As humans, our bodies require sleep for optimal health and wellbeing.
Sleep is not just essential in restoring and rejuvenating our bodies, but also for solidifying and consolidating information.
It is also easy to determine when humans are asleep, however with fish, this is slightly more difficult.
In this article we’re going to look at whether fish actually sleep, how to determine if fish are sleeping, the way in which different species of fish sleep, and how often fish sleep.
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Do Fish Sleep?
The very fact that the dictionary definition of sleep requires the animal to have their eyes closed, and part of the brain (the neo-cortex) must be shut down makes it difficult to answer the question ‘Do fish sleep?’ with a yes.
Fish do not have eyelids, nor do they have a neocortex. However, it’s important to also look at their behavior and not just their anatomy.
Just because fish don’t have the same anatomy as mammals, doesn’t mean they don’t need rest or “sleep”. It’s just a different type of ‘sleep’ to the kind we experience as humans.
A study carried out by the University of Zurich, evaluated and studied sleep in almost 200 species of animals including fish. It found that whilst the way in which animals sleep varies widely, all animals DO sleep in their own specific way.
Most animals, but not all, need time to rest and slow down their bodies. Whilst fish might not fall into the exact dictionary definition of what sleep is to us – they do still have periods of inactivity and slowing down which is their version of sleep.
What is noted in fish sleep is a reduced rate of movement, and a slower heartbeat. Both of these signs indicate that their metabolic rate has reduced and that they are conserving energy.
In the study mentioned above, sleep was defined using the definition which was first stated by a French psychologist in 1913. The definition of sleep was made up of the following criteria:
- A species specific sleep posture (e.g. lying down for humans, or sleeping in a cave for certain species of fish.)
- The maintenance of behavioral quiescence (inactivity or slowing down)
- They are difficult to arouse
- The state is reversible (they can be woken up again)
Using this definition, it is much easier to determine that yes, fish do in fact sleep.
How Do Fish Sleep?
Whilst it’s very easy to spot when most mammalian creatures are sleeping, it is a little more difficult at first sight to spot whether fish are sleeping.
Fish don’t close their eyes – they don’t have eyelids, and they also don’t show signs of REM (rapid eye movement) sleep.
Instead, fish become inactive; how inactive they become depends on the species of fish. Most fish hover or stay still and their gill movements slow down.
The majority of fish stay alert to danger even whilst they are sleeping, so they can make a quick escape if they need to.
In an aquarium, you’ll be able to tell when a fish is sleeping because it will have either slowed down or stopped moving all together, entering a hovering day-dreaming like state.
Why Do Fish Sleep?
We know the very basics of why we need sleep, and that is to give bodies the chance to rest and repair.
In human sleep, our eyes close, muscles relax, and we go through different stages of sleep; slow-wave sleep (deep restful sleep) and REM sleep (rapid eye movement sleep in which we sometimes remember our dreams).
It appears that the reasons humans sleep are much more complex to the reasons a fish sleeps, although research in this area is not extensive enough to say definitively.
The most likely and simple reason that fish sleep is to rest their bodies.
When we humans don’t have enough sleep, it starts to impact our ability to function properly, a similar thing has been observed in fish.
In a 2007 study, a group of zebrafish were sleep deprived during their normal 6 hour period of rest, the next day the zebrafish were much harder to arouse and their mouth and gill movements were reduced. The study also showed that they have slower breathing cycles in sleep, take naps and are governed by melatonin – very similarly to us as humans.
The National Sleep Foundation describes fish sleep as a day-dreaming state where their metabolic rate decreases which allows the body to restore itself.
When Do Fish Sleep?
Fish rely mostly on their eye sight to keep them safe and find food. Whilst their sense of smell is good, water currents can interrupt this sense and make it less reliable.
Most surface fish are diurnal, meaning they are active during the day and sleep at night. Due to their eyesight, it therefore makes sense that they are active during the day when they can see food easily, and sleep at night when their eyesight isn’t effective.
When fish sleep really depends on their environment and the type of species they are. For example, fish that live near coral reef tend to stay active during the day and rest at night to avoid predators.
Whereas some fish don’t even begin sleeping until they’ve reached adulthood, for example Tilapia don’t start showing any signs of sleeping until around 22 weeks old.
Most aquarium fish sleep when the lights have been switched off during the night.
Do All Fish Sleep In The Same Way?
The way a fish enters a ‘dream-like state’ varies depending on its species, where it lives and their activity level.
Some fish nestle into the substrate; others hide in caves or amongst coral while others just drift with the occasional flick of a fin to keep them steady.
Certain types of fish may look to mimic very similar sleeping patterns to those of humans.
The Parrot fish has a very interesting sleeping ability. It secretes mucus which surrounds its body providing it with a cocoon-type outer layer. This protects them from predators while they are resting.
Some fish, such as sharks, need to keep constantly moving in order to allow water to keep passing over their gills to extract oxygen. Whilst other sharks can stop swimming using buccal-pumping which forces water over their gills as they stay still.
Other fish wedge themselves between rocks or corals to keep themselves hidden and out of the sight of predators.
Brown bullhead catfish rest in 10-20 degree angles along the substrate with their tails flat and stretch out fins.
Some species of fish such as the mackerel and the bluefish show fewer signs of sleep. Even though they are less active during the night, they remain responsive to external stimuli and swim constantly.
There are even some fish that appear not to sleep at all such as tuna and some sharks.
Whilst the research into sleep in fish is still in its infancy, it is thought that the reason some fish appear not to sleep might be due to that fact that their scenery doesn’t change that much and they therefore don’t have much to process or consolidate into memories.
Observing sleep in large schools of fish out in the wild is also interesting. Part of the group will act as the eyes for the whole school while some fish get their rest, and then they switch over.
How to Tell If Fish Are Sleeping
The four main signs that fish are sleeping include:
- Inactivity for an extended period of time.
- A resting posture.
- A routine (sleeping at a similar time, in the same way).
- Decreased sensitivity to external stimulus.
You’ll be able to tell if the fish in your aquarium are sleeping by using the above criteria.
They might be drifting slightly, but they will look almost zoned out. Try not to disturb your fish while they are resting as this can startle them which can lead to stress.
Here are a few videos of popular species of fish sleeping:
Goldfish Sleeping (Click for Video)
Goldfish usually sleep when the lights of the aquarium are turned off, in a dark room. They are diurnal meaning they are active during the day and sleep at night.
Guppies Sleeping (Click for Video)
Guppies prefer sleeping in the dark and so sleep at night when all the lights are out. You’ll be able to spot if your guppy is sleeping if it is hovering in one place (usually just above the substrate). Sometimes they float at the surface – be aware that if this happens in the day time they are most likely not sleeping and it indicates a more serious problem.
Bettas Sleeping (Click for Video)
Some bettas float in one place while they’re sleeping, some like to rest on a surface such as gravel or a plant. Their gill movement will be significantly reduced while they are sleeping.
So, Do Fish Actually Sleep?
Whilst it is clear that fish sleep in a different way to most mammals, it is evident that they require periods of rest in which their bodies slow down, and it is undeniable that fish do enter a sleep-like state.
From the many examples we’ve given you in this article you should now be able to see that the way in which different species of fish sleep differs drastically.
From continuing to swim whilst sleeping, to floating close to the substrate, to hiding in caves or coral – fish have many different ways of sleeping, but the majority of them require switch-off time to allow their bodies to rest.
We’re all looking forward to more research being carried out into fish sleep to see exactly what is going on behind the scenes. Things are rarely as simple as they look.
Do you have any interesting stories about fish sleep? We’d love to hear from you in the comments section below…