Jellyfish sleep and nap like us. Studying them could help human brains

What do humans have in common with jellyfish and sea anemones? You might be thinking, not a lot, but a new study published in Nature Communications shows they do sleep like us and that sleep has a big impact on their DNA health. Lior Appelbaum is one of the researchers on that study, also a professor in the Faculty of Life Sciences at Bar-Ilan University in Israel. Welcome to the show.

LIOR APPELBAUM: Thank you very much for inviting me.

KELLY: So it's not like you can see a sea anemone or a jellyfish close their eyes. How do you know if they're sleeping?

APPELBAUM: Yeah. So we used various behavioral criteria. So we, for example, see that they reduce their activity at night - the jellyfish, while the sea anemone reduce their activity during the day. But the main criteria is simply to apply stimuli from outside and test whether a jellyfish or the sea anemone will respond fast or slow.

APPELBAUM: So, for example, we can apply a light stimuli. We also just apply food. So the sea anemone will respond to the food. If they sleep, they just will respond slower.

KELLY: So you're waking them up (laughter) is what... APPELBAUM: Exactly. We try to wake...

KELLY: They're trying to sleep, and you're annoying them.

APPELBAUM: Exactly. Like human, when we hear a sound. So we will not respond for a very weak sound if we sleep because our arousal threshold is increased.

KELLY: Is this surprising to you?

APPELBAUM: No. So already, a few years ago, people showed sleep-like state in jellyfish and in hydra (ph), and we were the first to define sleep in sea anemone. And what was surprising is the sleep patterns. So it was very interesting that both animals sleep more or less eight hours per day. What also was very exciting to see that the jellyfish nap at midday. So they sleep during the night, but they also require, like, one hour or two hours of nap.

KELLY: I'm jealous. I would love to sleep for an hour or two every afternoon.

KELLY: So why? I mean, if they are sleeping and napping, it must be because sleep is beneficial to these organisms. Do we know how?

APPELBAUM: Exactly. So sleep traditionally is sought, and many examples show it, that it's important for the brain. But here we show that the sleep is important way before, even millions of years before even the brain was developed in jellyfish. So jellyfish still have a very simple nerve net. So we test whether sleep is important to the nerve net for single neuron, basically. And what we found is that during wakefulness, cellular stress and DNA damage is accumulated in the neuron. And sleep is important to reduce the cellular stress and reduce the DNA damage and start the new day fresh and ready to go.

KELLY: So I ask this respectfully, but why does it matter? Why study this? Why does it matter if we know whether a jellyfish or a sea anemone sleeps?

APPELBAUM: First, we want to understand how sleep has evolved. I mean, we all spend one-third of our life sleeping and presumably wasting our time. But obviously, this is very crucial, and it was necessary even to a very simple animal like jellyfish. Second of all, I mean, we all know that sleep disturbances - I mean, this is what shown many times - is highly linked to neurodegenerative disease. And what we suggest here that maybe one of the reasons that the risk to neurodegenerative disease is increased because sleep disturbances increase the amount of DNA damage, which may be accumulated even in the human brain. And eventually, after chronic and long-term sleep disturbances, can lead to neurodegenerative disease. So whenever you study basic mechanism like that - why do all animals sleep or why a single cell requires sleep - it definitely help to understand very complex disease in human, like neurodegenerative disease.

KELLY: Lior Appelbaum, professor of Life Sciences at Bar-Ilan University in Israel. Thank you.

APPELBAUM: Thank you very much.

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