Do fish sleep at night? How to tell when they are resting underwater, even without eyelids
Normally, we think of night as the time for sleep.
But what about fish beneath the water's surface—do they sleep, too?
For me, having studied fish and written articles on the science of fishing, this is a question that feels familiar, yet one I have never properly organized.
Last time, I wrote about how standing by the water at night helps quiet a mind filled with work and plans. So, while we spend our time like that, are the fish sleeping? Are they waiting for food? Or have they begun to move in places we cannot see?
Tonight, I will take a look at the night from the perspective of the fish, not the human.
How can you tell if a fish is sleeping?
To give the answer first, at least some fish have states that have been confirmed as "sleep" based on their behavior. However, many fish do not close their eyelids like humans do. So, what should we look at to know if they are sleeping?
When we hear the word sleep, we imagine closing our eyes and lying down. Because that marker cannot be used for fish, sleep is confirmed not as a single gesture, but as a combination of several changes.
Even if a fish is sitting still at the bottom of a tank, that alone does not mean it is sleeping. It might be resting to avoid a current. It might be waiting for a predator to pass by. There is also the possibility that it is waiting for food to approach.
Therefore, researchers combine multiple clues.
Activity decreases for a sustained period in line with a daily rhythm
Resting in a posture or location specific to that fish
Reduced responsiveness to stimuli that would normally be noticed
Ability to become active again if stimulated
Increased rest after being disturbed
The last change is somewhat similar to how we try to catch up on sleep after being sleep-deprived. They increase their rest afterward, as if to make up for the time they could not rest.
Of course, this is not a universal checklist that can be applied to every fish. There are still not many fish species for which all these clues have been examined at once, and the criteria for determining sleep vary by study. Since some studies use a period of inactivity as the main criterion, it is necessary to think of rest, low activity, and sleep as separate things (Norman et al., 2024).
Rather than applying the exact same yardstick as human sleep, we look at the overlap of "when, where, what posture they take, and how their response to the surroundings changes." Through that accumulation, the difference from a state of simply not moving gradually becomes visible.
"Not moving" may be an entry point for thinking about fish sleep, but it is not the answer itself.
What Brown Trout Taught Us About "Fish Sleep"
There is research that has confirmed this distinction using fish familiar to anglers.
In 2024, a study was published observing juvenile brown trout caught in a river in Hokkaido, one by one in tanks (Furusawa & Koizumi, 2024).
While resting on the bottom, the fish showed a dulled response to external stimuli. Furthermore, when their rest was disturbed, they increased their resting time afterward. The researchers determined this to be sleep as a behavior by layering not just "being still," but also posture, responsiveness, and changes after sleep deprivation accordingly.
However, this was not a study observing fish sleeping on a natural riverbed. It was an experiment where fish collected in the wild were examined in tanks. In a natural environment with river currents, predators, prey, and relationships with other fish, the time and place for resting may change.
What is even more interesting is that the time usage of the 43 individuals was not uniform. Some individuals were biased toward activity at night, some moved throughout the day and night, and others were in between.
It has long been established for many fish species that activity times can change depending on the season, growth stage, light, temperature, food, and the risk of predation (Reebs, 2002). However, what we are mainly looking at here is "when they move." This is not the same as "when they sleep."
Even within the same species, they do not all follow the same schedule. The fact that it cannot be decided so simply is part of the fascination of fish at night.
Are fish that can eat in the dark active at night?
Here, an important distinction emerges for connecting sleep and fishing.
In an experiment using largemouth bass, a type of black bass, the success rate for catching small prey fish exceeded 95% even in weak light equivalent to moonlight. In darkness as dim as starlight, it dropped to 62%, and in total darkness, they were almost unable to catch anything (McMahon & Holanov, 1995).
Looking only at these results, one might be tempted to think, "Bass might be fish that are active even at night." However, what this experiment showed was their ability to find and catch small fish even in weak light. It was not a study measuring when they are most active in the wild.
