[♩INTRO].

It's easy to look at a sleeping dog, legs twitching as she snoozes, and imagine she's dreaming of chasing rabbits. And it's only natural for us humans to wonder if other animals dream.

But dogs' brains are relatively similar to ours. What about something way more different from us? Say, a fish?

We don't yet know exactly why our brains race through fantastical or scary scenarios at night, but there are plenty of hypotheses. Dreams may be ways for our brains to process emotions or memories, or to prepare for new scenarios. Human sleep occurs in a series of cycles.

We cycle through a series of phases over the course of the night, which can take anywhere from 70 to 120 minutes. Most dreams occur during a phase called rapid eye movement, or REM. As you might imagine, this type of sleep is characterized by rapid movements of our eyes.

It's possible these eye movements are connected to our dreams themselves. Researchers have suggested that each flick of the eye may correlate with a new image being encountered in a dream. But do other animals experience sleep the way we do?

Well, we can't exactly ask them, but we sure are curious. And this fascination has led us to study sleep in fish as far back as 1913. That study set out behavioral criteria for fish sleep.

In zebrafish, that means things like immobility, a preferred sleeping location, and a reduced respiratory rate. But it wasn't until a 2019 study that anyone defined what was happening inside the brains of fish while they sleep. And it took so long because it's hard to do!

One way to look inside human brains, including during sleep, is using an instrument called an electroencephalogram, or EEG. This typically measures activity in a region of the brain called the neocortex. Fish don't have a neocortex.

However, zebrafish do have something similar, called the dorsal pallium, which the researchers targeted in this study. They took advantage of the fact that the young zebrafish are see-through. You can see straight into their brains!

They inserted a gene into the fish that caused their neurons to glow when active. Specifically, it gives off light in response to calcium. Since calcium levels change as neurons send their signals, this allowed the researchers to watch brain activity while the fish slept.

Their results showed that like humans, fish cycle through sleep patterns. The researchers saw two main sleep states: slow bursting sleep and propagating wave sleep. And propagating wave sleep showed a number of similarities to our own REM phase, though the fish's eyes stayed still.

The researchers suggested these similar patterns of sleep activity could have evolved before fish and humans split, more than 450 million years ago. So, this doesn't prove that fish can dream, but it suggests that they do go through something similar to our own dream-filled REM sleep. So, that means fish dreams are a possibility!

Thanks to our patron Olan Kenny for asking this question. Our patrons submit and vote on questions that eventually get made into episodes like this one! So, if you want to be a part of that process, or just want to help SciShow keep making free educational videos for everyone you can check out patreon.com/scishow. [♩OUTRO].