How Do Fish Sleep? Marine animals sleep underwater... but how?!

A very influential study from the University of Zurich surveyed over 150 animal species across invertebrates, amphibians, fish, reptiles, birds, and mammals, and found that every animal sleeps, even though some do it very differently from us. Even microbes have a sleep-wake cycle. Though sleeping often involves lying down, marine mammals do it differently.

Dolphins are mammals, and don't have gills, so they have to swim to the surface to breathe. They actually maintain a breathing cycle by  shutting off one of the hemispheres of their brain (and closing the opposite eye) to enter unihemispheric slow-wave sleep. Though different species of dolphin can sleep differently. Some dolphins perform a behavior called "logging," a deep sleep where the animal floats at the surface like a log, allowing them to breathe and rest. Others, like the river-dwelling Indus dolphins "never stop swimming," and sleep for seven hours that way, sometimes only for seconds at a time.

Fish on the other hand, don't close their eyes -- they don't have eyelids -- and also don't have signs of rapid-eye-movement sleep; nevertheless, they do sleep... Studies done recently in Behavioral Brain Research, PLoS Biology and Nature, determined zebrafish have slower breathing cycles in sleep, take naps, can suffer from sleep deprivation, and are governed by melatonin (just like humans) which regulates their sleeping cycle based on ambient light.

The National Sleep Foundation describes fish snoozing as a "daydreaming state," where their metabolic rate decreases allowing the body to restore itself. Though how that manifests is obviously going to be different across species. The parrotfish, for example, covers itself in a cocoon of mucus. The exact reason is unclear, but they think it might be to protect from tiny parasites while it dozes off.

Sharks have to keep water flowing through their gills to breathe, so it's true that some sharks never stop swimming, but others can stop swimming to sleep without suffocating! Some very ancient sharks use buccal pumping to force water over their gills while sitting still. It's sort of like they're swallowing water constantly using their cheek muscles -- keeping air flowing and allowing them to be inactive for a time. But not all species do this: some have to swim constantly, and they'll enter a period of inactivity for a few minutes at a time throughout the day.

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