Human beings aren’t the only animals that need sleep. Other mammals do too, and scientists have found that many animals, including even insects and worms, regularly enter inactive states that look like sleep.
Neuroscientists have evidence that sleep plays a central role in the function of the brain. Sleep is known to be involved in clearing metabolic wastes from the brain, contribute to regulating the immune system, and is involved in the changes to the brain that happen during learning and the creation of memories. That’s why the discovery that animals without brains sleep too came as a surprise to researchers.
In 2017 a group based in the United States reported that jellyfish regularly enter a sleep-like state of inactivity, from which it is hard to arouse them. In 2020 another team, from Japan, reported a similar discovery in related animals called hydra. Hydra and jellyfish are both part of an animal group called cnidarians.
Cnidarians are simple animals whose bodies are controlled by a nerve network without a central brain. Though they lack a brain, the nerve cells of cnidarians are much like ours, and the hydra researchers found some important physiological similarities between hydra sleep and human sleep.
They showed that substances like melatonin, which promote sleep in humans, also do so in hydra. They also found that some of the genes involved in sleep regulation in humans perform similar jobs in hydra.
What this tells us is that sleep and presumably some of its functions are important for all animals, even if they don’t have a brain. The basic molecular mechanisms for sleep probably evolved long before brains and centralized nervous systems.