Hydra.CC BY-NC-ND 2.0 Jasper Nance / https://flic.kr/p/2JShUR
Sleep is one of biology's biggest mysteries.
Every species with a nervous system has some form of resting period, and so one theory for why animals sleep is that it helps maintain the brain — allowing an organism to reinforce or remove neural connections made in learning and memory.
But not all animals have a central nervous system with a distinct brain, and scientists have now found that even brainless animals have a sleep-like state.
The finding comes from a study by researchers at Kyushu University in Fukuoka, Japan, and Ulsan National Institute of Science and Technology in South Korea, where biologists studied Hydra vulgaris — a tiny jellyfish-like creature (1-3cm long) with a network of nerves but no centralized structure (brain).
The study showed that unlike many animals, whose body clock revolves around a roughly 24-hour-long circadian rhythm, Hydra follow a 4-hour sleep-wake cycle.
As sleep is typically monitored via brain waves but Hydra are brainless, the researchers used videos to track whether the animal was in a sleep-like state based on the amount of movement.
They also measured genetic activity after using temperature and vibration to create sleep-deprived Hydra, which revealed that sleep is controlled by 212 genes, including a gene the produces 'PRKG1' — a key protein that regulates sleep in everything from flies and nematode worms to mice and other mammals.
In the study, researchers looked at how the tiny aquatic creatures responded when given chemicals that affect sleep in more complex animals such as humans. Some molecules had a similar effect on Hydra — PRKG1 and the sleep hormone 'melatonin' encouraged the creature to sleep longer and more frequently, for example — while another chemical had the opposite effect: whereas 'dopamine' causes arousal in many animals, it actually prompted Hydra to feel sleepy.
The new study helps answer another big question in the evolution of animals: Which came first, sleep or the brain?
Because the common ancestor of all animals probably resembled something like a 'primitive' hydra, the above findings suggest that the origin of sleep predates the brain, meaning it evolved before nervous systems became more sophisticated.