Sleeping Fish

When you’re inside water, sleeping is not so easy.

Sleeping Fish

When you’re inside water, sleeping is not so easy.

At first glance, it looks like just a ball of baked mud. The scorching sun has been shining down on the land for months, turning the very bed of the lake as dry as a desert. No water is to be found anywhere, not even tucked away underground.

When the rains finally arrive, the ball will remain still. It will wait as the water lashes against the earth and is soaked in instantly as by a sponge.

Finally, slowly, the lake will begin to fill. It will become a wet splash, then a tiny puddle, and finally a growing pond. That’s when the ball will start to change. Soaked with water, the outer layers will begin to crumble. They’ll begin to peel away, in flecks and flakes.

But the ball still won’t vanish altogether. The dissolving mud will give way to a leathery cocoon; one that was once dry and hard but is now becoming saturated with water.

It will take more than a day for the ball to finish soaking. The mud will be washed away by then. The leathery cocoon, too, would have all but vanished, leaving the ring it was covering plain to see.

Before long, the ring will unfurl. It will become a short, twisted line; one which hasn’t finished unfurling yet. It will lie at the bottom of the lake. Soaking in the water, filling itself out. Inside the line, veins and vessels, low on water for so long, will be taking in as much as they can.

Finally, the line will straighten out. It will swish its tail.

That means the lungfish has finished drinking. Now it is ready to eat.


Every small bit of extra electricity used has its impact. I remember an old statistic saying that, if all the computers currently left on standby were powered off, Germany’s three smallest nuclear power plants could be closed down immediately.

The standby or “sleep” mode seems especially wasteful, because it doesn’t really do anything. All operations are stopped. The only reason for using it is that it keeps all your programs open, so that when you use them again you can continue where you left off. But while the sleeping is actually going on, the computer is doing nothing at all.

(Nowadays, people have smartphones which are always on too. Those devices are busy checking for notifications, news alerts, and cat videos. But that’s a different kind of nothing).

Does it make sense to use so much electricity, just to save a few minutes of waiting for things to start? Isn’t it a waste to spend so much time and energy simply sleeping?

Some people think the same way about themselves, too. But people are different. When they’re asleep, they’re actually doing something.


As your mind goes to rest, your body starts working. It’s the time when your muscles don’t have to worry about shifting around, and your cells don’t need to burn energy to keep you thinking and moving. Instead, that energy is spent on healing and servicing your cells and muscles, on repairing what needs to be repaired, and helping parts of your body grow.

Proteins are generated: things which help in growth and repair, including special “antibodies” that protect your body against disease. Studies have shown that sleeping properly helps your immune system stay stronger.

Sleep gives your brain a rest, too. The brain is the most energy-consuming organ in your body, taking a fifth of all the energy you use while awake. That’s because it has to keep track of all the many other parts of your body, process information from your senses, and think whatever you tell it to think.

While you’re sleeping, your brain still has to remain active, but its workload does reduce a lot. Nearly half of its blood supply is diverted in one stage of sleep, and sent over to other parts of your body where things are growing or healing.

In another stage of sleep, your brain works on sorting out your thoughts. Extra blood gets sent to the parts dealing with memories and emotions. Your eyes move rapidly, and you get dreams which you may remember when you wake up. People who don’t get regular sleep end up finding it harder to learn and remember new things — and that happens even if they try to “catch up” by sleeping for extra time later.


When you’re in water, sleeping is not so easy. Often, you have to keep awake just so that you can stay afloat.

The sleep habits of fish are quite different from those of mammals and birds. To start with, they don’t close their eyes. Why not? Because they don’t have eyelids to close them with! That’s why it’s a bit hard to tell whether a fish is sleeping or not.

But the scientific definition of sleep isn’t just about eyelids: it also includes a certain wave-pattern in a part of the brain known as the ‘neocortex’. Unfortunately, that doesn’t help much either. Fish don’t have a neocortex.

As you can see, the idea of “sleep” works slightly differently with fishes. All we can say is that they go into state where they power down their brains and bodies. Or rather, some of them do. And of those that do, they don’t all do it in the same way.

