The short answer
Most aquarium fish rest rather than sleep the way we do: with no eyelids to close, they go still, breathe more slowly and react less, on the bottom, in caves, near the surface or among plants, says Aqueon, an aquarium products maker. Most rest at night with the light off; some species are nocturnal and rest by day.
This guide is written for freshwater tanks; marine examples are labeled.
What Counts as Sleep for a Fish
NOAA, a US federal agency, says: “The nature of fish “sleep” is an area of active research. While fish do not sleep in the same way that land mammals sleep, most fish do rest.” It adds that “fish may reduce their activity and metabolism while remaining alert to danger.”
Michael Heithaus, a marine biologist at Florida International University, writes in a children’s Q&A column that “Most fish don’t have eyelids, so their eyes don’t close.” A 2024 review in the Journal of Experimental Biology (JEB) lists four criteria for behavioral sleep: “(1) circadian-regulated periods of quiescence, (2) an increased arousal threshold, (3) species-specific posture and (4) homeostatic regulation.” It says “only a few fish species have been empirically tested against all criteria for sleep”.
In adult zebrafish (zebra danios), a 2007 PLOS Biology study describes “reversible periods of immobility, increased arousal threshold, and place preference”, and a “sleep rebound” after rest deprivation by gentle electrical stimulation.
Aqueon, which notes that “Scientists haven’t done a lot of research on fish slumber”, says fish “need to sleep on a regular basis to stay healthy and live long, happy lives.”
Where Fish Rest: Research Postures by Species
Aqueon says where fish sleep “differs from species to species”. The JEB review says postures “have been observed and described in a small number of fish species”, and: “Whether these specific sleep postures are accompanied by the use of specific sleep sites is unknown, as their observation is mostly limited to laboratory settings.” Aqueon’s rows are general descriptions, not study results.
| Fish | Resting position | Source |
|---|---|---|
| Zebrafish (zebra danio) | Floating head down, or horizontal on the bottom | JEB review |
| Cardinal tetra | On the bottom (anecdotal) | JEB review |
| Banded knifefish | Head up, at 30° to the bottom | JEB review |
| Several barbs (Barbus) | Tipped at 45-60° | JEB review |
| Certain catfish and plecos, some knifefish, loaches | Nocturnal: rest by day in a cave or crevice | Aqueon |
| Some parrotfish and wrasses (marine) | Inside a mucus cocoon | Aqueon |
Aqueon warns about keeping small tetras such as neons with “large-mouthed catfish such as Pictus cats”: “While the Neons are huddled on the bottom at night, the catfish are cruising around looking for an easy meal.” See our checklist for choosing fish (tank mates) and our 10 commonly recommended beginner species.
Tip
Aqueon: “Provide plenty of cover in your aquarium as well, so all your fish have a place to feel safe while at rest.”
Lights, Timers and the Day-Night Cycle
Aqueon says “Most aquarium fish are diurnal, meaning they move about during the day and rest at night”, and that “The easiest way to make sure your fish are sleeping enough is to put your aquarium light on a timer, giving them a consistent day and night routine.” Chewy, a pet retailer, quotes aquatic veterinarian Jena Questen: “It is important that fish get some periods of darkness each day so they can sleep restfully and not be kept under bright lights 24 hours a day.” Aqueon’s disease page lists “leaving the light on 24/7” among stress sources, without linking it to sleep.
In adult zebrafish in a lab, the 2007 study found that “light suppresses sleep in zebrafish”, with “no evidence for a sleep rebound” afterward. Aqueon, summarizing zebrafish work, says fish kept under constant light for several days “slept normally when returned to a regular day/night cycle.” These are laboratory results; neither source tests a home tank.
No source here gives a light schedule for a home aquarium. JEB notes that in zebrafish “a 6-h night-time period is sufficient for homeostatic sleep” in lab conditions. The figures below are Aqueon’s advice for limiting algae (the planted figures are for plants), not sleep research, and its two pages differ:
- Algae prevention page: “A maximum of 6 to 8 hours of light is sufficient in unplanted aquariums, while planted aquariums need 10 to 12 hours of high-quality light per day.”
- Green water page: “Planted aquariums need 8 to 12 hours of light daily, while non-planted aquariums can get by on 6 hours or less.”
Heithaus’s column, which cites no study, says “Aquarium fish sleep between seven to 12 hours each day.” Chewy says “the duration varies between species.”
Our guide to adding new fish covers how long sources say to keep the light off while new fish acclimate.
Our planted aquarium light roundup compares timer lights for planted tanks, with each maker’s safety warnings.
Resting or Unwell?
Aqueon’s signs of a sleeping fish:
- “They remain motionless for periods of time.”
- “They lay on the bottom or an object in the aquarium.”
- “They don’t react to things going on around them.”
- “They exhibit this behavior at about the same time each day, usually when the aquarium light is turned off.”
NOAA says resting fish may remain “alert to danger”, and Chewy says responsiveness drops “slightly”. Chewy reports Dr. Questen’s view that a fish active a few hours earlier and now still is “most likely sleeping rather than dead or ill.” Chewy adds: “Sick fish might sit still like this all day long, which is a sign that something is wrong.”
Warning
These signs cannot tell you what is wrong with a fish. If a fish stays still through the day, or you are not sure, test the water and contact an aquatic veterinarian. Related guides: tank temperature by fish type, raising oxygen and keeping aquarium water clear.
More in our Tank Setup & Care section.
Frequently Asked Questions
Q. Do fish sleep with their eyes open?
Yes, for most fish. Chewy says that even so, “their body slows down and they become less responsive to their surroundings.”
Q. What time do fish go to bed?
Most rest at night, usually once the light goes off (Aqueon). Besides nocturnal fish, Chewy says “Others are considered crepuscular, which means they’re most active during dawn and dusk.”
Q. Do fish sleep on the bottom of the tank?
Many do: Chewy lists “Settling toward the bottom of the tank” as a sign, and the zebrafish study found “most sleep episodes occurred at the bottom or the top of the tanks”.
Q. Do fish sleep sideways?
Chewy says resting fish “may seem less alert or tilt against a fixture”. No source here describes a fish lying flat on its side; the table’s tilts are reported research postures. If you see it, test the water and contact an aquatic veterinarian.
Sources (9)
- NOAA National Ocean Service, Do fish sleep? (federal agency)
- Aqueon, Fish Sleeping Habits (maker)
- Aqueon, Prevention and Control of Nuisance Algae (maker)
- Aqueon, Green Water: Causes and Cures (maker)
- Aqueon, Freshwater Aquarium Disease Prevention (maker)
- Norman et al., Journal of Experimental Biology review (2024) (peer-reviewed review)
- Yokogawa et al., PLOS Biology zebrafish study (2007) (peer-reviewed study)
- FIU News, Michael Heithaus on fish sleep (university; children’s Q&A column)
- Chewy, Do Fish Sleep? (pet retailer)
