How Do Fish Sleep?

If you’ve seen the hit movie Finding Nemo, you’ve heard the phrase “just keep swimming.” This may have got you wondering: do fish ever take a break and just sleep? If so, what are the logistics of sleep for fish? How do they actually relax and sleep while underwater?

Since most fish don’t have eyelids, even if you’ve owned fish, the answers to these questions may not be clear. It’s natural to be curious about whether fish just go through life without ever catching some Zs.

Though we don’t know everything about the sleeping habits of fish, researchers have determined that most fish do sleep or have some physical process of rest and recovery. Even though it’s different from sleep in mammals, it still services an important biological role.

Why Do Fish Sleep?

Though this can’t be answered with 100% certainty, most experts believe that fish sleep in order to allow their bodies and organs to recover. In this way, sleep in fish promotes health like in people, however, the mental component is not the same: since fish have no neocortex in the brain, they don’t dream.

How Do Fish Sleep?

Because of the diversity of fish species, there is a wide range of ways that fish sleep. Some examples include:

  • Lying down: some fish sink to the bottom of their body of water where their bodies can slow down and rest. At times this can look like playing dead as in the case of the woolfish. Or for some parrotfish, it includes secreting a mucus layer as a bubble to help protect it from parasites.
  • Burrowing: some fish don’t just lie down, they burrow into their environment in a way that looks like how we get cozy with a mattress and blanket. In the case of coral reefs, some fish camp out in the coral, but their bodies keep moving like they are swimming while they sleep. This is part of a win-win relationship as the fish’s movement increases oxygen flow to the coral, which helps shelter the fish.
  • Drifting: with this type of sleep, fish more or less stop swimming and just drift. They may occasionally move part of their body in order to move a little, but compared to normal, their movement is very limited. Some fish that are kept in home aquariums sleep in this way, so you may have seen this type of fish sleep before.
  • With the school: some fish may be able to relax and rest within their school while other fish remain alert and keep the school together and safe.

During sleep, it is normal for many fish species to maintain a high level of alertness. This can help the fish make a quick getaway if they become threatened by predators. On the other hand, though, some fish, like the bluehead wrasse or Spanish hogfish sink into a deep stupor that allows them to be picked up without waking up.

Do Sharks Sleep?

Sharks belong to a type of fish species that has to swim in order to breathe. For this reason, sharks can’t just completely shut down to sleep. But recent research has indicated that sharks may be able to shut down part of their brain while still keeping their fins and tail moving, allowing them to recuperate in phases. But it’s also possible that sharks just don’t sleep at all, at least not in the way that we normally think about sleep.

Is Fish Sleep Consistent?

It seems clear that fish do not sleep with the same kind of consistency that most people do. While the Circadian rhythm (sleep-wake cycle) for most people is set firmly with their daily light-dark cycle, in many fish, it’s much easier for this to change. For example, fish may increase or decrease their sleep in response to their needs for migration, spawning, or parenting (such as guarding or monitoring eggs). The sleep cycle of a fish may also change based on water temperature, the presence of predators, food availability, and other factors.

Overall, while it appears that most fish have some kind of rest that allows them to recharge, it takes on a very different form than in humans and can demonstrate a huge amount of diversity given the great variety of types of fish.

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