Can Fish Sleep With Blue Light on? Does it Affect Their Sleep?

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As an aquarist, I enjoy watching my aquarium at night. There were times when I even used blue lights since I heard it calms down the fish. However, as I learned over the years, keeping the blue lights on at night may interfere with the fish’s sleep ability. To make sure you take the right approach in your tank, I decided to spread all my knowledge on this matter.

Fish cannot sleep with blue lights on since they require entirely dark conditions for sleeping. Keeping the lights on at night will eventually impact their health, behavior, and well-being. However, blue lights can induce sleep as a means of transition between light and darkness.

As we move forward, I will share a few tricks that will make sure you don’t accidentally leave the blue lights on at night. I will also show you how nocturnal fish, such as Cardinalfish, may react to those lights at nighttime. 

Will Fish Sleep With Blue Light?

If you want to know whether your fish will respond positively to blue light, you’re not alone. LED lights are attractive because you can get them in so many colors. However, you cannot expect every single color to benefit your fish. Some are better than others. 

The question of blue light matters because the color is quite popular among aquarists. In many cases, it is packaged as the ‘Moonlight’ mode of an LED lighting system. Aquarists flock to it because they believe that it mimics the moon, allowing their fish to transition from day to night and vice versa.

Whether or not it achieves this objective doesn’t matter. There is a big difference between using blue light to aid your fish’s transition from day to night and keeping it on all night:

1. The Effect of Blue Lights on Sleep in Fish

Blue lighting is popular because it produces a calming effect. Fish like it because, as was mentioned above, it mimics the moonlight. But can you keep it on all night? No, you can’t, not unless you want your fish to suffer in the long run.

Blue light performs several functions. First of all, most aquarists install blue lights because they want to see their fish at night. If you understand the importance of switching bright aquarium lights off at night, but you want to observe your fish, all the same, a blue light will illuminate the tank without disturbing the fish.

Secondly, blue lights will enhance the appearance of your fish, allowing their colors to pop and creating a beautiful ghostly environment in the water without straining your eyes or ruining the fish’s attempts to relax. They are perfect for aquariums that are located in bedrooms.

Third, your fish can use blue lights to transition from daytime to nighttime, as was noted before. The calming effect of the color will compel them to slow down. It is also worth noting that, in the wild, the lighting changes gradually. Suddenly switching the lights in an aquarium off is just as likely to induce panic. 

You are better off dimming your ordinary lights or using blue lights to guide your fish from daytime to night time. However, you need to switch those blue lights off at some point. If you don’t, they will impede your fish’s ability to sleep. Even though blue lighting induces calm in fish, most aquarium species require several hours of darkness every day.[1]

Blue lights are dim, but they can still have a disruptive influence on your fish’s sleep. Many aquarists leave their blue lights on by accident. They turn them on because they want to observe their fish, but they doze off an hour or two later.

You can expose your fish to blue lighting overnight every so often without causing any lasting harm. But you cannot make it a habit. That is why timers are so important. They will ensure that your fish acquire the hours of darkness they need to rest.

2. Blue Lights and Nocturnal Fish

Blue lighting is a problem for fish that follow a regular day/night cycle. You cannot keep such lights on all night long. That being said, blue lights are perfectly acceptable for tanks with nocturnal fish. Their calming effect will encourage shy nocturnal fish to emerge from their hiding places, swim, eat, and go about their business.

That is why the color is so standard in bedrooms. Blue aquarium lights are dim. You can keep them on all night in a dark bedroom without impeding your own sleep. But again, that is only appropriate for tanks with nocturnal fish. If your fish are awake in the day and asleep at night, switch the blue lights off before you sleep.

3. Alternative Lighting Options

As was mentioned, the best way to avoid sleep disruptions is to use a timer. This way, you can forget your aquarium while ensuring the required dark hours for your fish. The one that I personally use is the BN-LINK 24 Hour Plug-in Mechanical Timer (link to Amazon). All you need is to set the lightning hours and plug in the LED’s cable.

However, if you want to keep your aquarium lit at night to observe your fish and you wish to avoid blue lights with a timer, invest in some red LED lights. That is highly recommended if the idea of accidentally leaving your blue lights on all night frightens you.

First of all, in terms of their impact on your tank’s appearance, red lights are just as attractive as blue lights. They will make your fish’s colors pop. Secondly, it has been theorized that fish cannot see red light. Or, at the very least, they cannot differentiate it from other colors.[2]

Therefore, for fish, a tank lit with red LED lights might as well be dark. The lights won’t impede their sleep. If you are determined to light your aquarium at night, use red LED lights. I personally recommend getting the NICREW ClassicLED (link to Amazon). That device covers white, blue, red, and green LEDs with a spectacular performance.

