If you keep fish, you have probably asked yourself: do aquarium fish sleep? And if they do, how can you tell? Being a fish owner myself, I never knew that they sleep, until one day I noticed that they do, at different times of the day, and each of them seems to have its own sleep schedule.
From my own aquarium: I first noticed my fish sleeping when I saw that they did not all rest at the same time. Some settled down for the night as soon as the lights went off, while others rested at different times of the day. Since then, I have watched my fish’s routines more closely before deciding something is wrong.
Do Fish Sleep?
Fish do not sleep like humans, but they do need rest. They rest by reducing their activity and responsiveness. Since fish have no eyelids and no neocortex, the signs we use to spot sleep in people and other mammals do not apply, and scientists look at behaviour instead. A fish is considered to be sleeping when it stays inactive for a while, rests in a typical posture or place, follows a daily cycle, and reacts less to what is going on around it.
Research on zebrafish, a common aquarium fish, shows that their brains show patterns similar to slow-wave and REM-like sleep in mammals, according to a Stanford Medicine study. The same research notes that fish sleep is not identical to human sleep. Scientists are still learning how widely this applies, and a peer-reviewed review points out that fish sleep has been under-studied.
Recommended reading: Free or Confined – Is it Cruel to Keep Fish in Tanks?
How Do Fish Sleep?
Most fish sleep by slowing down. Their movements, breathing and responses all slow, and they stay in a safe spot. They stay alert enough to react if something threatens them, so they are easier to wake than most land animals.
Where a fish rests depends on the species:
- Hovering in place in open water or among plants, with only tiny fin movements to stay balanced.
- Resting on the bottom or on the substrate.
- Hiding in caves, crevices, ornaments or thick plants.
- Wedging into a leaf or decoration, which is common in bettas.
A few fish never fully stop. Some sharks must keep swimming to push water over their gills, and they rest while moving. According to Britannica, fish also differ in how alert they stay depending on how much danger they face.
Did you know? Some marine fish, such as parrotfish, are known to wrap themselves in a mucus cocoon at night, which is thought to protect them from predators and parasites.
When Do Fish Sleep?
Most fish follow a regular daily rhythm, like other animals. What that rhythm is depends on whether the fish is diurnal, nocturnal or crepuscular:
| Type | Active | Rests | Examples |
|---|---|---|---|
| Diurnal | During the day | At night | Bettas, angelfish, tetras, goldfish, zebrafish |
| Nocturnal | At night | During the day, in hiding places | Many catfish and plecos, many loaches, eels |
| Crepuscular | At dawn and dusk | Mostly in the dark hours and less in full daylight | Some predatory and reef fish |
Not every fish follows the pattern. Some young fish are reported to sleep very little until they are several months old, and cave-dwelling populations of the Mexican tetra, a blind cavefish, sleep far less than their surface-dwelling relatives, probably because there is no daylight cycle in their caves.
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How Long Do Fish Sleep?
Most sources put the sleep of aquarium fish at roughly 8 to 12 hours a day, though it varies by species, age and tank conditions. Fish tend to rest in one stretch rather than in many short naps, although some, such as bettas, also take naps during the day.
Fish in a tank do not follow the natural sunlight cycle of the wild. They follow the tank’s light schedule, which means you control when they sleep. A regular schedule helps them settle into a rhythm. Aquarium writers such as Portland Aquarium suggest about 8 to 10 hours of light and the rest in darkness, using a timer.
Tips for a Better Fish Sleep Schedule
✅ Put the aquarium light on a timer, so it comes on and goes off at the same time daily
✅ Keep the tank dark at night, including blue “moonlight” LEDs, unless you keep nocturnal fish
✅ Give fish hiding places: caves, plants, leaves or decorations
✅ Avoid tapping the glass or switching on room lights suddenly
✅ If you keep day and night fish together, make sure each has a hide where it can rest undisturbed
How Can You Tell if Your Fish Is Sleeping?
It is usually easy to tell. Look for:
- Staying still for long periods, with only small fin movements
- Resting near the bottom, near the surface, among plants or in a hide
- Reacting slowly to movement around the tank
- Doing this at about the same time every day, usually when the lights go off
- Slow, steady gill movement
Do Fish Sleep at the Bottom of the Tank?
Often, yes. Many fish rest near the bottom, but others rest in the middle of the tank, on plants, on decorations or near the surface. Staying at the bottom does not always mean sleep: bottom-dwelling species such as many catfish simply live there, and illness or stress can also keep a fish low.
| Sign | Probably Sleeping | Possibly Unwell |
|---|---|---|
| Timing | Same time daily, usually when lights are off | Stays low all day, even with lights on |
| Response | Wakes and swims normally when lights come on or food appears | Ignores food and movement |
| Fins and body | Relaxed fins, possibly paler colour | Clamped or ragged fins, white spots, swelling |
| Breathing | Slow, steady gill movement | Fast gasping, especially at the surface |
| Position | Settled calmly, upright or on a leaf | Floating sideways or upside down, struggling to stay level |
If your fish stays at the bottom for long periods, check the basics first: test the water for ammonia, nitrite and nitrate, check the temperature, and look for signs of disease or bullying. Poor water quality is a common cause, and our guide on how to lower nitrates in your aquarium can help. If the fish does not improve, consult a vet who treats fish or an experienced aquarist.
Are Betta Fish Nocturnal?
No. Bettas are diurnal: they are active during the day and sleep at night. They are not high-energy fish, though, so they take plenty of naps during the day.
Their long fins on a small body make swimming tiring, so bettas like to rest on soft surfaces such as a broad leaf, a hammock or a decoration. Without something to rest on, a betta may settle at the bottom or float near the surface. Some also hide inside a decoration.
A betta resting on its side occasionally is not always a problem, but if it does so often, breathes fast, or cannot stay upright, it may be unwell and a vet who treats fish should see it.
Where Different Aquarium Fish Rest
| Fish | Usual rhythm | Where it often rests |
|---|---|---|
| Betta | Diurnal, with daytime naps | Leaves, decorations, the bottom or near the surface |
| Tetras | Diurnal | Hovering among plants or near the substrate, often paler at night |
| Angelfish | Diurnal | Hovering in a quiet spot among tall plants |
| Goldfish | Diurnal | Near the bottom or mid-water, motionless |
| Plecos and many catfish | Mostly nocturnal | Caves, wood and shaded spots during the day |
| Loaches | Often more active in the evening | Hides, sometimes lying on their side or upside down, which can look alarming |
Species vary, so watch your own fish and learn their routine. Rest places and times are general guides, not rules.
Frequently Asked Questions
Why do fish sleep with their eyes open?
Is my fish dead or sleeping?
Can fish sleep on their sides?
Do fish sleep better in the dark or in the light?
Should I turn off the aquarium filter at night?
How many hours do fish sleep?
Closing Thoughts
Do aquarium fish sleep? Yes, though not like we do. Fish need rest, and knowing how and when your fish rest helps you spot what is normal and what is not. Watch your own fish, keep a steady light schedule, and give them places to hide, and they will reward you with calmer, healthier behaviour.
Key Takeaways
- Aquarium fish sleep, usually for about 8 to 12 hours, by going still and responding less. They do not close their eyes.
- Most common aquarium fish rest at night, while many catfish, plecos and loaches rest by day.
- Use a light timer, keep the tank dark at night and provide hiding places so fish can rest well.
- Resting at the bottom can be normal, but staying there all day, gasping or floating sideways may signal illness or poor water quality.
- When in doubt, check your water and consult a vet who treats fish or an experienced aquarist.
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