Do Corals Sleep At Night? (With Two Surprising Facts)

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In my early days of fish keeping, I discovered that fish require several hours of darkness to sleep.

But I knew little then about corals. Do these creatures sleep like other organisms in reef tanks? Do they need as many hours of darkness as humans?

Since I heard many fish owners asking themselves the same questions I had, I decided to devote an entire article to this topic.

Let’s dive right into it.

**Beautiful orange Cup Corals at night**

Do Corals Sleep At Night?

No, corals don’t sleep at night in the traditional sense because they lack brains, but they may have some sort of circadian rhythm and experience a regular day/night cycle.

However, that doesn’t mean you can eliminate the day/night cycle from your reef tank. You should keep these two factors in mind:

1. Corals Don’t Have Brains

Corals are animals.

Many newcomers classify them as a type of plant because they think the creatures photosynthesize. After all, corals require light to thrive and survive.

However, light only matters to zooxanthellae, the microorganisms that live in a coral’s tissue.

Zooxanthellae use light to make nutrients for corals. These microorganisms are plant-like.

Corals are animals, and most animals have a circadian rhythm. In fact, most living things have a circadian rhythm.^[1]

However, corals are unique, particularly in comparison to other animals, because they don’t have brains.

Coral polyps feature a nerve net, a primitive nervous system that can’t even register pain. Although it allows the corals to find food.

Some of you may wonder how brain corals fit into this equation. Despite their name, brain corals don’t have brains.

These organisms, which can live for 900 years, are famous for their Meandroid tissue integration.

Their polyps are highly associated with each other, and the tissues are more closely connected.^[2] But this doesn’t mean they have brains.

**Although they look like they have it, brain corals don’t actually have a brain**

Common sense will tell you that living organisms require a brain to sleep. According to The American Brain Foundation, the human brain works with the body to regulate sleep.^[3]

Therefore, you wouldn’t be wrong to assume that any creature that lacks a brain cannot sleep.

However, the jellyfish contradicts that assumption. These creatures don’t have brains, either. They use a radially distributed nervous system.^[4]

And yet, jellyfish sleep.

A 2017 study in Current Biology (Ravi D. Nath, Viviana Gradinaru, Lea Goentoro, Paul W. Sternberg, Micheal J. Abrams, Claire N. Bedrook) proved as much.^[5]

Researchers found that jellyfish were not only inactive at night but also more difficult to rouse.

Therefore, you shouldn’t be so quick to assume that corals don’t sleep simply because they don’t have brains.

2. Corals Experience A Regular Day/Night Cycle

Corals in the wild experience a period of darkness routinely. The sun rises and sets every day.

Additionally, researchers have found that artificial light at night is a threat to corals. This shows the vital role nighttime plays during a coral’s development.

Does this mean corals use the darkness to sleep? No, it doesn’t. However, it strongly suggests that corals have some sort of circadian rhythm.

But again, that doesn’t mean they sleep in the traditional sense.

Don’t expect deep-sea corals to manifest the same response to a day/night cycle as their shallow counterparts.

After all, the sun cannot reach them. Deep-sea corals spend most of their existence in relative darkness.

Why Do Some People Think Corals Can Sleep?

Corals won’t necessarily sleep at night. However, you will notice certain reactions that some individuals may blame on sleep:

1. Color Changes

This creates confusion among some newcomers. They expect corals to become more or less colorful at night.

However, a coral’s colors will vary depending on the light in the environment. For instance, corals may glow if you expose them to blue LEDs in a dark room.

The coral’s colors haven’t actually changed. You’ve merely altered its appearance temporarily by applying a specific color light.

Interestingly, many deep-sea corals are fluorescent. They generate a glow that attracts prey. Either way, this is not some symptom or manifestation of sleep.

Don’t be fooled by stories regarding sleeping corals in the wild.

Those so-called sleeping corals are an entirely different case. Yes, they lose their color and even appear dead. But that is a response to high temperatures.

The corals in question look dead because warm waters have bleached them. However, studies have found that those corals are not actually dead.