4. Fish and Sleep

First of all, you need to understand that fish require sleep. They don’t sleep like ordinary people; that is to say, they are accustomed to living in rife conditions with predators. As such, some of them cannot afford to sleep as soundly as humans do.

Many species have brains that remain relatively active during sleep. That allows them to stay alert to nearby dangers. Others will continue to swim during these periods, especially species that migrate in large schools.[3] 

Some fish will shut down altogether, especially those creatures in aquariums that can count on the relative safety of their environment. They will settle at the bottom, becoming so still in some cases that an amateur aquarist might confuse their sleeping state for death.

Regardless of your fish’s sleeping preferences, most aquarium fish require a period of rest every night. A failure to provide the sort of environment that permits sleep will result in the manifestation of symptoms related to stress, such as lethargy and loss of appetite.

Is Blue Light Good for Fish at Night?

Blue lights are good for fish at night. Because they mimic the moonlight, you can use them to create a transition from dusk to dawn and dawn to dusk. Usually, the goal is to keep the lights on for an hour or two to signify the shift from daytime to nighttime and vice versa.

Lighting matters more to fish than it does to humans. On the one hand, fish do not necessarily require light to navigate. On the other hand, light allows them to create a routine and identify a day/night cycle that will govern their habits.

In an aquarium, fish cannot differentiate between daytime and nighttime. Some people resolve this issue by positioning their aquariums near the window. That sounds like a practical solution, but it is dangerous because the tank is exposed to direct sunlight that can cause drastic temperature changes.[4]

If your tank is protected from the sun, your fish need aquarium lighting to create a day/night cycle. That will regulate their eating and mating habits. The lighting is even more critical where their sleep is concerned. That is because fish don’t have eyelids.

As such, they don’t have the option of escaping bright lighting by closing their eyes. This is why aquarists are encouraged to define and create a proper day/night cycle in their tank. Fish need roughly 8-12 hours of darkness every night.[5]

Without this period of darkness, they cannot sleep properly. And without sleep, your fish will struggle with crippling and potentially fatal stress. They will also be more susceptible to diseases and bullying tankmates. 

Does Blue Light Affect Fish?

Blue light and artificial lighting, in general, affect the way fish behave. It impacts their sleeping and eating patterns. It can even affect their reproduction. Some industries use artificial lighting to delay the sexual maturation of their fish.[6]

People are so keen to reproduce moonlight in aquariums using blue light because fish in the wild react to moonlight. Those that spend most of their time at the bottom of water bodies will surface in response to moonlight. This is also seen in tanks. 

Shy and timid nocturnal fish find blue lights soothing, and they are more likely to swim into the open when you switch the blue lights on. Studies that were done on Nile Tilapia found that blue light could prevent stress, validating claims that experienced aquarists have made regarding the ability that blue lights have to induce calm.[7]

Everyone agrees that blue lighting has a positive impact on aquarium fish. However, as was mentioned earlier, it just isn’t a good idea to leave your blue lights on all night. The better approach would be introducing blue light as a transitional period before nighttime.

Is it Okay to Leave Aquarium Lights on at Night?

It is not okay to leave aquarium lights on at night. Keeping the lights on 24/7 will prevent your fish from sleeping. Fish that can’t sleep will eventually struggle with stress, aggression, loss of appetite, timidity, and numerous other symptoms that will cause distress in your fish.

If you have incandescent bulbs (or metal halides), keeping them on all night is an even more significant hazard because they produce a lot of heat. They could raise the temperature of your water drastically. Yet, the biggest challenge is algae.

Aquarium lighting encourages algae to grow and multiply. Keeping your lights on all day and all night will cause your algae infestation to overwhelm your aquarium, not only out-competing your plants for resources but ruining the appearance of your aquatic environment.[8]

Blue lighting cannot save you from this problem. Algae can grow in blue light as well, which is why keeping your blue lights on all night is a bad idea. Not only will it impede your fish’s sleep, but it will exacerbate your algae problem.

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Just like humans, fish require dark conditions to sleep correctly. In the wild, they do have the moonlight to light up the water during nighttime. However, in specific depths, they enjoy sealed, dark conditions.

Therefore, keeping blue lights on all night could harm your fish. That is even though blue lights are meant to resemble the moonlight. Continuous illumination may end up with lethargic fish that are more vulnerable to bullying and diseases.

If you wish to use blue lights to observe the tank at night, I highly recommend considering a timer. That will make sure you won’t accidentally leave the lights on once you fall asleep. You may also consider red lights. There are assumptions that fish cannot see those and may sleep as if the tank is completely dark.