Mr. Boxy
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Yes, and no. Fish rest, but whether this is true sleep is debatable. They don't have the sleep states that humans do, as their brains are much simpler.
Fish don't have eyelids, so they never quit seeing when they're in this rest state. Many fish go into a sort of daydream state: they stop moving, but they still remain alert for predators. If you've ever seen a motionless fish floating in the corner of an aquarium, it's in this state. Some species of fish will find an area safe from predators, such as a log or a crack in a coral reef, before taking a nap.
Most fish can stay still when sleeping, but sharks must constantly move to pump water through their gills to absorb oxygen. For example, this shark is opening and closing its mouth to push the water through:
Creepy, isn't it?
How long species of fish sleep varies widely. Some fish sleep whenever there is an opportunity, while other have a regular sleep cycle, with some sleeping at night, and others during the day. Fish can also suffer from sleep-related illnesses. Stanford University did a study on zebrafish with extra hypocretin receptors, the same receptors linked to narcolepsy in humans. These fish exhibited signs of insomnia, sleeping a third less than normal fish, and acting erratically when they did sleep. Regular fish who were kept awake showed signs of sleep deprivation, catching up on rest when they were able to the following day.
 Posted 5430 day ago
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