Yes, aquarium fish do sleep, but not all species sleep at night. Nocturnal fishes like spend most of the day hiding (Learn why aquarium fish hide) under plants and rocks kuhli loaches and some snail species only coming out at night to feed.
How long your aquarium fish sleeps will also depend on the species as some are bigger lazy-bones than others, though most species rest for a duration anywhere from 8 hours to 12 hours.
Of course, this period is not entirely true for all fish, plus there is no expert documentation of the exact number of hours that fish sleep. Even so, considering most aquarium fish are diurnal, that time frame is a pretty reasonable estimate.
Interestingly, some fish species in the wild do not sleep at all including blind, cave-dwelling fish and deep-water varieties who swim continuously.
Please read on for more insight on this rather unusual but pretty necessary topic.
How Much Sleep Do Aquarium Fish Need?
I believe I have already said, although briefly, aquarium fish are diurnal, meaning they require an average of 8 to 12 hours of rest a day.
From my experience, most species will be active during the day when light is in abundance and only retire into their caves and what have you at night.
However, nocturnal fishes will rest during the day and only come out at night when the lights go out. In fact, when keeping nocturnal bottom feeders, it quite easy to think a fish is dead because they tend to hide in the plants for days on end, and the only way you’ll see them is at night with a flashlight.
Though in the recent past, new technologies in aquarium lighting have brought a solution in the form of moonlight LED bulbs, which are good for aquarium displays at night.
The lights are subtle in a way they do not stress any fish sleeping at night but are bright enough for you see your nocturnal fish go about their life in darkness.
Long story short, to give your fish enough rest at night, leave your aquarium lights on all day (assuming there is not enough sunlight to light you tank), then switch them off at night to allow your fish at least eight hours of rest. Better still, put your aquarium lighting on a timer to give them a consistent day and night routine.
That is especially important for you fishes health and well being. A good night sleep, same as in humans, will keep your fish less stressed during and swimming all over the tank; just the way you like it!
How Can You Tell If A Fish is Asleep?
While people retire to bed, close their eyes, and lie motionless from dusk till dawn, or like me, dusk to midnight to dawn, this is a luxury your fish cannot afford…
…but they still manage to get some good napping all night long.
So, how do they do it?
Well, they lie still mostly at the bottom of the tank or near the water surface. While they sleep, the fish will be conspicuously slow to react to things going on around them, and that’s why it’s important to have plenty of plants for fish to hide from belligerent members in the community should they try to seize that nap time to attack.
If you are keen enough, you may also notice a reduction in the rate of breathing characterized by a slower movement of your fish’s gills.
However, do not expect to see your fish closing its eye to sleep because fish don’t have eyelids. They don’t need them in water because there is no dust or dirt particle to protect their eyes against.
And how does your fish know when its time to sleep you ask?
They do have a biological clock system same way human beings do, and the clock is set to reference the diurnal-cycle, where lights mean day and darkness mean night or sleep.
Consequently, once you turn off the aquarium lights, the fish take that to mean bedtime, becoming less active and remaining still to rest.
But remember there are nocturnal (bottom-dwelling) fishes that come alive at night and don’t mimic the normal day cycle as surface and mid-level fish do.
So, instead of plunging your tank into total darkness at night, consider getting moonlight LED bulbs for your nightly fishies to about their activities.
This rather conveniently brings us to another common question “Do aquarium fish need darkness to sleep?".
Technically, YES, fish do need some period of darkness to rest and unwind. As I said, most time this will coincides with the normal daylight cycle, meaning eight to 12 hours of darkness at night are required by most aquarium fish.
For more insight on how darkness plays into your fishes resting and sleeping patterns, please have a look at this post.
Where Do Fish Sleep In A Tank?
Where you fish sleep will depend on which part of the tank the prefer to live in. It is unlikely that you’ll find you loach, pleco, or catfish resting near the surface of the water.
Likewise, your betta and other top and mid-level fish will not sink to the bottom for a nap. They will sleep closer to the water surface, the same way bottom-dwellers will sleep at the bottom of the tank.
However, while some species have no problem sleeping in the open water, most will prefer to hide within the plant foliage and rocks, just so they are safe from attackers.
Some loaches even virtually live within the foliage only coming out to feed at night.
Still, some fishes like cichlids will prefer to bunker in caves, driftwood, or rock formations while they sleep.
Just in case you have a fish in the tank that is not moving, and you are not sure whether it’s sleeping or sick, look for occasional movements like a flip of their fin or a movement of the gill, which fish use for balance while they are not actively swimming.
Also, a sleeping fish will exhibit sleeping behavior around the same time every day, any durations of ‘rest’ outside this time frame could very likely be caused by other reasons.