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.
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.
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.
Some species of fish stop sleeping at certain periods in their lives, such as when they are caring for their young , or when they are migrating. 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.
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 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.
When humans sleep, we typically close our eyes and relax our bodies. Our fish friends are the exact opposite. Most aquarium fish just hover or stay still with their eyes open. Of course, these habits vary between species. Some fish enjoy cozying into their environment, hiding away in corals or in the plants, during sleep.
Other fish lay down on the tank floor to rest or drift with the water. Even while asleep, most fish remain alert and aware of their surroundings. This helps them escape from potential dangers.
There are some fish species like the Spanish hogfish that go into a deep slumber—you can even pick them up without waking them! Aquarium fish are typically active during the day time and sleep at night.
Fish rely on their eyes to eat, swim and be themselves so they need daylight to perform these activities. It is easy to Understand when we humans are asleep, with fish, on the other hand, it is slightly more difficult. From several surveys conducted on zebrafish it has been discovered that they just have a different way of sleeping compared to that experienced by humans.
Since there are no eyelids, the fish never have their eyes closed, so do not hope to recognize a sleeping aquarium fish by observing the eyes. You can for example look at is its pace, if it is phlegmatic and the animal is almost stopped, it will probably be sleeping; the same if it becomes less sensitive to external stimuli.
Try it with your goldfish by feeding it when it is stopped on the bottom behind a decoration: more likely it will have very slow reflections and will not run towards the food! Slow movements and reduced heart rate show that its metabolism has decreased and that the fish is consequently saving energy to rest the body.
Some fish, such as tuna and sharks, continue to swim even though they are sleeping, as this is the only way to get the water to pass through the gills and thus continue to breathe.
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