Instead, the polyps have retreated and settled into a state of hibernation during the summer.

They will get smaller and pull away from the skeleton for a little while before rejuvenating once the temperatures drop.^[6]

Scientists are optimistic that corals can use this technique to survive climate change.

However, you shouldn’t expect the corals in your reef tank to manifest a similar response at night.

The behavior noted above is a defensive mechanism, not a type of sleep.

2. Shriveling Polyps

This is the most tangible response you will record. The corals will seemingly close up and shrink. Their tentacles will shrivel.

You can test this response by keeping the lights on at night. The corals will stay open.

Actually, they may stay open in the face of red and blue lights even though these colors are supposedly safe for reef tanks at night.

But the corals only shrink at night due to a decrease in photosynthesis and an increase in their respiration rate, not because they are sleeping.

Since there is no light, there is no reason for the coral to stay wide open. By closing, the coral conserves vital energy.

**Some corals curl their polyps at night**

3. Extended Tentacles

Some aquarists have seen their corals shrivel up and close at night, only opening in the morning when the light returns. They call this ‘Sleep.’

But others have observed the opposite. The polyps come out at night and extend the tentacles, using them to catch microscopic prey.^[7]

They do this because light is absent, and photosynthesis cannot occur. Zooxanthellae are incapable of turning light into sugars for the corals to consume.

Therefore, the corals will supplement their diet by catching plankton.

**Some corals extend their tentacles at night to catch microscopic prey**

What Do Corals Do At Night If They Don’t Sleep?

Sleep doesn’t serve a purpose or benefit for corals.

However, coral spawning is a significant event that primarily happens at night. This is where corals release egg and sperm bundles into the water.

Fertilization occurs at the surface once these bundles rise. In the event of successful fertilization, the eggs will settle at the bottom, slowly developing into corals.

This so-called eruption can last twenty minutes at the Great Barrier Reef.^[8]

But it only happens at night, reducing the chances of coral egg and sperm bundles falling prey to predators.

The moonlight is a cue that gives the corals the green light to spawn.^[9]

This tells you that nighttime matters to a coral’s reproduction, but not necessarily sleep.

Artificial light at night can create delays in a coral’s reproduction. Additionally, the organisms will stay open at night instead of shriveling and retreating.

But their health won’t necessarily suffer.

Can I Leave The Light On At Night In A Coral Tank?

By now, we clarified that corals don’t sleep in the traditional sense.

They are not that active during the day, to begin with. Therefore, you can’t expect them to become significantly less active at night.

They don’t have eyes that close either. More importantly, exposing the coral to light at night will not overwhelm them with stress as it does with fish.

But that doesn’t mean you should expose your corals to 24 hours of light. Even though they don’t sleep, corals need at least six hours of darkness.

There are a few bad things that might happen if you expose your corals to light at night:

Algae will excessively grow on your corals.

Reduced coral polyp extension, which is necessary for feeding and growth.
The coral’s natural circadian rhythm will be disturbed.
Increased stress on the coral, which can lead to bleaching or death.

Reduced oxygen levels in the water due to increased photosynthesis, which can harm the corals and other organisms in the tank.

**Exposing your coral to too much light can cause bleaching**

Pro Tip: Feeling uncertain about how to light your coral tank? Don’t worry, I’ve got you covered with my ultimate guide on the topic.

Conclusions

If you’re in a hurry, here’s a quick rundown of the key points mentioned above:

Corals do not sleep in the traditional sense because they lack a brain. Although they have a primitive nervous system, they cannot feel pain and do not have the ability to register sleep.
Corals do, however, experience a regular day/night cycle, and the darkness of night is essential for their development and reproduction.
Color changes and shriveling polyps are not symptoms of sleep in corals. Rather, they are responses to changes in their environment.

Corals that appear to be sleeping in the wild are actually hibernating in response to high temperatures, a defensive mechanism rather than a type of sleep.
Although jellyfish can sleep without a brain, it is unclear if corals can do the same, and more research is needed to understand the circadian rhythm and behavior of corals.

References

https://nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx