The long-standing puzzle of whether fish can sleep has been solved by a study that has shown that they like a lie-in after a disturbed night.

Like most other species of fish, zebrafish, Danio rerio, do not have eyelids and it has been difficult to establish if they are asleep when inactive or merely resting.

Researchers have now been able to show not only that the fish sleep, but that they can suffer from sleep deprivation and insomnia.

By repeatedly disturbing the fish using mild electric shocks, researchers were able to keep the popular aquarium species awake at night. Those fish that had suffered a disturbed night were found to catch up on their sleep as soon as the opportunity arose.

Some of the fish used in the study had a genetic mutation to the neural receptors for hypocretins, a substance that helps to promote wakefulness. A lack of hypocretins in human beings has been linked to narcolepsy.

Zebrafish with the mutation suffered from insomnia and it was found that the time they were able to sleep was cut by 30 per cent compared with fish without the mutation. “Fish lacking this receptor demonstrate short and fragmented sleep in the dark,” the research team reported in PLoS Biology, an online journal.

The study has given researchers insights into the function of molecules that regulate sleep and they hope further research into zebrafish, which were selected because they have a similar central nervous system to mammals, will help them to understand sleep disorders in human beings.

“Sleep disorders are common and poorly understood. Further, how and why the brain generates sleep is the object of intense speculations. In this study, we demonstrate that a bony fish used for genetic studies sleeps,” the researchers said.

Fish monitored by the research team from the United States and France were observed to have a drooping tail fin and stayed at the surface or bottom of the tank when asleep.

Emmanuel Mignot, of Stanford University in the United States, who was involved in the study, said: “This will likely give us important clues on how and maybe why sleep has been selected by natural evolution and is so universal.”

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