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A Blue View: Sleep with the Fishes

Just keep swimming, just keep swimming, just keep swimmi…

Published January 26, 2016


For most fish, that line Dory utters in the popular Disney movie “Finding Nemo” is no exaggeration. Sounds exhausting, right? But you’re unlikely to catch a fish closing its eyes for a quick catnap.

That’s partly due to the fact that most fish don’t have eyelids. But it’s also because fish don’t technically sleep—at least not like we humans do.

In a watery world of predator vs. prey, deep sleep isn’t really an option. Instead, most fish find ways to reduce their activity and metabolism while still remaining alert.

We humans spend our time a little differently. In fact, nearly one third of our lives are spent asleep, but we’re hardly the heavy-hitters of the animal kingdom. Plenty of other species enjoy a good snooze.

Here are just a few of the animals prioritizing a good night’s sleep:


For a fish, rest is more akin to a dreamy haze than a deep sleep, minus the dreams of course. Some fish will find a hideaway in the sand or wedge themselves between coral to avoid predation.

In addition, most fish have to keep moving throughout their sleep state, passing water over their gills for oxygen. For some fish the occasional flick of the fins might suffice, while others must continue swimming slowly through the water.

Episode Transcript

If you’re like me, your eyelids start to droop when you feel tired. Left unchecked, your eyes gently close as you nod off, and soon its ‘off to la-la land.’

Now imagine that you don’t have eyelids, so you can’t close your eyes. Add the caveat that, in your world, sleeping soundly probably will get you killed. Just for good measure, imagine too that the very medium in which you live is in constant motion, 24/7.

Welcome to the world of fishes, who don’t technically sleep, but do, like us, still need their rest. Some bury themselves in the sand. Others drift motionlessly, looking for all the world like flotsam. Still others wedge themselves into a secure spot in the mud or the crevices of a coral reef. Perhaps one of the weirdest is the parrotfish, which fashions its own cozy sleeping bag by secreting a cocoon from its own mucus, keeping parasites and other nibblers at bay.

The whole concept of sleep in fishes and other undersea dwellers is very different from our own. Almost universally, they sleep with open eyes, remaining eternally alert to predators and other dangers.

For fish, sleeping is more like suspended animation, a sort of trance state that we might view as daydreaming. Except they don’t dream or experience so-called REM sleep like us, either. And yet, marine biologists do indeed study the field of fish sleep…or non-sleep…for a very good reason: understanding this aspect of their lives can actually help us better understand the importance of sleep in our own, very different lives.

Putting it into perspective, remember that we humans spend as much as one-third of our lives asleep, or at least we’re supposed to. Lack of sleep is a significant public health problem that’s been linked to chronic depression, compromised immune systems and impaired learning. Inversely, proper sleep helps us repair our bodies, grow, create memories, and process information. In other words, it helps us thrive. Yet, an estimated 50 million to 70 million American adults suffer from sleep disorders.

Which brings us to the striped zebrafish. This tiny, transparent fish is used extensively in research as a model for human sleep studies, with a brain and nervous system that can actually be observed. Diurnal like us—that’s awake and active during the day and asleep at night—the zebrafish exhibits fundamental behavioral and neurochemical characteristics of sleep and arousal that are strikingly similar to mammals.

Zebrafish research has helped reveal the genetic causes of narcolepsy. In another study, scientists studying zebrafish showed that the chemical melatonin is required for the circadian regulation of sleep, and without it, they become sleep-deprived. Like us… In fact, they even suffer from sleep disorders and insomnia. In one lab experiment, zebrafish barely slept at all when kept in a constant-lit tank, which can be even worse if that light is blue-light…exactly what our LED-powered smartphones emit. Uh oh.

And, just as we try to catch up on our sleep on the weekend, zebrafish, too, have been observed to attempt to refill their overdrawn sleep bank by taking frequent naps.

Calm and restorative rest reboots the brain. It’s essential for the well-being and survival of nearly all animals, and without it, our health falters. How that need evolved, and why, are among the many tantalizing questions that we have yet to answer. It’s kind of amazing to think one of our best mentors in this quest may just be a 2-inch long fish from streams in India.

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