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Do Corals Need Light At Night? (3 Facts To Consider)

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Most fish owners know that fish do not do well with light that lasts all night. Since they have no eyelids, they are not able to sleep.

But little is known about corals and how they react when exposed to light all night, be it blue, white, red, or green.

Since this has occupied my mind quite often in the past, I decided to devote an entire article discussing this topic.

Let’s dive right into it.

A reef tank illuminated with white light at night.

Do Corals Need Light At Night?

No, corals don’t need light at night to survive. Leaving the lights on at night can disturb fish and disrupt their circadian rhythm, leading to stress and potentially death.

You can’t use anecdotes and testimonies to answer this question because every aquarist you consult will confuse you by presenting contradictory evidence.

Some people leave their lights on all night, and their corals thrive. Others have seen their corals die because they forgot to turn the lights off at night.

If you’re struggling to make a lighting schedule for your reef tank, you should keep the following in mind:

1. Corals Do Not Receive Light At Night In Nature

To understand a coral’s lighting requirements, you must look at the conditions they encounter in the wild. This will show you what to do.

Some corals live in shallow, clear water that exposes them to the brightest lights

Additionally, they live in tropical regions where the sun is particularly intense. Naturally, their exposure to moonlight is also significant.

But what about the corals at the bottom of the ocean? Well, they have evolved to survive in low-light environments.

First of all, the water attenuates the sunlight during the day, permitting only dim colors like blue to reach the corals.

Secondly, moonlight cannot reach these corals. The bottom of the ocean is pitch black at night.

According to the American Museum of Natural Science, the aphotic zone (the region without light) starts at 200 meters.[1] Here, you only find blue-green light too weak to support photosynthesis.

That soft blue-green light doesn’t exist at night. And yet, you will find so-called cold-water corals at those depths.[2]

Roughly half of all coral species are cold-water organisms living deep in the ocean.[3] They continue to survive despite the absence of light at night.

What does this tell you? Well, in short, corals don’t really need light at night to survive.

You may argue that shallow-water corals are different because they are close enough to the surface to receive moonlight every night.

But what happens on days when the moon doesn’t shine or when it hides behind a thick cloud cover? Shallow-water corals survive all the same.

These organisms get all the light their algae require for photosynthesis during the day.

Even corals that live in shallow water barely get light at night.

2. Blue Light Is Not Necessary At Night Either

Some people add blue LEDs to reef tanks at night to mimic the moon. They normally transition from white daylight bulbs to blue LEDs when the sun sets.

Reef tanks can also tolerate red LEDs at dusk. But aquarists favor blue because it does a better job of boosting photosynthesis without disturbing the fish.

However, the moon only appears blue on TV. If you’ve ever looked at an actual moon in the sky, you know it has many colors, ranging from gray and white to silver, red, and yellow.[4]

Few people would call the actual moon blue. Therefore, suggesting that a blue LED mimics the moon is inaccurate.

This shouldn’t stop you from adding blue LEDs to the tank, so long as you remember to limit the intensity.

A blue light bright enough for you to read by is too intense.[5] Cut back. When deployed appropriately, moonlight bulbs offer several benefits, including:

  • They allow you to view the tank’s inhabitants without disturbing them.
  • They allow nocturnal animals to feed and hunt at night.
  • They improve your tank’s appearance.

3. Leaving The Lights On At Night Will Disturb Your Fish

Corals are not your only concern. Don’t forget that your reef tank houses fish. Even if your corals can withstand 24 hours of light, you can’t say the same for fish.

Fish have a circadian rhythm. They need a period of darkness at night to sleep, and light disrupts their rest because they don’t have eyelids.

In other words, they can’t just block out the light. Exposing them to 24 hours of light will lead to stress, and stress can kill fish.

Some people would argue that fish cannot see blue light, and therefore can sleep while it is on. But that couldn’t be further from the truth.

Again, you can’t base your decision here on anecdotes because they will only confuse you.

Some people keep blue LEDs on 24/7, and their fish and corals thrive. Others have killed their fish by keeping the blue LEDs on all night.

