What Jellyfish Can Teach Us About Sleep

There seems to be an endless fascination with sleep: how much should we get, how do we get more, what happens when we don’t get enough and especially, how do we get better sleep? But despite the growing body of scientific studies, researchers are still scratching their heads over why it is, exactly, that we sleep. What purpose does it serve? While the physical benefits of sleep are evident, scientists still don’t have one conclusive answer for why we do it. But theories abound in the research communities and there has always been an assumed brain-sleep connection that lies at the base of most hypotheses. Recent evidence has even indicated that during sleep our brains are active and involved in mental housekeeping that helps with memory consolidation. So clearly the brain is involved in our need to sleep, right? Maybe not. A recent research study at the California Institute of Technology set out to gain a greater understanding of sleep by taking a closer look at the nighttime habits of the Cassiopea jellyfish – a creature that has no brain at all.

According to the Harvard Medical School Division of Sleep Medicine, sleep has some specific characteristics: it is a period of reduced activity with a decreased responsiveness to external stimuli that can be relatively easy to reverse. Some scientists would also extend the criteria to include an increased drive to make up for lost sleep. With this definition in place, three grad students from the California Institute of Technology – Ravi Nath, Claire Bedbrook, and Michael Abrams – decided to test the theory that sleep required a brain, however small.

The team chose the primitive Cassiopea jellyfish as the subject of its experiment. Instead of a brain, Cassiopea jellies only have a diffuse net of nerve cells distributed throughout their bodies. In fact, they barely classify as animals – they have no mouths, instead sucking in their food through pores in their tentacles and actually engage in a symbiotic relationship with photosynthetic organisms that live in their cells. The jellyfish typically spend their days floating upside down near the ocean floor, pulsing their tentacles in a steady rhythm. So the researchers used this pulsation as a metric with which to measure activity.

First, to assess the possibility of these brainless creatures actually sleeping, the team observed the jellyfishes’ pulsing behavior and noted how the pulsation slowed considerably at night. But when food was introduced to the tanks, the jellies roused, indicating that the sluggish state was easily reversed.

Next, the scientists tested the animals’ arousal threshold, or the amount of time it took for them to respond to a change in environment. At random periods during the night, the researchers would move the jellies from their preferred resting spot at the bottom of the tanks. Those jellies with increased pulsations swam immediately back down to the bottom while slow-pulsers would strangely float around, taking longer to reorient themselves.

Lastly, the research team wanted to evaluate how jellyfish behaved after long-term disruption to their rest periods. To keep the animals awake, the scientists squirted the jellyfish with pulses of water every 20 minutes for up to 12 hours, effectively keeping them awake all night. The next day, the jellies were noticeably more sluggish with reduced pulse activity while the following night, the jellyfish slept even more deeply to make up for the lost sleep the night before.

Now comes the question: why is this important? Most prominently, such an experiment demonstrates that a brain is not necessarily required for all of the criteria of sleep to be met. Further, as Cassiopea are ancient, the study is indicative that sleep evolved as a very primordial process that has been in existence for millennia, prior to the evolution of organisms with brains or central nervous systems. This analysis opens the doors to other questions like what other simple organisms might sleep? How about plants?

If nothing else, this research shows that as developed and intelligent as we may be, we still share a fundamental need with even the most minimally evolved species.

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This article originally appeared in Sleep Retailer eNews on October 26, 2017.

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