You can tell a fish is sleeping if it’s in one place, not moving, and staying very, very still. But that fish will still be alert to danger: make a loud noise or try to touch it, and it’ll wake up immediately. Enemies may be coming from anywhere, so fish can’t afford to sleep as deeply as mammals do.

Unless they’re Spanish Hogfish. That particular species sleeps so deeply, you can even lift it out of the water without waking it up.

Not all fish sleep surrounded by water. Some go down, wedging their bodies into a crack in the mud or a nest of coral before taking their nap. Some borrow under the sand to avoid predators.

And some fish don’t sleep at all.

There are the fish that swim all the time, that have to keep paddling to stay afloat. Migrating fish like the Sockeye Salmon can pause their sleeping habits during their journey. And then, there are blind fish living in caves that never seem to need sleep at all. Perhaps its because life is slow and there’s not much to see, so their brains and bodies don’t get so tired out anyway.

What about dolphins? They’re not fish — but they live in water and have to stay awake to swim and breathe!

That’s why they do something called “unihemispheric sleep”. One side of their brain powers down, while the other side stays awake to do all the work. Then, it’s the second side’s turn to sleep while the first side takes over. You can see it happening, too: when one side of their brain is resting, the eye on their opposite side gets closed.

This trick is used by seals, sea-lions and some flying birds as well, giving a whole new meaning to the term “half asleep”.


Sleeping is different from hibernation.

Hibernation is when you body goes unconscious for a long, long time. It usually happens during the winter time, to get past the horrifically cold months that plague many regions of the world. The idea is to use as little energy as possible in a time when food is scarce, so that you still have enough of it left to get going when you finally wake up.

Hibernation doesn’t do all the healing and memory-sorting and body-growing that sleep does. That’s why hibernating animals don’t usually hibernate all through the winter. They pause their hibernation so that they can sleep.

Computer hibernation is even more efficient. It works like computer sleeping, but it saves the state of your computer onto the hard-disk. So you can turn off the power completely, and still have your files and programs open the next time you turn it all. Unlike the ‘sleep’ mode, hibernating your computer doesn’t use any electricity at all.

So if you want to save electricity and still get back to your work, you now know what to do.


Hibernation is different from æstivation.

While animals hibernate to wait out the cold, æstivation is what is done in hot, dry places to escape the heat. Animals usually find a cool or shady spot, and go into a hibernation-like state to conserve their energy.

Snails move into shady areas, or climb up bushes and trees to avoid the ground. Crabs burrow down into their holes, to places the heat cannot reach, while water-holding frogs wait under the sandy ground along with their precious, saved-up supply of water.

Few animals æstivate longer than the African Lungfish.


It all started some 390 million years ago, in the middle of the Devonian period. That was when the rains slowed down. Ponds, lakes and streams began to dry up, and many fishes died.

Its ancestors were lucky. They were able to breathe out of water, directly from the air. They could use their fins to paddle from one mud-puddle to another. Finally, they learned to lie still and wait out the drought, even in puddles that had no water at all.

So when the rains stopped and the water levels dropped, it knew what to do. It ate as much as it could, digesting the precious food that would be its only source of energy for months to come. As the lake became a pond, and the pond a puddle, it splashed through the water and slithered along the land to stay in the wettest place possible.

Finally, it realised its time had come. It burrowed down into the still soft ground, to make a hollow safe from the scorching heat. It covered itself completely. Only one small hole was left on top. It lay there curled up into a ring, tail over nose to stop moisture from escaping. Its covered nose was pointing towards the small hole, to breathe.

A thick layer of mucus oozed out and covered its body entirely. This eventually hardened to make a thick, leather layer, protecting it from the outside world and helping to keep the inside moist.

There it lay, for month upon month. It lay in a state deeper than sleep, using as little energy as it possibly could. That little energy came from its muscles, which slowly became thinner and thinner over time. And there it still lies, waiting for the day the rains return again.

At first glance, it looks like just a dry ball of baked mud.

But if you knew the full story, you would realise there was something fishy going on.


Feeling sleepy? Every year, the 3rd of January is celebrated as the Festival of Sleep. It’s a time for you to catch up after all the New Year celebrations, and it’s quite easy to take part in. Just sleep in longer, take some naps, or curl up in bed with your favourite book. And don’t forget to let us know how it goes!

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