Hydra reveal role of sleep in the evolution of the nervous system
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.
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