Do fish sleep?

Do we know if fish need sleep and if they actually sleep? They do not close their eyes, but is that a clue?
A brief overview of the (still fragile) knowledge on this subject...

What do we know about the question of sleep in fish?

12% of French households have an aquarium, and the number of fish in these aquariums is estimated at about 30 million.

We can therefore assume that these animals are well known, studied, and understood.

 

And yet, on this subject as on so many others, scientific knowledge is close to zero.

These animals, so numerous in our homes and around us, are very poorly known.

I will therefore try to summarize the current knowledge on the question.

Yes, fish do sleep. To date, no animal species is known that does not sleep.

But their sleep is, at the very least, extremely different from ours.

 

Various experiments have established that fish do have rest phases that can be called sleep.
When a fish "sleeps," it is lethargic, its metabolism slows down, and a daphnia can pass right under its nose without it reacting, which leaves no doubt!

Scientists have even managed, on unfortunate danios, to detect sleep cycles very close to those of mammals and therefore humans.

Like many, the Platy variatus, a very hardy fish in aquariums, also shows easily observable rest phases.

 

But this sleep takes various forms depending on the species.

Fish generally sleep at night, but not all.
Some, like the moray eel, wait precisely for the night when others sleep to go hunting, taking advantage of their lethargy.

Generally, fish sleep is associated with a certain stillness, but not always!
Sharks do not have gill slits and therefore cannot create a water current in their gills except by swimming constantly. If they stopped moving forward, they would die of suffocation. So we see them sleep while swimming, quite mechanically of course.

 

Fish do not close their eyes to sleep, simply because they do not have eyelids. Humans do not sleep with eyes closed only to block out light, but mainly so their eyes do not dry out when eyelid blinking stops.

But fish live in water, and their eyes do not face this risk.
It is these always-open eyes that long led people to believe they did not sleep.

Some fish change color when sleeping, adopting less bright and more discreet shades. When sleeping, it is not wise to attract predators!

A well-known example in our aquariums is the "pencil fish" (Nannostomus marginatus). As soon as it sleeps, its lines change direction and it looks very different.

 

In the vast majority of cases, fish sleep on the bottom or on a plant, move very little, and take on somewhat duller colors.

Sleep, for fish as for other animals, is essential and vital.

But I find it interesting that, on such a common subject, our knowledge is so patchy, even poor.

I see this as further proof that fish have long been considered as things, living beings little evolved and primitive, even secondary.

The organization of living beings with humans as the superior species above all others, then species less and less evolved the further away from humans, is a view that has passed.

It is false and is at the root of many abuses since time immemorial.

 

And now, what if we wondered... what do fish dream of?
Or how does a water louse (or a shrimp) sleep?
If a Blackworm feels sleep after a long day?
And our snails: does a bladder snail sleep, and when, and how?

In your living room, in your aquarium, there are so many unexplored mysteries that... slumber!

And if we want to better understand their behavior, even feeding plays a role. tubifex are also an interesting subject of debate.

To go further on fish well-being, we can also question common practices, such as the presence of aquariums without fish or the abuses linked to goldfish (scandal)

Finally, to understand some nocturnal behaviors, it may be useful to revisit fish reproduction