Many pelagic fish species, such as bluefish, Atlantic mackerel, tuna, bonito, and some sharks, swim continuously and do not show signs, behavioural or otherwise, of sleep. ... Therefore, the role of sleep would be to periodically shut down sensory input to allow the brain to form memories.

How do you know if a fish is sleeping?

It's pretty easy to tell when fish are sleeping: they lie motionless, often at the bottom or near the surface of the water. They are slow to respond to things going on around them, or may not respond at all (see some sleeping catfish here). If you watch their gills, you'll notice they're breathing very slowly.

Is it normal for fish to sleep?

While fish do not sleep in the same way that land mammals sleep, most fish do rest. Research shows that fish may reduce their activity and metabolism while remaining alert to danger. ... These periods of "suspended animation" may perform the same restorative functions as sleep does in people.