Do Fish Sleep?

Apparently no one knows. None of the fish textbooks I've read even mentions sleep. I've seen my fish yawn from time to time, but it doesn't appear to indicate that they're sleepy.

It Depends

Like so many questions, the answer is, "It depends."

It depends on how you define sleep. My dictionary says sleep is "the natural periodic suspension of consciousness during which the powers of the body are restored."

I've also seen definitions that include things like closing one's eyes and alterations in brain waves.

As you're probably aware, fish don't have eyelids (actually, some sharks do), so that's not a helpful clue to whether they sleep.

There's been very little scientific work done on studying fish brain waves and what there is indicates that fish brain waves do not change during those periods that they appear to be sleeping.

As for losing awareness of surroundings, that certainly seems to happen with many types of fish. Some species of reef fish sleep so soundly that divers are able to handle them without them waking up. The divers can sometimes even remove the fish from the water without disturbing them.

If you define sleep as some type of resting, with some diminished level of awareness of surroundings, then it begins to be clear that fish do sleep.

Why Would They?

As the dictionary mentions, sleep helps to restore the powers of the body that were depleted during waking activities.

Sleep can also be a time to conserve energy. An efficiency move, if you will. It seems reasonable to assume that all mobile animals would benefit from resting occasionally.

But even this is not straightforward. There are at least a few types of fish, such as hammerhead sharks and tuna, that must constantly swim in order breathe. These types of fish are called "ram ventilators," meaning they need to swim in order to force water across their gills.

Fish who have a "buccal (mouth) pump" can force water across their gills independent of forward motion.

Since ram ventilators can't rest in the sense of ceasing motion, does that mean they can't sleep? There's a theory that they're able to sleep even while swimming. Apparently it isn't uncommon to find a fast species such as tuna in the stomachs of slower-moving sharks.

In fact, hiding from predators is a big drawback to sleeping. Many types of sleeping fish wedge themselves into nooks and crannies in an attempt to be safe. One type of parrotfish produces a mucus coating around itself at night as well as hiding in a crevice to sleep.

My tank of African cichlids has a large pile of rocks dominating the tank. At night and early in the morning there's not a fish to be seen in that tank. But if I use a flashlight and look carefully, I can spot little fish faces tucked into individual caves. I was fascinated to find that each fish sleeps in the same cave night after night. They've apparently picked out the "apartment" they like and moved in!

Other Types of Sleep

Then there's the question of hibernation. Some consider that a form of sleeping.

And what about estivation, a period of dormancy some fish go into during hot, dry periods as they try to stay hydrated. Didn't we all learn about the African lungfish in school? Is that sleep?

I also find myself wondering about catnaps. Do fish necessarily have to sleep for one long period each day? Could they catch little naps periodically throughout the day instead?

How much sleep does a fish need? This is another question I can find no answer for. Maybe they don't even need to sleep daily. Perhaps once a week is enough.

Triggers

What causes a fish to go to sleep? For diurnal fish (fish who normally are active during daylight and inactive during darkness) it seems to be as simple as the amount of light in the environment. Turn off the lights in an aquarium and within a short time the fish will appear to be asleep. Turn on the lights and usually within about 10 minutes the fish will be up and about again. So I regulate how much sleep my fish get by how I set the timers on my lights. To a point.

I've been asked if it's okay to leave aquarium lights on 24 hours a day. My opinion is that it's not a good idea because I do think fish need to sleep and I think they have trouble going to sleep with the lights on. (Maybe just a nightlight if they're very young.)

I've done some experiments to investigate this a bit. I've found that if you leave the lights on all night, eventually, say about six hours into their normal "night," the fish finally settle down and assume their usual night positions. I can't tell if they're cranky and sleep-deprived the next day (but I know I am!).

Another question that niggles at the back of my brain is how the sleeping habits of nocturnal fish compare with diurnal fish. Are there differences beyond the fact that one is awake during the day and the other is awake at night? The only difference I've noticed in my own fish is that the coloring of the nocturnal fish doesn't seem to change.

They Wear Pajamas!

My diurnal fish put on pale pajamas when they go to sleep. That is, their normal colors fade significantly during sleep. I wonder if this is to hide more effectively from predators.

That doesn't seem to be the case with clown loaches. They pale so much they look two-days dead. And they're fond of sleeping flat on their sides, right out in the open. Like many loach owners, when I was new to keeping them it scared the daylights out of me when I thought I'd found my whole group dead one morning.

Perhaps the paleness is just another version of resting. Maybe they can conserve more energy when they don't have to maintain that color.

Some species of fish don't just fade; they change colors more dramatically when asleep. Blue tangs don white stripes at night.

Another fish that has scared me with its sleeping habits is the oscar. Mine seem to lose their equilibrium when they sleep. They apparently don't care quite so much which direction is up. They will sleep listing at a 45-degree angle. They don't settle to the bottom; they usually just drift around the tank.

I have one albino oscar who is fond of sleeping on his side stuck into a large plastic plant. Even now he still scares me sometimes when I'm up early. I gasp in horror for a split second before I realize, "Oh, yeah, it's him."

Why Do We Care?

So why do we care whether or not fish sleep, other than to decide whether we have to buy a timer for our aquarium lights?

Some scientists are finding it helpful to estimate fish populations by counting them at night. Fish that by day are hidden so effectively that they're seldom seen can be found relatively easily with flashlights while they slumber.

I became interested in the subject when I mysteriously lost some fish out of my koi pond. The pond was designed deep and with steep sides specifically to deter predators. Raccoons and blue herons typically won't wade into water deeper than two feet, so I felt safe. And I was for years.

But last summer I had three fish disappear one night. The fish ranged from 9 inches to about 13 inches in length. I thought it was most likely a raccoon because it happened after dark, but usually if a raccoon's been by there are signs of trampled plants or rocks knocked in. There were no signs of anything except that the rest of the fish were spooked, as they usually are after a predator visits.

Two nights later I lost one more fish, again with no signs other than the fact that the other fish were jumpy. By now I was worried enough to lose sleep over it. So the next night I went outside at about 2 a.m. to check on the pond. I was horrified!

The fish were all fine; it was the way they were sleeping that upset me. I'd always assumed that the pond fish slept on the bottom of the pond because my indoor koi seem to do that. Not so!

Pond Fish Are Different

Some of the koi were sleeping on the bottom. But quite a few were sleeping at the surface and several of those surface fish were lined up against the front edge of the pond. They appeared to be sleeping and didn't move when I approached them. I could easily have reached down and grabbed them!

What good are depth and steep sides if the fish are going to present themselves within such easy reach at the very time they're least aware of their surroundings!

I've since installed around my pond one of those short, folding wire fences they sell in gardening centers. It's dark green and very thin, so it's almost invisible. This is nice because it doesn't spoil the look of the pond.

I haven't lost a fish since I put in the fence. It certainly isn't strong enough or tall enough to stop any animal, but my theory is that it hinders them for at least a moment as they bump it and gives the fish a chance to wake and dash away.

The Silver Lining

I can say one good thing about the experience. I've learned that if you're unable to catch your koi, try it at night with a flashlight. It's much easier. Even if you don't want to catch them, night is often the easiest time to get a close look if you want to check them out. The glare of the flashlight doesn't seem to affect them once they've gone to sleep.

So Do Fish Sleep?

You'll have to decide for yourself if you think fish sleep. I believe they do. Which leads me to the last and most important question . . . where do fish sleep?

Why, in a waterbed, of course.