Fish reveal four distinct sleep states, including three with eye movements

What Fish Can Teach Us About the Science of Sleep

Imagine lying awake at 2 a.m., staring at the ceiling, wondering if your body is actually getting the deep, restorative sleep it needs. Most of us think of sleep as a uniquely complex human experience. But new research suggests the biological roots of sleep, including states that look a lot like our own REM cycles, go far deeper in the animal kingdom than anyone expected.

Scientists have pinned down four distinct sleep states in fish. Three of them involve eye movements. This could seriously change how we think about sleep across all species, including us humans.

Why Fish Sleep Matters to Human Health

Here's the thing. Fish don't have eyelids. So for a long time, researchers assumed studying sleep in fish would be nearly impossible. How do you track sleep stages in an animal that can't close its eyes?

Turns out, you watch the eyes anyway. And what researchers found was remarkable.

A study dug into zebrafish, those little guys used a lot in neuroscience. Turns out, they cycle through a few sleep stages. Three of those stages have unique eye movement patterns. One stage looks a lot like REM sleep in humans. That's where we dream big and lock in memories.

Research backed by the NIH is pushing our understanding of sleep evolution in vertebrates forward. Fish are stepping up to fill in the blanks.

Breaking Down the Four Sleep States Found in Fish

So what exactly did researchers find? Let me break it down simply.

Slow bursting sleep: Characterized by slow, irregular brain activity. This appears to be a deep, quiet phase with minimal movement.
Propagating wave sleep: Brain waves move across the fish's brain in organized patterns, similar to slow-wave sleep in mammals.
Brief wake-like activity: Short bursts of brain activation during sleep, somewhat analogous to light sleep stages in humans.
REM-like sleep: Rapid eye movements occur. The brain shows high-frequency activity. This is the state that drew the most attention from the scientific community.

To be fair, calling any fish state "REM sleep" is still debated. But the structural similarities are hard to ignore.

The REM Connection: More Ancient Than We Thought

We all know REM sleep is when dreams happen and memories get sorted out. But here's the kicker—not many realize that REM-like behavior in fish suggests this stage might've evolved hundreds of millions of years ago. That's way before mammals even showed up.

That's a big deal.

If fish, which split from our evolutionary path about 450 million years ago, have something like REM sleep, it means these states aren't just for mammals. They might be a basic part of vertebrate nervous systems. PubMed's got a pile of research digging into this evolutionary sleep stuff.

Honestly, the textbook view that REM sleep was a "mammal thing" looks increasingly outdated.

What This Tells Us About the Purpose of Sleep

Here's the thing, this might actually change how we see everyday health. If fish go through multiple sleep states, including something like REM, it hints that these stages aren't just filler. They're there for a reason, thanks to evolution.

Sleep isn't just downtime. It's when the real work happens. Your brain kicks into gear, clearing out waste, locking in memories, and sorting out hormones. Miss those key stages, even now and then, and you're throwing a wrench in the works.

Look, most adults in Western countries are chronically underslept. And not just in total hours. Many people are missing out on adequate deep sleep and REM sleep specifically, which are the stages where the most restorative work happens.

That fish research? It's hammering home what sleep gurus keep telling us: quality of sleep matters just as much as how much you get. You want to hit all the stages, not just aim for eight hours on the pillow.

How Sleep Stage Disruption Affects Your Body

When your sleep stages get disrupted, the effects aren't subtle. Here's what the research shows:

Impaired memory consolidation and learning ability
Elevated cortisol levels and increased stress reactivity
Reduced testosterone and other hormone regulation (particularly during deep sleep)
Weakened immune function
Higher risk of metabolic disorders including obesity and type 2 diabetes

That last point connects more than people realize. Hormonal regulation during sleep is deeply tied to overall vitality, energy, and even sexual health. Poor sleep disrupts testosterone production, which is one reason some men exploring ED supplements ranked by effectiveness often find that lifestyle changes, including sleep, matter more than they expected.

Practical Takeaways From the Fish Sleep Research

So what do you actually do with this information? A few things are clear.

First, protect your sleep cycles. Alcohol, late-night screens, and inconsistent schedules fragment your sleep stages. Especially REM. These aren't minor inconveniences; they're biological disruptions.

Second, don't just track hours. If you're using a sleep tracker, pay attention to stage data when available. Consistently low REM or deep sleep percentages are worth discussing with a doctor.

Third, take the evolutionary evidence seriously. The fact that even fish maintain structured sleep stages tells you this isn't optional biology. Your body needs these cycles. Every night.

Frequently Asked Questions

Do fish actually sleep?

Yes, fish do sleep, though their sleep looks different from ours. Research has shown that fish enter periods of reduced activity and responsiveness that share characteristics with mammalian sleep, including distinct brain states and, in some species, eye movements similar to REM sleep.

What is REM sleep and why does it matter?

REM (rapid eye movement) sleep is where your wildest dreams play out and your brain's busy filing away memories. You cycle through it several times a night, and it's crucial for keeping your emotions in check, learning, and thinking straight. Miss out on REM too often, and you're looking at anxiety, memory blips, and bad decision-making.

How many sleep stages do humans have?

We go through four sleep stages: three are non-REM (light sleep, deeper sleep, and slow-wave deep sleep) and one is REM. A full loop takes about 90 minutes. Most of us hit four to six of these cycles a night.

Can poor sleep affect testosterone and hormonal health?

Yes, sleep deprivation directly reduces testosterone production. Most of it gets released while you're snoozing, especially in those deep sleep stages. If you're constantly running on empty, your testosterone tanks. And yeah, your sex drive takes a hit too. That's why the sleep chat always kicks off the men's health talk.

What can I do to improve my sleep stages?

So, the go-to moves? Keep your sleep schedule solid. Cut back on booze and caffeine, especially at night. Ditch the bright screens before bed. Make your bedroom a cool, dark cave. These tricks? They help keep your deep sleep and REM cycles in check.

This article is for informational purposes only and does not constitute medical advice.