Do Fish Sleep? How do They Sleep?

Ever caught yourself wondering “do fish sleep?” You probably have if you own a pet fish. These interesting creatures never seem to close their eyes and take a nap… or do they?

Scientists who had a similar question in mind decided to study fish and it turns out they do sleep. But how this water-dwellers sleep is what makes it interesting.

With over 30,000 species of fish, these vertebrates which are considerably older than human and land animal species, have various sleep styles.

Stick around to find out how yours sleeps.

Does fish sleep?

In order, to answer if fish sleep, scientists had to redefine sleep. Unlike humans and land animals, where sleeping means to shut the eyes and slow down the neocortex of the brain. Fish don’t have eyelids or a neocortex making it difficult to determine how they sleep.

But with long studies, scientists finally established that fish sleep but through behavioral characteristics. These characteristics include slowing down, swimming in place, floating, or hiding in objects.

Not all fish sleep. Others like tuna, sharks, and blind fish don’t sleep at all. They keep swimming throughout the night though they slow down their movements.

Some fish sleep during the day and others during the night.

How Do Fish Sleep?

While we do sleep, various species on earth have different sleeping styles and fish are no exception.

Different species of fish, sleep differently. Some settle at the bottom of the water in the rocks, corals, or sand. Some float on water with their bellies up. However, if this happens during the day, your fish could be indicating all isn’t well.

Some sleep in groups. Some lay still with slow fin movements to maintain proper oxygen flow over their gills. Some like the parrotfish like to sleep with a blanket. They produce a jelly-like substance that protects them from the enemy while they sleep.

Dolphins, on the other hand, sleep with one eye open and others log (half of their bodies float on the surface of the water like a log).

Why fish sleep the way they do?

Fish don’t have eyelids, so they never shut their eyes even while sleeping. Some might say that this is kind of creepy. But their eyes don’t need moisturizing as ours do. So, they have no need for eyelids.

Fish’s circadian rhythm is not similar to humans. Some just rest for some time like how human’s daydream, while others sleep so deeply they might not feel when touched. Others, sleep but in an alert mode, so that they can swiftly escape when threatened. This types of fish mostly live in coral reefs.

However, no matter how they sleep, sleep performs the same function in fish as it does in human beings.

How do you tell when a fish is sleeping?

This is how you can tell a fish is sleeping. It has a resting posture. Sleeping almost the same time, the same way, and in the same location. It is less sensitive to external disturbance and shows inactivity for long periods.

Some might swim in place but look zoned out like a goldfish. Don’t disturb the fish during this time. It might lead to stress.

Why do fish sleep with their eyes open?

Fish sleep with their eyes open because they don’t have eyelids, except sharks. Sharks have eyelids to protect their eyes during an attack.

Fish don’t need eyelids because their eyes are already moisturized from spending all that time in the water. Eyes require moisture to keep them fresh.

This characteristic of fish makes people think that fish don’t sleep.

Why do fish move while they sleep?

Some fish sleep while moving. Why is this? For most, it’s to keep a constant flow of water moving past their gills to maintain proper oxygen levels. Some fish flip on their side to stabilize their bodies. This is because they lack pelvic fins that help stabilize their bodies.

How different fish sleep?

GoldFish– When goldfish sleep they hover in one spot to keep themselves steady. This is usually near the bottom of the tank. With their head slightly tilted downward and their eyes looking zoned out.

Guppies– Guppies sleep by lying motionless at the bottom of the tank. They can also float at the surface or rest near objects in the tank. When they float and stop swimming they sometimes appear dead but are just resting.

Bettas– Bettas are diurnal and sleep during the night like humans. Though they take the occasional day nap as we do. This they do by lying at the bottom of their tank. Though others prefer to float. For this type of fish, it’s best to get a floating log for them to sleep in.

Zebra Fish– Zebrafish sleep by lying near the bottom of the tank and dipping their tails. Or by sleep just a short distance under the surface of the water. Zebrafish have been the subject of many sleep studies and it was discovered they can suffer from insomnia. A substance called hypocretin found in mammals, helps them fall asleep. The same is available in zebrafish and when taken out it is unable to sleep or rest.

Sharks– Sharks sleep while constantly swimming. This is usually to allow proper airflow over their gills for oxygen absorption. During this time a part of their brain rests while the other remains active. Whereas some sharks don’t sleep at all..

Conclusion

The brain knows to slow down when it’s dark and wake when it’s light. These same cues apply to fish. As such, it’s important to switch off the lights at night for your fish to rest.

Just like humans, fish don’t sleep when they are new moms. They stay up to take care of their babies. If you notice your fish skipping sleep after birth, don’t worry 😉 they are just being a mom.