Essentially every animal needs sleep or similar periods of rest. Sleep or rest is usually manifested by periods of reduced metabolism, decelerated respiration, and lessened brain activity.
So, a question arises do fishes also sleep like the rest of animals?
The answer is Yes! Fishes do experience sleep, but the pattern is quite different from the rest. They do not close their eyes as they primarily lack eyelids. Latest studies have shown that fishes do have significant internal clocks that manage their sleep timings.
Let’s discuss in detail the patterns, behavior, and quality of sleep fishes experience.
How do fish sleep?
The basic criterion of sleep is adequately fulfilled in fishes such as slow breathing, and reduced motility. A major setback is that fishes do not show changes in brain activity. Lessened brain activity is an important measure of sleep in mammals, reptiles, birds, and many amphibians.
The sleep or period of rest experienced in fishes is “Behavioral Sleep”. The epic description of this model is that it’s “a reversible state of reduced responsiveness”.
Fish going through periods of behavioral sleep experience short periods of slow and delayed responses to stimuli. This pattern is most commonly observed in fishes.
What is the pattern of fish’s sleep?
There are mainly two patterns of sleep seen variably in different species of animals around the world.
- Sleep as changes in brain activity:
- Sleep as changes in behavior:
Changes in brain activity are seen in all animals except fishes. The reason behind the lack of changes in brain activity is that fishes lack Neocortex the brain area that usually manifests changes in sleep. As this area is absent in fish thus no changes in brain activity are usually recorded in fishes.
Stages of sleep:
Normally there are five stages of sleep seen in most animals as Wake, light sleep, deep sleep, REM, and repeat process.
Fishes do not go through Rapid Eye Movement Sleep (REM) as they do not close their eyes.
What does a sleeping fish look like?
Sleep can look different in different fish species. Some common sleeping habits seen in some species are as follows.
The sleeping pattern of zebra danios:
Zebra danios are one of the wildly used species for research purposes. Their sleeping habits resemble closely that of humans. They usually experience sleep at night manifested by a complete lack of motility. NO swimming reduced breathing and lying low on the bottom of the tank.
The sleeping pattern in sharks:
The sleeping behavior seen in sharks is quite distinct. Sharks usually rest during the daytime in short intervals. Sharks exhibit a unique pattern of sleeping called “Sleep Swimming”.
During the sleeping phase, sharks continue to swim to keep a constant flow of water through the gills and keep up with the oxygen supply. There is no concept of deep sleep or REM sleep in sharks.
Sleep swimming is accompanied by the least responsiveness and reduced activity, but some level of activity is always present. Sharks are most responsive and active at night.
The sleeping pattern in parrotfish:
Parrotfish shows quite an impressive pattern of sleep. They fancy a sound sleep with comfort. Parrotfish activity secretes a jelly-like mucous-filled bag that essentially acts as a sleeping bag.
This sleeping bag serves to protect them from predators and helps them sleep with comfort for long periods.
The sleeping pattern in cavefish:
Cave-inhabiting species such as Mexican tetra fish have a unique pattern of sleep. They lose their eyesight as there is essentially no sunlight in the caves. Moreover, during periods of starvation, they decrease their sleep.
Where do fish like to sleep?
Depending on the nature of fish and its surrounding environment every fish can find its comfort zone to sleep. Some fishes such as sharks keep moving around while sleeping as well thus, there is no specific location for them.
There are others such as cavefish that need a specific location with caves and rocks to hide behind. Some species take benefit of their camouflage ability to hide inside the sand or coral reefs to protect themselves from any danger while sleeping. Such species lay completely motionless and take a sound sleep.
As in the case of parrotfish already mentioned above, they look for a rock or reef. When they find the appropriate rock or reef, they usually make a cocoon around themselves and sleep.
Why is it difficult to study sleep activities in fish?
There are certain limitations when it comes to conducting sleep studies in fish.
- They lack an essential part of the brain called the Neocortex. The absence of this region makes it difficult to understand their sleeping pattern.
- As fishes are an inhabitant of water thus, it is difficult to determine their underwater brain activity(waves).
- Fishes do not close their eyes or show any sign of complete immobility, which makes sleep a definite conclusion.
Thus, it is quite difficult to conduct sleep studies on fish. Still, there is much research work going on.
How does fishes sleep differ from ours?
The sleeping pattern in fishes and humans is quite different in some manner.
Firstly, fishes do not close their eyes as they lack eyelids. Whereas humans close their eyes completely while sleeping.
Secondly, fishes do not show the REM stage of sleep, whereas REM is an essential stage of human sleeping patterns.
Moreover, fishes are always aware of their surrounding environment and the dangers awaiting them. Deep sleep in the case of humans is nearly unconscious.
Lastly, there is no change in brain activity in the case of fish. In humans, there is a marked reduction of brain activity during sleep.
When do fish usually go to sleep?
Interestingly enough, fishes have their alarm system set internally to keep their sleep-wake cycle in balance. This internal rhythm is similar to the circadian rhythm seen in most animals and even in plants.
After periods of prolonged restlessness or disturbances in the sleeping schedule, this internal clock is responsible for bringing the sleep back to normal.
Most fish species usually sleep at night, and this is managed by their internal sleep-wake rhythm.
Factors affecting fish’s sleep:
Many factors affect the quality of sleep a fish experiences. Some of them are mentioned.
- Excessive light can make it difficult for the fish to sleep or rest.
- Crowded places are one of the most disturbing factors in sleep. Every fish needs its own space to feel comfortable and sleep adequately.
- Hiding places such as rocks, coral reefs and decorations make the fish feel safe and protected. This feeling of protection also improves their sleep.
- Water temperature is another contributory factor. Warm water fish can not sleep or rest well in cold water.
- Strong water currents can also disturb fish’s sleep.
How to help your aquarium fish sleep better?
If you want to make your fish feel at ease and experience great periods of rest, you can improve the aquarium conditions. Nice and appropriate water temperature, adequate space, and dim lighting can improve your fish’s sleep.
Make sure to switch off the light at night and also slow down the water pump to provide resting conditions.
Frequently Asked Questions (FAQs):
According to the latest research fishes are Diurnal and require at least 10-12 hours of sleep or rest every day. These prolonged periods of sleep are usually experienced at night.
Studies are being conducted on fish’s sleeping manners. Studies have shown that some fish might experience dream-like situations at periods of rest. But research is still active on this topic.
Thus, to conclude all this it’s clear that fishes do experience sleep or rest. The manner, pattern, and timing of sleep may differ from one species to another. The sleeping pattern of fishes is quite distinctive from that seen in humans.
To make sure that your fish takes sound sleep there are some relieving factors mentioned above.
We hope that this article answers all your question regarding fish’s sleeping pattern
Best of luck with your fish!!