When we wonder about our friends in the sea, the first question we ask ourselves is: Do fish sleep? This is a legitimate question because if there is something that no one has ever seen, it is a fish (whether it is small or big) clutching its little fins and nestling in the torpor of its sweet dreams.
We see them all the time in movement, they never really stop swimming and always have their eyes wide open, does that mean they don’t need to sleep? Enough jokes and too long introduction ankylosing those who have been endowed with eyelids that get heavier as the lines go by.
Do fish sleep?
Yes, fish sleep and if we never see them close their eyes it is because they do not have eyelids because their eyes do not need to be moistened like ours. If we don’t see them lying down to rest either, it’s because they don’t need to and because some fish like sharks are obliged to keep swimming while they sleep in order to be able to collect the oxygen found in the water.
In order to answer this question, a group of scientists measured the life frequencies of fish and found that during sleep, the heart rate and brain activity of fish decreased. Like all other animals, fish also need to rest and to abandon themselves in the delicate arms of Morpheus! It is interesting to know that some fish like rays bury themselves in the sand to sleep or that others take shelter in small caves to hide from predators!
Now that you know that fishes sleep, don’t hesitate to say good night to them
How do fish sleep?
All animals need to sleep, or at least to enter a state of rest where the experiences of the previous day are consolidated and their bodies rest. Not all animals sleep in the same way and do not need to sleep the same number of hours.
For example, prey animals, such as ungulates, sleep for very short periods of time and may even sleep standing up. On the other hand, predators can sleep for several hours, they are not always having deep dreams but they are in what is called a “half sleep”, a clear example is the cat.
Animals that live in the water, like fish, also need to enter this state of sleep, but how do fish sleep? Because if a fish slept like any other land mammal, it could be swept away by the currents and eaten.
Transition between sleep and wakefulness
A few years ago, it was demonstrated that the transition between sleep and wakefulness, that is, between a state of sleep and a state of wakefulness, is regulated by neurons located in a region of the brain called the hypothalamus. These neurons release a substance called hypocretins and its deficit produces narcolepsy.
In previous research it has been proven that fish also have this neuronal nucleus. So we can say that fish sleep or, at least, that they have the tools to do so.
How do fish sleep – interesing fact
First of all, it should be said that it is extremely difficult to understand the sleep of fish. To calculate the sleep of mammals and birds are used techniques such as the electroencephalogram. This technique is related to the cortex of the brain, a structure that fish do not have, and its use in an aquatic environment is not possible.
To recognize sleep in a fish, we must pay attention to certain behaviors, such as:
Prolonged inactivity. When a fish remains motionless for a long time, at the bottom of a reef for example, it is because it is sleeping.
Use of a shelter. When a fish is resting, it will look for a shelter or a hidden place to take cover while it sleeps. For example, a small cave, a rock, an algae….
Decreased sensitivity. When sleeping, fish reduce their sensitivity to stimuli, so they will only react to events around them if they are very violent.
In many cases, fish decrease their metabolic rate, thus decreasing their heart rate and respiratory rate. Therefore, even if you don’t see a fish sleeping like you might see in any of your pets, it doesn’t mean that a fish isn’t sleeping.
When do fish sleep?
It is common for people who wonder how fish sleep to also wonder when fish sleep. Fish, like many other living things, can be nocturnal, diurnal or crepuscular animals and, depending on their nature, they will sleep at one time or another.
For example, the Mozambique Tilapia (Oreochromis mossambicus) sleeps during the night, sinks down, decreases its breathing rate and immobilizes its eyes. On the other hand, the brown bullhead (Ameiurus nebulosus), is a nocturnal animal that spends its day in a shelter with its fins loose and relaxed. It will not react to sound, stimuli or contact and has a very slow pulse and breathing.
The tench (Tinea tinea) is another night fish. This animal sleeps during the day, lying on the bottom for 20 minute periods. In general, fish do not sleep for long periods, the cases that have been studied are always a few minutes
Do fish sleep with their eyes open?
A popular belief is that fish do not sleep because they do not close their eyes. To think that this statement is true is a big mistake. Fish simply can never close their eyes because they have no eyelids. For this reason, fish always sleep with their eyes open.
However, some types of sharks, have what is called a nictitating membrane. It’s a third eyelid that serves to protect their eyes. Even though they do not close them to sleep either.
Unlike other fish, sharks cannot stop swimming. Indeed, in order to breathe, they need to be in constant motion for water to pass through their gills. Therefore, while they sleep, sharks remain in motion, even if it is very slow. Their heart rate and breathing rate decrease, as do their reflexes, but as predatory animals, they have nothing to fear.
If you want to know how other aquatic animals sleep, don’t hesitate to consult the article how dolphins sleep?
References
Adamantidis, A. R., Zhang, F., Aravanis, A. M., Deisseroth, K., & De Lecea, L. (2007). Neural substrates of awakening probed with optogenetic control of hypocretin neurons. Nature, 450(7168), 420.
Appelbaum, L., Wang, G. X., Maro, G. S., Mori, R., Tovin, A., Marin, W., … & Mignot, E. (2009). Sleep–wake regulation and hypocretin–melatonin interaction in zebrafish. Proceedings of the National Academy of Sciences, 106(51), 21942-21947.
Reebs, S. (1992). Sleep, inactivity and circadian rhythms in fish. In Rhythms in fishes (pp. 127-135). Springer, Boston, MA.
I’m Benjamin, an aquarium, and fish hobbyist for more than three decades now.
My passion for aquarium fish never wavered after so many years. I keep consuming everything about fish tank, aquariums and alike.
As a result of my experience with aquariums and fish, I have decided to build this blog.
My goal is to answer all your questions – and even those you haven’t asked yourself yet – so you can take care of your fish and aquariums. From one passionate to another passionate!
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