Most animals need to sleep – or rest – at regular intervals to rejuvenate their bodies, process information from the day and "recharge their batteries". You can usually tell when most animals are sleeping; they close their eyes, lie down, and don't respond to gentle stimuli. Scientists have determined that a part of the brain – the neocortex – shuts down when mammals sleep. The problem with fish is that they don't have eyelids so they can't close their eyes, and they don't have a neocortex.
Which begs the question "Do fish sleep?" And if they do, how, where and when do they do it, and how do we know when they're sleeping? Researchers have determined that although fish don't sleep in the conventional way we think of, most species do in fact go into a restful state where they remain still and experience reduced breathing and metabolic rates, as well as lower brain activity. This promotes good health and disease resistance. So, in essence, fish do "sleep", it's just a different form than what we're accustomed to in other animals.
How Do Fish Sleep?
In most cases, when fish sleep, they remain still, their breathing slows down and some can even be picked up in your hand. Researchers at Stanford University discovered that Zebra Danios sleep much the same way we do. Using state-of-the-art technology, they monitored brain and body activity in the fish, and were able to identify slow-wave sleep and paradoxical sleep (deep sleep), just like mammals, birds and reptiles. The only difference was that during paradoxical sleep they did not exhibit Rapid Eye Movement (REM) like humans and other animals, and of course, they don't close their eyes because they don't have eyelids.
While most fish remain motionless when they're sleeping, certain species of sharks must keep moving, even while at rest, to ventilate their gills. On an even more interesting note, some species of marine Parrotfish and Wrasses surround themselves with a mucus cocoon when the sleep. Scientists believe this slimy "sleeping bag" might help protect them from predators or even parasites.
Where Do Fish Sleep?
Where fish sleep differs from species to species. Some lay on the bottom or even bury themselves in the sand, some hide in caves or under grottos, some hover or drift along motionlessly near the surface, and some nestle into plants, driftwood, corals or other objects and just sort of hang there, occasionally flicking a fin to stay balanced. Seeing our fish laying on the bottom or seemingly "stuck" in an ornament or hollow log is enough to panic any aquarium-keeper, but many aquarium species do this when they're sleeping.
When Do Fish Sleep?
Scientists believe that most fish have regular sleep schedules just like humans and other animals. Most aquarium fish are diurnal, meaning they move about during the day and rest at night. However, some species are nocturnal and prowl at night, spending daylight hours sleeping in a cave or crevice. Certain species of catfish and plecostomus, some knife fish, loaches and others fall into this category. This can pose a problem if you keep small tetras like Neons with large-mouthed catfish such as Pictus cats. While the Neons are huddled on the bottom at night, the catfish are cruising around looking for an easy meal. That doesn't make the Pictus cats mean or aggressive, they're just hungry!
Many fish don't sleep when they're caring for young, and scientists have also discovered that some fish, like Tilapia, don't start sleeping until they are 5 to 6 months old. In addition, wild fish don't sleep when they're migrating. (It's probably safe to say aquarium fish don't migrate unless you move their tank to another room or a new house.) Finally, research suggests that blind cave fish do not sleep, possibly because they're constantly in the dark and don't really know when it's day or night.
How Do I Know If My Fish Are Sleeping?
Signs that your fish are sleeping include:
- They remain motionless for periods of time.
- They lay on the bottom or an object in the aquarium.
- They don't react to things going on around them.
- They exhibit this behavior at about the same time each day, usually when the aquarium light is turned off.
Do Fish Suffer From Sleep Disorders?
Scientists haven't done a lot of research on fish slumber, but in studies on Zebra Danios, it was discovered that when they were deprived of sleep for a few days using a mild electrical current, they slept more when they were returned to a normal day/night cycle and not disturbed. However, when they were kept under constant light for several days, they slept normally when returned to a regular day/night cycle. Researchers think that light might inhibit the sleep-promoting hormone melatonin in Zebra Danios, but much more research is needed in this area.
How Do I Make Sure My Fish Get Enough Sleep?
The easiest way to make sure your fish are sleeping enough is to put your aquarium light on a timer, giving them a consistent day and night routine. Provide plenty of cover in your aquarium as well, so all your fish have a place to feel safe while at rest. If you use an aquarium in a child's bedroom as a night light, turn the light off during the day and keep the curtains drawn so your fish think it's nighttime.
Like all animals, fish need to sleep on a regular basis to stay healthy and live long, happy lives!
