Deep Sleep

Sleep is an essential process thought to be especially important for the brain, most notably by facilitating updates in neural connections, required for learning and memory. Yet recent research has uncovered sleep-like states in cnidarians, such as upside-down jellyfish (Cassiopea spp.) and now Hydra vulgaris (pictured), animals lacking a brain or central nervous system. Monitoring Hydra with video cameras revealed regular periods of reduced activity, in which Hydra were also less responsive to stimuli, key criteria for defining sleep. When sleep was disrupted, sleep-deprived Hydra later spent longer in their sleep-like state, much like tired humans catching up. Melatonin, a hormone which encourages sleep in humans, and dopamine, a neurotransmitter typically associated with wakefulness, both increased sleep-like behaviour in Hydra, highlighting similarities and differences in the mechanisms of sleep. Challenging our assumptions about the purpose of sleep, cnidarians provide new insights into the evolution and regulation of this critical behaviour.

Written by Emmanuelle Briolat

Emmanuelle Briolat

Writer

A researcher specialising in visual signals in Lepidoptera, camouflage and the effects of light pollution, Emmanuelle’s writing covers a range of topics, from biomedical research to children’s books about moth defences.

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• Image by Flickr user Proyecto Agua, published by the Encyclopaedia of Life (EOL)
• Research from School of Life Sciences, Ulsan National Institute of Science and Technology, Ulsan, Republic of Korea and Faculty of Arts and Science, Kyushu University, Fukuoka, Japan
• Image originally published with a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0)
• Research published in Science Advances, October 2020