On the other hand, a Canadian study published in 2026 attached small tags to 17 largemouth bass to measure their body movements. On average, movement was high at dawn and low at night. However, that does not mean they did not move at all at night. These values are not direct indicators of sleep, appetite, or ease of catching (Hlina et al., 2026).
The results of the tank experiment and the field study do not contradict each other. One looks at the ability to prey in dark places, while the other looks at the magnitude of actual movement at the water's edge. They are answering different questions.
They can eat in the dark. But that does not necessarily mean they are moving at that time.
Being willing to eat and reacting to a lure are also different things.
The presence of fish is different from them eating
At a night fishing spot, there are times when you can see the fish, but they do not react to bait or lures. At such times, if you think, "Since the fish are there, they should eat," you end up wanting to settle on a single reason for the lack of reaction.
However, just being there only indicates their position. The fish might be in a state of low activity. They might be resting. Even if they are waiting for food, they may not be in a state where they would pursue what is in front of them. Of course, there is also the possibility that they are simply not interested in the bait.
This is not a conclusion directly proven at the fishing spot by the research introduced here. These are multiple hypotheses that I want to consider separately when fish are visible but not reacting. I believe the biggest hint that sleep research gives to fishing is to rethink not just 'if fish are there,' but 'what state are they in right now?'
Rather than memorizing a single time when fish bite, look at the conditions that cause the fish's state to shift. That is the perspective that can be brought back from sleep research to fishing.
Even at night, there is more than one way to spend the time
At least some fish have sleep that has been confirmed through their behavior. However, it does not mean that all fish rest at the same time once it gets dark. Even within the same species, there are differences in the allocation of time between activity and rest.
It is said that night is a time for sleep. But as everyone who gathers in this magazine knows, there is more than one way to spend the same night. While we cannot speak of humans and fish as the same, the same darkness does not bring the same state to all fish.
When I look into the night from the fish's perspective, I realize that we were the ones making assumptions based solely on the time, thinking 'they rest when it gets dark' or 'they move if it is night.' The fish's night was not such a simple binary choice.
That is why, rather than guessing one time when they bite well, I want to distinguish and see what state the fish are in right now. The simple question of 'do fish also sleep?' led to the perspective I want to value in 'scientizing fishing.'
Night is not a bell that puts all fish to sleep at once.
There is a night for each of them underwater as well.
Next time, I will consider how the moon and streetlights reshape that night.
References
Furusawa, C., & Koizumi, I. (2024). Behavioural sleep in salmonid fish with flexible diel activity. Animal Behaviour, 209, 43–52. https://doi.org/10.1016/j.anbehav.2023.12.013
Hlina, B. L., Rous, A. M., Piczak, M. L., Midwood, J. D., Brownscombe, J. W., Portiss, R. J., Sciscione, T. F., Wells, M. G., Doka, S. E., & Cooke, S. J. (2026). Seasonal effects on the acceleration of largemouth bass and Northern Pike in Toronto harbour. Animal Biotelemetry, 14, Article 5. https://doi.org/10.1186/s40317-026-00444-6
McMahon, T. E., & Holanov, S. H. (1995). Foraging success of largemouth bass at different light intensities: implications for time and depth of feeding. Journal of Fish Biology, 46(5), 759–767. https://doi.org/10.1111/j.1095-8649.1995.tb01599.x
Norman, H., Munson, A., Cortese, D., Koeck, B., & Killen, S. S. (2024). The interplay between sleep and ecophysiology, behaviour and responses to environmental change in fish. Journal of Experimental Biology, 227(11), jeb247138. https://doi.org/10.1242/jeb.247138
Reebs, S. G. (2002). Plasticity of diel and circadian activity rhythms in fishes. Reviews in Fish Biology and Fisheries, 12, 349–371. https://doi.org/10.1023/A:1025371804611