Brain not needed for sleep? Study suggests animals needed shuteye before vital organ’s development

Cognitive functions can become arduous with a lack of sleep. Previous research shows the brain requires a minimum of four hours of sleep to survive, and up to nine for increased cognitive functions, such as memory and learning. One study, however, suggests that animals may have actually acquired the necessity of sleep prior to brain development. 

The study out of Kyushu University in Japan focused on the origin and evolution of sleep in animals, during which researchers found brainless marine hydras that exhibit a state of sleep. Although these tiny animals lack a brain, they seemed to respond to sleep molecules that have only been associated with higher evolved animals. “We now have strong evidence that animals must have acquired the need to sleep before acquiring a brain,” says study leader Taichi Q. Itoh, in a statement.

The species known as Hydra vulgaris was affected by chemicals that cause drowsiness in humans. Itoh, in collaboration with scientists at the Ulsan National Institute of Science and Technology in Korea, explains that a sleep-like state was also seen in jellyfish which are members of the same phylum.

“Based on our findings and previous reports regarding jellyfish, we can say that sleep evolution is independent of brain evolution,” says Itoh.

For the study, researchers tracked sleep and wake cycles in the hydras by using a video system. When the animal was asleep, the movement was reduced, and vice versa. The cycle was disrupted when light was added, usually in the form of a flashlight. 

Researchers found that the apparent sleep and wake cycle of the hydra is every four hours, opposed to the 24-hour circadian rhythm of most animals. They also reported significant similarities in the molecules that regulate sleep between animals with brains and the hydra. These similarities also could also be seen genetically. 

The hydras were exposed to melatonin, which increased their sleep amount significantly. GABA, another neurotransmitter that acts as an inhibitor, also increases sleep activity, as seen in many other animals. Dopamine, a neurotransmitter that increases arousal in humans, had the opposite effect by promoting sleep for the hydras.

“While some sleep mechanisms appear to have been conserved, others may have switched function during the evolution of the brain,” suggests Itoh.

Researchers also disrupted the sleep cycle of the hydras by changing the temperature and using vibrations. This caused sleep deprivation which led to longer sleep cycles and the suppression of cell proliferation. The lack of sleep resulted in changes in gene expression of 212 genes which included the gene that produces the protein PRKG that regulates sleep in many animals.

In an adjacent study, the disruption of similar genes in the fruit fly resulted in changes to the sleep cycle of the fly. Researchers plan to investigate these genes in hopes of identifying unknown genes related to sleep in animals with central nervous systems.

“Taken all together, these experiments provide strong evidence that animals acquired sleep-related mechanisms before the evolutionary development of the central nervous system and that many of these mechanisms were conserved as brains evolved,” says Itoh. “Many questions still remain regarding how sleep emerged in animals. Hydras provide an easy-to-handle creature for further investigation of the detailed mechanisms producing sleep in brainless animals to help possibly one day answer these questions.”

This study is published in Science Advances.

Report written by Amanda Christmas

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