Sleep varies greatly across species and can be very different to what we humans experience. So what’s it like for fish?
Most research on the topic has involved mammals and has shown that the brain’s electrical activity is distinct during sleep. Fish do not produce this kind of electrical activity, which has led us to believe that they do not sleep.
Sleeping with both eyes open
There is one way to tell that an animal is sleeping: its eyes are shut. But fish don’t have eyelids! So we have to look for other signs. We know that, like all animals, fish have periods of rest during which they react more slowly to sensory stimulation. Many fish stop moving, while others burrow into the sand for the night. One thing is clear: their rest period looks nothing like that of humans.
Sleeping and seeing
For fish, the most significant brain activity is reserved for processing visual information. Blind fish, which live in caves, do not have a rest period because they have no visual information to process.
Sleeping and swimming
Some fish can rest without swimming, while other must swim non-stop. Species without spiracles need to keep swimming continuously. Spiracles are openings on the side or top of the head of certain fish, which allow water to reach the gills so they can breathe. Fish without spiracles must be moving at all times. This is the case for most sharks. As they swim, sharks take in water through their mouths; as it exits through their gills, the oxygen is retained and the carbon dioxide is expelled. Even when asleep, sharks keep swimming. If they were to stop, the water would no longer flow through their gills and they couldn’t breathe.
Do fish dream?
Dreams occur during rapid eye movement (REM) sleep; in mammals, this phase is characterized by neural activity similar to that during waking hours, but it does not occur in fish. Current research therefore seems to indicate that fish do not dream. One day, more advanced research techniques may prove otherwise!