine whether zebrafish could develop insomnia, the scientists had to first figure out whether the fish actually fell asleep. Though Mignot's lab has 1,400 fish tanks with 10 to 20 fish in each, the study used only several hundred zebrafish, which the researchers tried to keep awake by playing music and tapping the tanks.

"The tapping was not so good because they get used to it," Mignot said.

The scientists switched to using a light electrical pulse to keep the fish awake, then tested how much of a current it took to awaken the sleep-deprived fish when they were allowed to nod off. Sometimes not even six volts would wake the fish, who normally leap at a single volt.

"The fish were just fine afterwards," noted Mourrain. "We were not frying them. We love them." In fact, the lab still has the last narcoleptic dog from their former project, which was adopted as a pet, Mignot said.

Among the zebrafish, scientists were able to identify several with genetic mutations that produced insomnia, causing them to sleep almost 30 percent less than other fish. The mutant fish lacked hypocretin receptors, the same deficiency that makes dogs and humans narcoleptic, Mignot said.

The finding enables researchers to use zebrafish as tools for further sleep and behavioral research. Previously the fish were only used to study developmental biology, Mignot said.

"No one has really used this fish to study behavior yet," he said. "We are among the first to do that."

Mignot said zebrafish have "huge potential" to reveal more about "basic wakefulness" and how insomnia and sleep apnea function in humans, as well as on more varied topics such as memory formation and fear conditioning.

Meanwhile, both researchers say they have few problems sleeping at night.

Mignot said he suffers from a bit of sleep apnea, but concluded, "I probably just have to lose some weight."

Mignot's research will be published in today's issue of Public Library of Science-Biology.