Do Fish Sleep?
If you’ve ever had a pet fish you probably saw it darting around or floating about in its little bowl a lot. Perhaps you once tried to catch it sleeping and ended up staying up for a really long time. Young you might have come to the conclusion that fish don’t sleep–especially since you were probably looking for your pet’s eyes to close. Intuition says everything has to sleep. Heck, even bugs have times of day where they are more or less active. Fish probably aren’t as concerned with nighttime though, so what’s up? Do fish sleep?
How Do Fish Sleep?
Turns out the answer is far more complicated than you might think. But we’ll talk about the stuff that doesn’t give scientists an existential crisis first.
We have yet to find a fish that sleeps in the same way we and most other mammals do. For starters, it’s not like all fish can close their eyes. Fish do enter a state of suspended animation where their brains are less active, though. Which is the closest thing to sleep we’re going to get. For the fish that do this, they kind of float still, so you’d probably think they were dead if you didn’t know about the whole upside-down thing.
Smarter fish will do a little more than just float still though. Some of them will cram themselves in between some rock. Others might cover themselves in mucus–kind of like you with a sinus infection but way worse.
To us, this sleep would look more like a “power saver” mode, especially since fish are still alert for danger when they’re in this state. But for the purposes of this post it’ll be synonymous with sleep.
Fish That Don’t Sleep
As we’ve alluded to, not all fish sleep. Some of them straight up don’t even have this “power saver” option. We’ve kind of grown to understand that pretty much everything needs sleep, even bugs have their own power saver. So if even your favorite fuzzy bumblebees need to take a nap sometimes, how are some vertebrates NOT doing it?
Well, of the fish we’ve seen to not sleep, they can’t really see us back. They’re genetically blind (since they live really deep underwater or in caves).
When it comes to sleep, there often comes an associated state of sensory deprivation. Think about it, you close your eyes when you head off to take a nap. So the theory basically holds that cutting off the need to process visual input saves enough “power” for the fish to never need rest.
We aren’t sure how convincing this is, but sleep science is far less understood than the layperson might think it is.
Okay, we know dolphins are mammals and therefore are not technically fish. But dolphins also live in the water, so how do they sleep?
Dolphins, some birds, and even sea lions have this neat little thing called unihemispheric slow-wave sleep (USWS). If you’re down with sleep, you’re aware that both hemispheres of the brain are unconscious. But for animals that exhibit USWS, this isn’t the case. Only one hemisphere of the brain goes unconscious, which means these animals sleep with one eye open.
The benefits are obvious, birds can half-sleep while making their migratory flights, and there’s the added benefit of almost always being able to track potential predators.
Feel the need to see animals sleep traditionally? Try this sleepy animal quiz here.