Michelle Dickinson: nanotechnologist on the new research into jellyfish sleeping habits

An upside-down jellyfish drifts gently in a shallow lagoon. At first glance, it looks like it’s just floating, but scientists have discovered something quietly astonishing - around midday, the jellyfish takes a brief nap to recover from a disturbed night.

The fact that jellyfish, which do not have a brain, are sleeping is amazing, and that discovery may completely reshape how we understand why all animals, including humans, need sleep.

A new study published in Nature Communications tracked sleep-like behaviour in two simple sea creatures - the upside-down jellyfish (Cassiopea) and the starlet sea anemone (Nematostella).

Both belong to an ancient lineage that dates back more than 500 million years, long before brains, backbones, or even eyes evolved.

Despite their simplicity, these animals clearly cycle between periods of activity and rest. When they’re resting, they respond more slowly to food or flashes of light, a key biological hallmark of sleep seen across the animal kingdom.

For decades, scientists have debated what sleep is actually for.

Is it about saving energy?

Consolidating memories?

Clearing out metabolic waste?

In humans and other vertebrates, sleep is known to help repair damage in the brain.  

While we’re awake, tiny breaks form in DNA inside our neurons, during sleep, repair mechanisms kick in and fix that damage.

Until now, scientists thought this kind of 'neural housekeeping' required a complex brain, but the jellyfish just proved otherwise. 

The researchers found that while jellyfish and sea anemones are active, DNA damage builds up in their nerve cells -when they rest, that damage is repaired.

When scientists deliberately kept the animals awake by altering water currents, the DNA damage increased and so did the amount of sleep the animals needed afterward, mirroring the 'sleep rebound' humans experience after a bad night. 

When researchers exposed the animals to UV light (which damages DNA), they slept more. Once they’d rested, the damage decreased and their normal rhythms returned.

Even more surprising? Adding melatonin, the hormone many humans take for jet lag caused these brainless creatures to fall asleep at unusual times.

That suggests melatonin’s sleep-inducing role evolved far earlier than scientists thought.

Put together, the findings point to a powerful idea: sleep may have started as a cellular defence mechanism.

Being awake is stressful for nerve cells as sensory input, movement, and metabolism all create molecular wear and tear. Sleep creates a quiet window of reduced stimulation where essential repairs can happen safely.

If even jellyfish need that repair time, then sleep likely evolved before brains, before complex nervous systems, and before animals even had left and right sides.

This doesn’t mean sleep has only one purpose.

In animals with more complex brains, functions like memory consolidation and learning likely layered on top of this ancient repair role, but the study strengthens the idea that protecting DNA may be the most fundamental reason we sleep. 

The study offers a sobering perspective on modern life, as chronic sleep deprivation in humans has already been linked to neurodegeneration and increased DNA damage. If creatures as simple as jellyfish can’t function without sleep, it’s a reminder that rest isn’t a luxury, it’s a biological necessity baked into life itself. 

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Episode Transcript

Speaker 1 (00:06):
You're listening to the Sunday Session podcast with Francesca Rudgin
from News Talks EDB.

Speaker 2 (00:12):
Dr Michelle Dickinson. Good morning, Hello, lovely to see you.

Speaker 3 (00:17):
It's so nice to see you. Did you have a
good night sleep?

Speaker 2 (00:20):
No, no, I don't. I should show you my sleep app.
It's very sad reading often it's four or five hours,
but that's because of the job. So no, I did
not have a good night's sleep. But we are going
to talk sleep and we're going to talk dreaming, which
is why we're going to start off with the question
why do jellyfish sleep? And what do they dream about?

Speaker 3 (00:39):
Well, the bigger question is why does anything sleep? So
you had a poor night sleeper according to your app?
Do you think it's important? Do you know what why
we even sleep?

Speaker 2 (00:49):
I would. I would presume it's because we need to
refresh ourselves. We need to refresh our functions, our curllular functions,
our brain functions. We need a little bit of risks,
we need to get back on top of it all.

Speaker 3 (01:02):
Great, So most people think it's because we need to
rest our brains and help it to repair. And you
said exactly the thing that we have heard, which is
why this jellyfish studies interesting because jellyfish don't have brains.

