We know mammals love to get some shut-eye but do fish sleep as well? If you've invested in the best fish tank and have found yourself spending hours watching your finned friends moving about, it's a question that you may have found yourself pondering. And the general consensus is that, yes, they do. Even if they are not sleeping in the exact same way as humans and other mammals, we certainly know they like to get some rest!
One of the biggest challenges in assessing how fish sleep is their lack of a neocortex. When humans sleep, this area of the brain becomes less active so sleep patterns are clear to see. But it's not possible to check this out in fish so scientists look for other patterns, such as how their metabolic processes slow and activity reduces. There is also evidence of waves in the dorsal pallium part of a fish's brain.
Of course, sleep can make a fish more prone to predation so they remain alert to danger while also seeking safe spots where they can. One fish – the parrot fish – goes a little further than that and (gross alert!) spends an hour creating a mucus cocoon around itself. This offers a level of protection from parasites.
Below, you'll find everything you've ever wanted to know about the sleep patterns of fish, including how to know if your fishy friend is pushing up zzz's and how long they tend to sleep for. Let's dive in...
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How do you know if a fish is sleeping?
It's not easy to determine if a fish is sleeping because they're not tucking themselves under covers, snoring loudly or even closing their peepers (simply because they don't have eyelids!).
But they do become less responsive and, in some cases, you can clearly see that they are simply not moving. If you're staring at a fish for a few minutes and they haven't stirred, chances are they are asleep. Likewise if you're trying to feed a fish and they don't immediately respond, then they're likely to be at rest.
A fish's position also generally indicates a period of sleep. There's no set “bed” for every fish but if you see one motionless at the bottom of their tank or floating towards the top (usually with their bodies tilted upwards by a small degree), then they are generally sleeping. And if you look really closely at their gills, then you may see that their breathing has significantly slowed.
But how are they ensuring water flows over the gills so that they can take in oxygen to pass through their body? Well, the gills are moving at just the right speed to ensure the fish will remain alive during this restful period.
In some cases, they will also use their pectoral fins which are just behind the gills to ensure a sufficient movement of water. With all that said, some fish (pelagic species such as tuna and sharks) do need to keep swimming while they sleep and, during this time, they shut down half of their brain at a time to ensure they get sufficient rest.
When do fish sleep?
Fish are very much like humans. They have an internal clock and display circadian rhythms. Many are also diurnal which means they sleep more during the night than the day. Some such as catfish, knife fish and loaches are nocturnal, however, but it nevertheless shows that they slip into a pattern.
What this means, though, is that the majority of those little swimmers are getting some sleep when the lights are turned off and the household is going to bed. If you want to see this with your own eyes, you'll have to get up in the middle of the night and take a look inside the tank – you'll find the fish are not moving.
For this reason, you should help your fish stick to their sleep cycle by keeping the lights off at night (saving yourself some money in the process!). By keeping the lights on, you could end up depriving your fish of some sleep, forcing them to catch up at some other point which isn't healthy over a period of time and not advisable.
How long does a fish sleep?
Most of what scientists know about fish sleep patterns comes from studying zebrafish. They have a nervous system and genetics similar to humans and it's also possible to look at their brain cells via a microscope because their skin is translucent.
Through such studies, scientists have found evidence of sleep patterns similar to our own including slow-wave sleep rapid eye movement, or REM. They also sleep for roughly the same amount of time: about 10 hours (well, roughly the same if we get a nice lie-in at the weekends!).
Can fish sleep upside-down?
If you've ever owned a loach fish, then you may have become alarmed at seeing it resting upside down. But this fish is known to sleep (and even swim) in this position and it's more likely to do so when it feels comfortable in its environment.
Lots of other fish will also sleep on their side, such as the Betta fish and you should only really become concerned if fish are upside down or on their side for too long a period of time because it can be a sign of illness.
It's also worth pointing out that most fish will sleep upright. A key test to determine if something is wrong is to see how easily they are stirred and become responsive again. That said, never startle a sleeping fish. Just drop some of the best fish food in the tank or switch on the light.
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David Crookes has been a journalist for more than 20 years and he has written for a host of magazines, newspapers, websites and books including World of Animals, BBC Earth, Dogs and Canines, Gadget and The Independent. Born in England, he lives in a household with two cats but he’s also keenly interested in the differences between the huge number of dog breeds — in fact, you can read many of his breed guides here on PetsRadar. With a lifelong passion for technology, too, he’s always on the lookout for useful devices that will allow people to spend more time with their pets.