If you have children, or ever interact with them, the chances are you have heard them ask about whether or not fish sleep. On the surface, this seems like a very simple question, but trying to give them an accurate answer can be more difficult.

One of the first challenges is figuring out exactly what qualifies as sleep. Nature defines it in the following way:

“A state characterized by a reduced responsiveness to sensory stimuli, suppressed locomotor activity and rapid reversibility to wakefulness.”

That is a pretty simple definition, and it works quite well for mammals. Mammals, however, are very different from fish. And even within the water world of fish, there are a lot of differences between the various species.

Some fish likely do engage in something that is very similar to sleep. A 2019 study at Stanford University had researchers set up an aquarium and record both the brain and the body activity of zebrafish. These fish did go into a state that resembled sleep, with measurable changes to their brain activity.

The team even identified two distinct types of brain activity during these periods of rest, which are similar to the various stages of sleep that humans go through.

Of course, these fish don’t have eyelids, so they don’t close their eyes, but at the very least, they do go through something sleep-like.

Fish that live near the bottom of the water will often find a place within coral or other places where they can hide from predators in order to engage in these restful periods. Not all fish, however, can do this. Some species live their entire lives in the middle section of the ocean, and many of them even need to keep moving in order to have water passing over their gills.

If this type of fish were to stop to sleep, they would die.

Some analysis of these fish has been done, however, and it seems that while they do keep swimming, they may go through periods of reduced brain activity. Some fish, for example, swim in large schools made up of hundreds or even thousands of individual fish. When that is the case, some of the fish in the group seem to essentially go into autopilot mode where they just follow along in the group without doing any decision making of their own.

Just how much of a reduction in brain activity takes place is not known, but some believe that schooling allows the fish to experience similar benefits as sleep, even if not everyone would classify it as such.

It is also important to note that there are  over 30,000 species of fish that we know of, and only a very small number have been studied for their sleep behavior. Also, you can get into other sea life like crustations to see how they respond.

So, the next time you are asked whether fish sleep, maybe the correct answer would be, “Yes…maybe…Its complicated.”

If you thought that was interesting, you might like to read a story that reveals Earth’s priciest precious metal isn’t gold or platinum and costs over $10,000 an ounce!