Nearly all animals and birds sleep. Depending on their biological clocks and their requirement of rest, they sleep for different numbers of hours, during the day or night.
When humans, birds and animals sleep, they close their eyes and lie motionless. That is one important identification mark of sleep. However, there’s one kind of aquatic animal that cannot close its eyes. It’s fish!
You may have learnt in your biology lessons that fish don’t have eyelids. How do they close their eyes, then? You might even wonder: without eyelids, do they even sleep?
The answer is yes! Fish do sleep with their eyes open! So, in case you see a fish with its eyes open, you need to check twice whether it’s asleep or awake. And their sleep patterns are similar to many other animals and also humans.
Similar to other animals, some fish are diurnal (sleep at night) and the rest are nocturnal (sleep during the day). They also get around 8 to 12 hours of sleep everyday.
Their beds could be a place in the sea or river. Some fish secure themselves into a cosy and safe spot in the mud or coral, and some even move into a nest. These periods of rest and slow movement (that we call sleep) may perform the same restorative functions as sleep does in humans.
You are a bit puzzled now, right?. We just told you that fish sleep with their eyes open. That would have led you to think – how do we identify sleeping fish?
It’s pretty easy! They lie motionless near the surface of water or near the bottom of the sea. They are also slow to respond to the things going on around them (their predators should probably not know this!).
Another sign is in their gills( their breathing organs). One close look at them will show you that they breathe very slowly, like how we breathe when we sleep.Some of you with fish bowls or aquariums at home may have also noticed these signs in sleeping fish.
When the lights go off, the fish are a lot less active. Even if you turn on the light, they are less active. It means that they are asleep.
Fish have a biological clock that tells them when to sleep. So, regardless of whether it is night or day, when their internal clock tells them, they fall asleep.
However, the presence of light makes their internal clock tuned to it. Scientists have studied this aspect in fish for a long time.
In fact, they have observed that fish in very dark caves, where there is little to no light have a regular sleep cycle, but it’s very different from those fish that are exposed to light during the day.
Now that you know how fish sleep and their sleep patterns, let’s dive into the sleep patterns of various species of fish.
One interesting aspect is that different species of fish sleep differently, depending on various factors. And mind you, there are over 82,000 species of fish! So there are several surprising facts about their sleep patterns.
Here are some examples:
Zebrafish – These fish live in freshwater, private ponds and aquariums. Their sleep patterns are very similar to humans’. Scientists, after a decade of research, found that the evolution of sleep cycles as seen in humans, began in fish.
Cavefish – These fish sleep for just 1.5 hours a day. There are fewer options for food in those dark caves, which means that these fish spend longer periods of time looking for food and spend less time sleeping.
Parrot Fish – These fish are found in coral reefs and among other bigger varieties of aquatic animals. Every night, they secrete a layer of mucus around themselves and sleep inside that cocoon to protect themselves from predators (like eels) and dust.
Dolphins and Sharks – They don’t sleep as deeply as other fish do, but they have their own active and resting periods. Sharks, for instance, sleep while swimming. Dolphins sleep with a half-alert brain to ensure they are alert.
Can you find more species of animals that have unique sleeping patterns? Share your answers with us in the comments section.
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