Do Jellyfish Sleep? (How Jellyfish Sleep and Quick Facts)

Who doesn’t like a good sleep? It is natural to wind down after a long day and enjoy quality rest on your bed. But have you wondered about other organisms like jellyfishes? Renowned for not having brains, do these creatures ever sleep? Do they stay awake their entire life?

Jellyfishes sleep too – especially at night. Scientists proved this with the Cassiopea, more commonly referred to as the upside-down jellyfish. Studies on this jellyfish showed that its activity levels – demonstrated in the rate at which it pulsed its bell – reduced during the night. In some intervals, the jellyfish stopped pulsing entirely. Overall, jellyfishes are 30% less active at night, exhibiting behaviors strongly reflective of sleep.

Learning that the jellyfish sleeps would feature among the weirdest thing you have heard in a while. But how does it manage to sleep without a brain? How long does it even sleep? Does it dream when it sleeps?

We will answer these questions with proven scientific findings. Read on!

How Does a Jellyfish Sleep?

Most animals – humans included – naturally experience declines in their activity levels when night comes. You will agree the energetic and bubbling you almost inevitably get passive and then drowsy when it gets dark.

Jellyfishes demonstrate just the same behavior. A team of researchers discovered this at the California Institute of Technology.

A study conducted on a unique jellyfish –called the Cassiopea or the upside-down jellyfish – triggered this exciting discovery.

Three researcher friends were at the heart of this study: Ravi Nath, Michael Abrams, and Claire Bedbrook.

Studying 23 jellyfishes in Abrams’ laboratory for six consecutive days and nights, the trio found that jellyfishes had the rate at which they pulsed reduced when illumination reduced.

Specifically, when night came, the jellyfishes under investigation had their activeness drop by an overwhelming 70%.

This alone wouldn’t have been sufficient to authoritatively declare that jellyfishes slept. But the decline in activity levels during nighttimes satisfied three core characteristics of sleep.

Let us talk about them.

The jellyfishes became less responsive at night

For context, let us talk about when you sleep. You get a bit numb to surrounding impulses: to the TV around, to conversations, and overall events around your immediate space.

This is your responsiveness dropping. It would take a really loud bang, for example, to jolt you awake.

The same was seen in the jellyfish.

During the six nights the jellyfishes were studied, they displayed substantial levels of unresponsiveness when disturbed – typical of sleeping.

The researchers placed the jellyfishes in a PVC pipe fitted with a screen bottom. Next, the pipe was lifted and dropped, consequently suspending the jellyfishes mid-water.

They found that while a typically awake (or active) jellyfish would quickly swim to the bottom, the jellyfishes during the night tended to freely float when roused as described.

This indicated that they were not as active and responsive as they would when awake during the day.

The jellyfishes could reverse their inactivity

Just like sleep, when an animal is in a coma, its activity reduces. But unlike a coma, where it is a total loss of consciousness, the jellyfishes under study could manageably revert to being active when disturbed.

Reversible inactiveness is a core parameter of sleep. By regularly disturbing them, the otherwise inactive jellyfishes would gradually rev back to full activeness (just as they would during the day) and start swimming.

The jellyfishes displayed signs of sleep deprivation

Remember how grumpy you would look in the morning when you didn’t sleep well? The jellyfishes did just that when they were seriously agitated through the night.

The three researchers blasted the jellyfishes every 20 minutes with water jets during the night. When blasted, the jellyfish’s active pulsing rates would be restored only to drop back after a period of no disturbance.

By repetitively blasting the jellyfishes with water jets all through the night, the researchers noticed that the jellyfishes were groggy during the next day.

This showed the jellyfishes responding to their disruptive rest times during the night with dropping activity levels during the next day.

Interestingly, when these fishes were blasted during subsequent days with water jets, they were minimally unsettled as they were already active.

How Long Does a Jellyfish Sleep?

A jellyfish sleeps intermittently. Within these periods, its pulse rates reduce, dropping to intervals when the jellyfish fully stops pulsing. Just after this, the pulsing would resume some moments later.

The researchers found that the jellyfish’s span stopped fully pulsing ranged between 10-20 seconds.

Do Jellyfish Sleep Upside Down?

This is a popular contradiction we urgently need to clear up. All jellyfishes don’t sleep upside down.

It is specifically the Cassiopea – the jellyfish species studied during the research – that majorly sleeps upside down.

This jellyfish species is not an avid swimmer, preferring to settle down in an inverted shape at the bottom of its tank (when in captivity).

Do Jellyfish Dream When They Sleep?

Yes, Jellyfish sleep, but they don’t dream as humans do.

To understand why jellyfishes don’t dream, we would have to talk about the REM (rapid eye movement) sleep stage where dreaming occurs.

There is a significant level of brainwaves during such REM state, with brain chemicals being emitted. This is what scientists strongly point to as an explanation for dreaming.

But jellyfishes don’t have brains in the first place. This means they can’t enter the REM state.

You may ask how come the jellyfish responds to stimuli when it sleeps. Yes, despite not having a brain, jellyfishes have a quasi-nervous system built from sensory organs radially distributed through the fish.

This synchronized packet of nerves enables the jellyfish to respond to stimuli like temperature, touch, salinity, and light when even when it sleeps.

The rhopalia is one of the most recognized nerves the jellyfish is furnished with. This helps it in detecting light.

Do Jellyfish Sleep at Night?

Yes, jellyfishes sleep at night. While they would not snore like humans or dogs, their pulse rate will reduce.

Overall, you would notice the jellyfish become 30% less active as it drifts to sleep, becoming completely inactive when fully sleeping.

When Do Jellyfish Sleep?

Jellyfishes in the wild have their biological clock optimized to sunrise and sunset.

But you can also induce sleep or inactivity in a jellyfish by shutting off its lighting. In such darkened environment, its activity levels will start dropping.

You can also chemically induce sleep in jellyfishes just as you would in man.

Melatonin is a chemical renowned for triggering drowsiness when injected into humans or ingested in the form of a drug.  Similarly, when the researchers dosed jellyfish with melatonin, they noticed that the jellyfishes’ pulses notably dropped.

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