You’ve asked us, “Do fish sleep?”
Well, the simple answer to that is YES; most fish do sleep… although they don’t sleep in the same way land mammals do.
Most fish may reduce their activity and metabolism while remaining alert to danger. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. These periods of “suspended animation” may perform the same restorative functions as sleep does in people.
While most fish don’t have eyelids, they do have a regular period of reduced activity and metabolism. This does not exactly serve the same purpose as a good night’s sleep for you, but research has shown that fish sleep does serve a restorative function, saving energy and refreshing memory circuits in the brain. Though they are in a restful state, many fish are still acutely alert for danger – sleeping, in essence, with one eye open. Others, like the Spanish Hogfish (Bodianus rufus), sleep so soundly, that they can be lifted all the way to the surface before eliciting a response. Some species of parrotfish actually make their own “mosquito net” out of mucus to protect themselves from parasites so that they can sleep in peace.
Some fish don’t sleep
Fish that swim continuously and blind cave fish do not sleep in a traditional sense. That’s because these types of fish may have less need to process sensory information, particularly visual information, and, as a result, their brain does not need to rest and reset in the same way that other fish do. For example, sharks and tunas that are obligate ram ventilators must swim to breathe so they cannot rest or sleep. These sharks and tunas also live in open water pelagic habitats, with very few visual features to register. Schooling fish, similarly, require less sensory processing because most fish are inside the school and rely upon directional information from other individuals. Blind cave fish, by virtue of being blind, do not register visual information at all.