You can experiment. Keep the moonlight bulbs on at night for a few days, and take note of the reaction from your fish.

If the creatures manifest adverse side effects, revert to a regular day/night cycle (8 to 12 hours of daylight and an equal amount of darkness).

Naturally, this consideration doesn’t matter as much if your reef tank has corals and nothing else.

A beautiful reef tank lit up at night.

How Many Hours Of Darkness Do Corals Need?

First, we must ask how many hours of darkness corals get in the wild. After that, we should try to provide a similar amount in the aquarium.

Aquarists with coral-only tanks are tempted to keep the lights on all night.

However, a 2020 study from Bar-IIan University noticed that artificial lighting at night could disrupt a coral’s reproductive cycle.[6]

The research team observed delayed gametogenesis in coral species exposed to supplemental lighting from sundown to sunrise.

On the other hand, the control group lived under conditions with natural solar light and moonlight phases.

They were not forced to tolerate supplemental light at night. As a result, their reproductive cycle progressed without delay.

This should encourage consumers to limit a reef tank’s light. Give them six or more hours of darkness.

How Many Hours Of Light Do Corals Need?

You shouldn’t overwhelm your corals with light. They won’t survive (especially those that require low to moderate light).

Some people think that warning only applies to nighttime lights. They are correct to apply caution at night.

Dror Avisar, Inbal Ayalon, Jennifer I.C. Benichou, Oren Levy, and Laura F. de Barros Marangoni published a study proving that artificial lighting at night causes oxidative stress and lower photosynthetic performance in red sea corals.[7]

But that doesn’t mean intense lighting during the day is acceptable.

Excess light raises the temperature and leads to coral bleaching. Stick to the tried and tested 8 to 12 – hour rule. It persists because it works.

It is worth mentioning that you can experiment with other colors during the day. Some corals respond very well to green. You might find red useful as well.

Like fish, corals need about 8 to 12 hours of light followed by a similar period of darkness.

What Happens To Corals At Night?

Corals do more at night than people realize.

First, coral polyps emerge, using their stinging tentacles to catch and eat any prey they come across. They swallow and digest these critters.[8]

Secondly, many aquarists understand that corals use the nutrients zooxanthellae provide to make mineralized skeletons in a process called calcification.

But they probably don’t realize that calcification increases at night because of a gene called STPCA (which makes an enzyme that turns carbon dioxide into bicarbonate), which is twice as active at night.[9]

Additionally, photosynthesis (which requires light) removes hydrogen atoms during the day, and hydrogen contributes to the calcification process.

The absence of light at night allows the hydrogen atoms to accumulate.[10]

What Is The Easiest Way To Properly Light A Coral Tank?

Many aquarists forget to turn off the light when they go to sleep. Others forget to do that gradually, especially at dusk or dawn.

Since this has happened to me countless times before, I was looking for an easy solution to automate the entire process.

That’s when I found the Hygger 24/7 Lighting Aquarium Light (link to Amazon). This device works absolutely perfectly. You don’t have to remember anything.

It is set to the following schedule by default, providing mostly white light during the day, and blue light towards night. This schedule is also beneficial for fish:

8 am – 6 pmWhite color
6 pm – 10:50 pmBlue color (imitating the moonlight)
10:50 pm – 6 amLights are off
6 am – 8 amOrange color (imitating the sunrise)
8 am – 6 pmWhite color again, and so on

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.


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

  • Corals in the wild don’t receive light at night, so they don’t need it to survive.
  • Corals at the bottom of the ocean have evolved to live in low-light environments, so there’s no need for light at night in reef tanks.
  • Blue light isn’t necessary at night either, but blue LED lighting can be used for viewing the tank’s inhabitants, feeding nocturnal animals, or improving the tank’s appearance during the day.
  • Leaving the lights on at night can disturb corals, which have their own circadian rhythm.
  • A study showed that artificial lighting at night could disrupt the coral’s reproductive cycle.
  • The number of hours of darkness required for corals can vary, but you should aim for at least 6 hours on average.