Home Fish Science What do Fish do at Night? (Helpful Examples)

What do Fish do at Night? (Helpful Examples)

by gvald

Generally speaking, we could think that fishes go to “sleep” once the night falls, these last ones having no eyelids, they cannot really close their eyes. So what do they do at night?


What do fish do at night

As explained above, most fish do not sleep in the same way we do, it is their metabolism that is slowed down. For domestic fish, most come to the surface and swim very slowly to rest.


In the sea, it is different, since they become vulnerable to more or less any prey, some manage to protect themselves better than others, changing color, finding shelter, going into the algae …


Some researchers have observed that most fish sleep for a few seconds, give a flipper stroke, then go back to sleep for a few seconds. No deep sleep, the creatures of the deep sea!


There are also some fish that start hunting at night, obviously, it is the ideal time to attack its prey when it least expects it.


Do fish sleep at night

Fish do not sleep in the same way as other animals. They generally do not have eyelids, so they do not close their eyes when they rest. However, even without eyelids, their eyes are not likely to dry out because they live in water.


In fish, we do not speak of sleep, but rather of a period of inactivity where their reactivity is diminished. In order to rest, fish either lie on the bottom or swim more slowly, depending on the species. Some sharks, for example, must swim continuously, even during their resting phases, in order to oxygenate their gills. Fish rest either diurnally (during the day) or nocturnally (at night). The zebra shark rests on sandy bottoms during the day and hunts more at night.


However, like many species, during their periods of inactivity, the fish become vulnerable to predators. They then set up stratagems to protect themselves from possible attacks: secretion of mucus to form a protective envelope (some parrot fish), camouflage by changing color, or hiding in a shelter.


Can we really say that fish are sleeping?

It is difficult to observe fish sleeping because they never close their eyes and some even continue to swim slowly while they sleep, but still they are sleeping! Generally speaking, we are talking more about a slowing down of the metabolism than a resting cycle.


If they don’t close their eyes, it’s simply because they don’t have eyelids. Since they live in the water, their eyes are not likely to dry out and, unlike land animals, they do not need eyelids. Some sharks are forced to keep swimming while they sleep to maintain a constant flow of water around their gills, an organ they use to collect oxygen from the water.


Researchers were able to prove that the fish are sleeping by measuring their heart rate and the electrical activity of their brain. Like most animals, fish need to sleep to recover, their heart beats slower, and they consume less energy. Generally they sleep during the night, but there are also predatory fish that rest during the day and leave their hiding place at nightfall to hunt, such as moray eels.


During their sleep, fish must protect themselves from their predators. To do this, they have developed several strategies: some fish hide at the bottom and bury themselves in the sand, as for example some rays. Others withdraw into rocky crevices, caves or under stones. Some very colorful coral reef fish change their colors during their sleep to adopt a more discreet outfit. Parrot fish that also live in tropical waters around coral reefs even use a kind of sleeping bag. They surround their bodies with mucus (slime) that protects them from their enemies.


Do fish need darkness to sleep

No, fish do not need a dark place to sleep, as explained earlier, they do not have eyelids, so it is impossible to ignore the light when it is present. Some species are even used to sleeping during the day to go hunting at night.


However, for domestic fish, it is preferable to recreate a day/night cycle, leaving the light on for a maximum of 12 hours. It is possible to leave a blue light on at night if you still want to enjoy your fish.

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