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Fish may not have eyelids, but they do sleep, and some even suffer from insomnia.

California scientists studying sleeping disorders in humans found that some zebrafish, a common aquarium pet, have a mutant gene that disrupts their sleep patterns in a way similar to insomnia in humans.

Zebrafish with the mutant gene slept 30 per cent less than fish without the mutation. When they finally drifted off, they remained asleep only half as long as the normal fish, the researchers at the Stanford University School of Medicine said.

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The mutant fish lacked a working receptor for hypocretin, a neuropeptide that is secreted in normal fish by neurons in the region of the brain that controls hunger, sex and other basic behaviour.

Zebrafish, also known as zebra danio, have become popular research subjects because they are cheaper to breed than mice and they have a backbone that better represents the human nervous system than fruit flies.

The researchers, led by Emmanuel Mignot, said they would look for fish that have a mutation that causes them to oversleep or never sleep in the hope of discovering if sleep-regulating molecules and brain networks developed through evolution.

"Many people ask the questions, 'Why are we sleeping?' and, 'What is the function of sleep?'" Mr. Mignot said. "I think it is more important to figure out first how the brain produces and regulates sleep. This will likely give us important clues on how and maybe why sleep has been selected by natural evolution and is so universal."

The study was published in Tuesday's edition of the Public Library of Science-Biology.

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