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Humans & Jellyfish Are The Same: They Both Need Sleep

S u m m a r y :
Humans and jellyfish are fundamentally different, but they are both susceptible to sleep, says a new study published in the journal Current Biology. This finding might help scientists understand the origin of sleep.

Sleeping Beauties

You could think of a million differences between humans and primitive jellyfish Cassiopea. For one, the latter comes without brain and spine. Its activities are, obviously, drastically different from ours—all but one. Regardless of all the barriers that separate the two organisms, they are bound by their need to sleep, purports the joint endeavour that has brought together researchers from three Caltech laboratories.

The authors, thus, conclude: given that the jellyfish date back eons ago, sleep is apparently an ancient behaviour that has been preserved by evolution for thousands of years.

How Old is Sleep? Do Plants Sleep?

Sleep is an established need of creatures. So, it might not be much surprise that jellyfish sleep as well. However, they constitute “the most evolutionarily ancient animals known to sleep”, argues study author Ravi Nath. Is sleep that old? The new findings bring in new questions concerning the origin of sleep, and its possible occurrence among plants.

What Is Sleep, Anyway?

We take sleep for granted, but do we really know the definition given by scientists? According to them, organisms meeting three criteria will have been ascertained to sleep.

These 3 characteristics are:

  1. Quiescence, a period of decreased activity
  2. A lower response to stimuli during quiescence
  3. An increased urge to sleep when sleep-deprived

The jellyfish exhibit all three of these.

“When humans sleep, we are inactive, we often can sleep through noises or other disturbances which we might otherwise react to if we were awake, and we’re likely to fall asleep during the day if we don’t get enough sleep,” says study author Claire Bedbrook. “We might seem extremely different from jellyfish, but we both exhibit a similar sleep state.”

Watching Jellyfish Sleep

Bedbrook and her team used cameras to monitor a group of jellyfish. They observed that the latter would be inactive for periods during the night; they only pulsed around 39 times per minute, as opposed to their usual 58 times during the day. Quiescence confirmed.

The jellyfish would also be less responsive to conditions that would have otherwise stimulated reactions during the day. The investigators put them on a higher position in a tank before removing the platform from underneath them. This should have normally caused alert jellyfish to swim to the bottom of the tank. However, the sleeping ones remained floating in the water for a few seconds, after which they woke up and reoriented themselves.

The jellyfish on the floor of their tank. Photo credits: Caltech.

Next, to confirm the third trait, their water was pulsed every 10 seconds for 20 minutes which prevented them from sleeping. Afterwards, the organisms were more likely to go into quiescence during the day when they would have otherwise been active. The team, thus, showed that the jellyfish would have a greater drive for sleep after having been deprived of sleep.

Jellyfish & Human Sleep Compared

Sleep in animals is promoted by certain compounds, like a substance named melatonin. When the latter was tested in jellyfish, it was found that it did affect jellyfish in the predicted ways. This implies that jellyfish might be having a similar sleep mechanism as humans’.


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