Speaker 2 (01:13):
No, this is true.

Speaker 3 (01:14):
So they're one of the few really ancient species who
don't have brains. And so the theory is we sleep
as humans and many mam will sleep because our brains
need cleaning through the night from all of the mess
that's in there from dealing with the day. And so
the scientists will went, well, if that's true, why do
jellyfish sleep and what are they doing? So this is

(01:36):
a lovely study if you want to follow along. It's
published this week in Nature Communications. And they took these
jellyfish and first of all, how do you know if
a jellyfish is sleeping?

Speaker 2 (01:47):
I can't tell. The eyes don't seem to even be open.

Speaker 3 (01:50):
They don't have eyes.

Speaker 4 (01:52):
So the first part of.

Speaker 3 (01:53):
The study is going, well, how do we actually know
they're asleep? So they worked out that jellyfish do have
a circadian rhythm and jellyfish pulse. So you may have
seen a jellyfish. They're pulsing. It's almost like a heartbeat
for a human. And they notice that at night they
pulse less. So they went down to about thirty to
thirty two pulses from over thirty five in the day.

(02:13):
So they went, ah, well, maybe that's a sleep function,
but how do we tell And then they said, well,
if it's a circadian rhythm in that they're awake in
the day and they're sleep at night. These are all
tank jellyfish, we're going to mess with their UV lights,
so we're going to change the way that it's daytime
to them and not. And they found that if they
made it the daytime much much longer and then gave
them a night, the jellyfish slept for longer, so they

(02:36):
caught up on their sleep like humans catch up on
their sleep. So they figured out, okay, they must be sleeping.
And then they said, well, if they're sleeping then and
they don't have a brain, then it can't be a
brain function. That means that these creatures sleep, So what
is it? And they ended up finding and it's very
similar to what you said, that they are repairing DNA
damage in all of their cellular function. So it's not

(02:57):
just their brains, it's actually every single part of them.
And so we know that DNA damage builds up in
our nerve cells. And they were able to measure that
this image is repaired. And so these poor jellyfish were
kept away and then they were poked and prodded, and
then they were left asleep and they were allowed to
have a sleep in. But it's actually really interesting. And
then what they did and I don't know if you've

(03:18):
ever taken anything to help your sleep, but they put
melotonin in the water. So we know that melatonin is
a sleep hormone and some of us take it to
help us fall asleep. Well, apparently it works on jellyfish too,
So they drop melotonin in the.

Speaker 4 (03:32):
Water and all the jellyfish are like, oh, so, what
I love about the study is they were able to
induce sleep in jellyfish that you can't even really tell
of sleeping.

Speaker 3 (03:43):
But yeah, So what it has shown us is even
in creatures that don't have brains, we still need sleep,
which implies that sleep actually evolved before brains erevolved in mammals,
and it's a critical function for DNA repair in all
of ourselves.

Speaker 2 (03:59):
I always remember the effectoid that learned years and years
ago that basically you I, well, we've completely changed ourselves
the course of our lives. We've regenerated so much of
our cellar's function and body that in fact, we're a
whole new person even though we look the same, sound
the same thing, the same, are the same. In fact,
it's a whole new body that is unlike the body

(04:21):
that we have when we were kids.

Speaker 3 (04:22):
Yeah, everything is replaced all the time.

Speaker 2 (04:24):
Wow. So yeah, your day it isn't until.

Speaker 3 (04:26):
The day it starts doing that. So the thing that's
going to accelerate that is not sleeping. So for those
of you, Andrew, I'm including you, is who are not sleeping, well,
please take some extra time. You need your DNA repair,
you need your on your own repair, so taken up
in the day. Just if you've got an app, it's
very good actually helping you to track what you're doing.
But make sure he gets some rest because it will
potentially help you live a healthier and longer life.

Speaker 2 (04:47):
Michelle, thank you very very much. I'm looking forward to
my nap as soon as i finished this job at
midday to day.

Speaker 1 (04:53):
For more from the Sunday session with Francesca Rudken, listen
live to News Talks it Be from nine am Sunday,
or follow the podcast on iHeartRadio.