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Do jellyfish sleep?

Do you need a brain to sleep? Riddhi Mane answers the question by delving into the weird and wonderful world of the jellyfish


Sleep remains highly elusive to both scientists and students. Questions, such as ‘why do we sleep?’ and ‘when did sleep first evolve?’ plague sleep scientists in the same way that questions such as ‘when will sleep come next?’ and ‘why is it two days past my bedtime?’ plague the general student population. Until very recently, we thought sleep was only reserved for mammals, flies, birds, or the weak, but new research undertaken at CalTech suggests that even organisms without brains like a certain type of jellyfish can display sleep-like behaviour.

even organisms without brains like a certain type of jellyfish can display sleep-like behaviour

Within the scientific community, a commonly held perspective is that sleep is required by relatively complex organisms to process memories, detoxify cells, and repair tissue. “The results of this study challenge certain commonly held beliefs,” says William Joiner, a neuroscientist at the University of California, San Diego, who was not involved with the work. “For example, that sleep requires a centralised nervous system and related neural circuits across evolution.” he adds. As the jellyfish is an evolutionarily ancient organism, this means that the need for sleep is older than the entire human species.

The key actors in this electrifying study were 23 jellyfish of the genus Cassiopea, which are affectionately known as ‘upside-down jellyfish’. In its element, this type of jellyfish sticks to the bottom of the sea floor on its bell with its tentacles pointing upwards, and avoids movement unless absolutely necessary. Once every second, it religiously pulses its bell in an effort to feed or get rid of waste.

Cassiopea Jellyfish. Source: wiki commons

The bell pulses of these jellyfish were closely monitored for almost a week, revealing that the pulse rate dropped by one-third as night-time set in – resembling a state of sleep. It was also found that waking up is a struggle regardless of whether you identify as a human or a jellyfish. When these ‘sleeping’ jellyfish were lifted off their surfaces and scrutinized, they found it harder to swim back to their original locations, and displayed the same level of responsiveness to stimuli as any caffeine-infused student during an 8.30 lecture.

At a cellular level, we all desire and require sleep

Finally, these jellyfish were deprived of sleep by spraying them with water every twenty minutes for half a day, which unsurprisingly resulted in reduced enthusiasm and energy in the daytime. The jellyfish were seen to compensate for lost sleep by sleeping at odd hours, making them one of the easiest organisms to relate to. Interestingly, when these jellyfish were given melatonin, a sleep-inducing hormone found in humans and one which is sensitive to light, they decided to fall asleep. Could this mean that humans and jellyfish, in spite of having evolved at completely different points in time, have similar underlying mechanisms to induce sleep?

Jellyfish. Source: Max Pixel

Ravi Nath, this study’s first co-author, explains, “This finding opens up many more questions: Is sleep the property of neurons? And perhaps a more far-fetched question: Do plants sleep?” He also thinks that if sleep is found in such a basic and simple organism, then its ancestral function is possibly also basic. The next challenge will be to find an even simpler ancestor in the tree of life and assess if it needs sleep as much as we do.

At a cellular level, we all desire and require sleep. However, when we sacrifice sleep to accomplish such meaningful tasks as hopping and swaying on sticky dance floors in a mentally compromised state, or when we load up on coffee to pull those nasty all-nighters, we must bear in mind that this is a less efficient way of digging one’s grave. Social jetlag – which is caused when social commitments take priority over sleep – is a crime which we are all guilty of, and one that can lead to an increased risk of cardiovascular disease and chronic fatigue. So, put those phones away, and remember this: if the going gets tough, almost everything will work if you switch it off and back on again.

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