Quirks & Quarks

Scientists discover jellyfish need sleep, too

Scientists say that discovering that jellyfish also doze off emphasizes how important sleep is for our neurons.
Cassiopea jellyfish. The animals prefer to orient upside in the water column. (Caltech)

The discovery

Sleeping. It seems so natural to us. For complex animals, sleep helps our brains learn and form new memories.

But what about animals without any brains, like jellyfish?

Scientists from Caltech decided to test whether one of our most ancient evolutionary relatives, jellyfish, need sleep. Jellyfish are among the first animals to have evolved neurons, but didn't evolve complex brains.

In new research a team including Ph.D candidate in biology and bioengineering Claire Bedbrook, found that, like us, jellyfish do sleep.

How they studied it

In order for the scientists to say the jellyfish were "sleeping," the animals had to meet three criteria.

First, they had to have a daily period of reduced activity, like we do at night when we sleep.

The scientists set up cameras to monitor the jellyfish around the clock and found that they pulse less at night than they do during the day.

Secondly, the scientists had to show that the jellyfish were slow to wake from the period of reduced activity — or quiescence. They did that by literally pulling the floor out from under the jellyfish they studied, which like to rest on a surface.

They observed that the animals would float in the water for a few seconds before waking up and reorienting themselves.

Lastly, they had to show the jellyfish displayed an increased drive to sleep when deprived of sleep. The researchers kept the jellyfish awake by pulsing water onto the animals every 10 seconds. After a night of that, the scientists saw the jellyfish were a lot less active — apparently sleepy — the next day.

Why this matters

This really just emphasizes how important sleep is for animals to survive. It also shows that sleep is a function not of complex brains, since jellyfish don't have them, but of neurons.