Yes, fish sleep. Fish sleep is the equivalent of lower brain function, despite basic external appearances that your fish might still be awake. Sleep is regulated very similarly to humans and other mammals, as shown in studies on zebrafish.
How Do Fish Sleep?
Unlike terrestrial vertebrates, within an aqueous environment, it is not possible for fish to rest in one place. Depending on their buoyancy and local currents, any fish that stops swimming and turns off their local awareness is at risk for drifting away from safety and turning into an easy snack.
Some species, such as the triggerfish, can anchor themselves into a crevice using specialized fin adaptations that lock them into place. Those fish that swim in schools can rely on their nearby neighbors to alert them to incoming trouble if necessary.
Sleep state in fish can be considered “low power mode.” Fish are still able to maintain breathing and minor external movements. Fish breathing strategies are broken into ram and pump ventilation. Ram ventilators, commonly many open ocean shark and large fish species, have to swim to pump water over their gills.
Sleep in these species requires forward movement or else they will not be able to breathe and likely sink to the bottom of the ocean. Pump or buccal ventilators, such as most reef and common aquarium fish species, open and close their mouths to breathe, meaning they can rest in one place provided they continue to open and close their mouth.
How Can You Tell When a Fish Is Sleeping?
Since they are not all tucked into bed to sleep, knowing when your fish is asleep and providing adequate night time is essential to the overall health of your fish. Studies have actually shown that fish can show signs of sleep deprivation just like in other vertebrate animals.
It is critical to provide your aquarium with a diurnal light and dark period in order to allow your fish to sleep soundly. You can use ambient lighting, a timed aquarium light, or turn your lights on and off manually in order to provide your fish with at least 8 hours of darkness.
Depending on the species in your aquarium, dark periods may vary. And not all species will appreciate bright lighting. Be sure you do your research before you potentially blind your lower-light species.
Fish do not have eyelids, so you cannot use closed eyes to tell if they are sleeping. Eyelids in terrestrial animals are responsible for keeping corneas lubricated, which isn’t an issue underwater, when eyes are constantly moist. So, just because your fish’s eyes are open, your fish may not be completely awake. They may be swimming and breathing, but they might not be aware of much else around them.