Raise your hand if you’ve ever turned on your tank lights in the morning only to find one of your fish stone dead. Now raise the other hand if the “deceased” was magically resurrected within five minutes, swimming around and begging for food as if it didn’t just almost give you a heart attack. Sounds familiar, doesn’t it? 

What in the world is up with sleeping fish? Why do our aquarium inhabitants spook us by snoozing in the strangest places? Why do some add extra drama by flipping themselves upside-down? Let’s find out.

Do fish actually sleep?

As you probably concluded after witnessing your first miraculous piscine resurrection: yes, fish do sleep—sometimes quite deeply.

Although what a fish considers to be restful repose has little to do with how we humans like to snooze, they need their downtime just like everyone else. To this end, their biological clocks are tuned into the planet’s environmental cycles. This means they’ve got circadian rhythms just like other animals: when it gets dark, melatonin is released, signaling it’s time to settle down (or the other way round, in fish that are active during the day).

To help ensure the rhythm never goes off-kilter, the body also registers the subtle temperature differences between daytime and nighttime. Even small fluctuations (as little as 3.6°F for some species) can help tell a fish whether it should go to sleep or not—even if it’s a cave fish living in complete darkness.

But how do we know when a fish is asleep if it doesn’t don pajamas and tuck itself into bed at night? In humans and other mammals, it’s easy to tell: hook them up to an EEG, and you’ll notice the typical slow-wave brain activity or twitchy REM state that tells you they’re out cold. The question of what constitutes sleep in other animals has been a topic of lively academic debate and the subject of many a scientific paper. Luckily for us, it has mostly been resolved in fish by now.

In addition to their responses to circadian factors, like light and temperature, there are five clear indicators that allow us to distinguish a sleeping fish from one that’s ill or just taking a little rest.

You won’t find fish in REM sleep, but you’ll find:

• Immobility: Fish don’t sleep the same way we do, but they’ll become inactive and even somewhat limp. 
• Increased arousal threshold: A sleeping fish won’t respond to stimuli the way it normally does. Scientists have found they can sometimes pick up sleeping fish from the bottom of their tanks*.
• Specific posture and/or location: Every fish species has its own way of staying safe and functional while asleep.
• Homeostatic regulation: If a fish has a bad (or short) night’s sleep, the body will generally compensate for this by sleeping longer or deeper during the next sleep cycle.
• Rapid reversibility: Once it has been disturbed or wakes up naturally, the fish quickly returns to its normal state. If it doesn’t, it’s more likely to be in a coma!

*Don’t try this at home, as waking up in a human’s clutches is obviously pretty stressful for a fish. 

Other characteristics of fish sleep that tend to spook aquarists include the way some species pale or darken when they’re resting, plus their tightly clamped fins, which during daytime would both be indicators of illness. If the fish is asleep or has just woken up, however, there’s no problem; rapid reversibility means it’ll be back to “normal” in a jiffy.

Strange places you may find your fish

As we saw above, all fish sleep—but they have different ways of going about it. With all these different sleeping instincts, it’s no surprise they fall asleep in the most unexpected places in our tanks.

In the wild, these sleeping styles play out in some surprising ways:

• Some filefish use their teeth to anchor themselves to corals to take advantage of their host’s structural safety and stinging cells.
• Some wrasses burrow into the substrate to sleep (which is why many species should be kept on a sandy substrate; sharp gravel can hurt them). 
• Other wrasses will build sleeping nests out of small reef rubble.
• Many fish wedge themselves into caves, crevices, or under corals for improved safety.

The actual sleeping posture varies very widely as well. A 2024 review found that some captive freshwater killifish float at the surface with their heads pointing up, while zebrafish may sleep in the same spot with their heads pointing down. Some tetras drop to the bottom of the tank, while other fish float right above it in a normal, horizontal position. 

What I’m trying to get at is that you could find your aquarium fish asleep anywhere. Its instincts dictate its preferred resting style, but how to express this in our home aquariums is up to the fish and the environment. You might find them in the sand, wedged behind objects in seemingly impossible positions, hidden between coral branches, laying motionlessly on the substrate, floating colorlessly around the surface as if long deceased, or even missing entirely (probably hiding between rocks; they’ll reappear when they wake up). They can look to be completely stuck in tank décor and then rapidly dart away—while giving you the side-eye—when you mount a rescue operation.

Their sleep styles may seem random at first, but if you keep an eye on your fish night after night, you’ll start to gain a better understanding of their normal preferences and patterns. This allows you to more quickly tell when something is actually wrong, so keep an eye on those fish!

But is my fish alive or not?!

Even when you’re aware of their strange sleeping styles, actually finding a snoozing fish can still be jarring. If you’re unsure whether yours is alive or just relaxing, check its breathing first. It may be limp and look completely off, but as long as its gill movements are slow and even, it’s probably absolutely fine.

Still unsure? You can consider turning on a room light or gently tapping the glass near the fish. Don’t poke or startle it, as this can cause it to bolt and crash into decor or the tank glass (or even get some airtime if the lid is open). You’re just looking to introduce a subtle stimulus that will hopefully cause the fish to shift slightly, confirming it is in fact not dead or actively dying.

Conclusion: Why do fish sleep in strange places?

With our newfound knowledge of how fish sleep, we can answer this question easily: they don’t think those places are strange at all. We might feel confused when we find one of our aquarium inhabitants wedged upside-down behind the filter—that can’t be comfortable?!—but the fish probably thinks it has done a pretty good job of avoiding predation tonight, thank you very much. 

If you find your fish asleep in a strange place in its tank and get to wondering why, your best bet is to think about how this sort of behavior might benefit it in the wild and how it got (mis)translated to captivity. Hiding yourself in reef crevices is a pretty good way to stay safe while you’re in a vulnerable sleeping state. If these crevices are not available, then the small space behind the filter can start looking mighty attractive.