New research from sleep scientists has shown that, contrary to popular belief, and despite their lack of eyelids, fish do sleep and some even suffer from insomnia.
Scientists at Stanford University School of Medicine studied Zebra danios, Danio rerio, in aquariums and recorded videos of their nocturnal behaviour using special night-vision cameras.
After watching hours of footage, researcher Tohei Yokogawa confirmed that the fish droop their tails during the night and sit motionless, either just under the water surface or on the floor of the aquarium.
Up all nightTo determine whether the resting fish were actually asleep, Yokogawa, needed to identify whether they experienced a process called "sleep rebound", a drive to get forty winks when suffering from tiredness.
In order to test this, it was necessary to deprive the fish of sleep to make them tired.
Tapping on the aquarium glass and playing loud noises into the aquarium through an underwater speaker failed to prevent the fish nodding off, but applying a "gentle electrical pulse" to the water did keep them awake.
Once the researchers had found the key to keeping the fish awake they developed a computerised system to stimulate the fish each time they tried to nod off.
When the sleep-deprived danios were returned to a peaceful, dark aquarium they compensated for their over-tiredness by taking longer naps.
"Originally, we didn't have the automated sleep-deprivation system, so I manually sleep-deprived them, becoming sleep-deprived myself," Yokogawa said.
The team's findings are due to be published later this month in the Public Library of Science-Biology journal.
Model organism for sleep scienceThe findings have allowed the scientists to use Zebra danios, or Zebrafish as researchers call them, to make new discoveries about the processes involved in sleep.
This is good news for the sleep scientists, because unlike mice and dogs, which have been used in previous studies, Zebra danios are quick, easy and inexpensive to breed in large quantities.
And, when certain types are identified that carry a certain sleep disorder, specific lines of fish carrying the trait can be produced to make studying the disorder much simpler.
"The fact that zebrafish larvae are transparent means you can look directly at their neuronal network, even in living fish," said Emmanuel Mignot, the lead author of the paper.
"The idea is to try to use this as an entry point to understand the neurobiology of sleep regulation."
InsomniacMignot's laboratory had previously found a gene which causes narcolepsy in Dobermans and Labradors in 1999, and found that neurons in the brain's hypothalamus, a part of the brain controlling behaviour, secreted a neuropeptide called hypocretin.
Dogs suffering from narcolepsy lack a working receptor for hypocretin, which means they can suffer from drowsiness and disturbed sleep.
The team managed to identify Zebra danios sharing the same kind of mutation seen in narcoleptic dogs, which means they'll be able to study the processes in narcolepsy much more easily.