The longer we stay awake without sleeping, the more difficult it can become to think straight. But our functioning is restored when we rest. Scientists have wondered whether the need for sleep arose after the brain evolved or before. New work has suggested that even animals that don't have central nervous systems, like hydras, must enter a sleep-like state. The findings have been reported in Science Advances.
Hydras are a few centimeters in length, and they carry a network of loosely linked nerve cells but don't have a central nervous structure like a brain. These animals don't have brainwaves that change when they enter sleep as people do, but their movements are reduced. Hydras in that state will start moving again if stimulated by light. Hydras also have a twenty-four-hour cycle of states of active and reduced movement.
"We now have strong evidence that animals must have acquired the need to sleep before acquiring a brain," said the leader of the study, Taichi Q. Itoh, an assistant professor at Kyushu University's Faculty of Arts and Science.
Sleeping-like behaviors have been recently observed in jellyfish, which are a hydra relative and members of the Cnidaria phylum. This work showed that molecules that can trigger drowsiness and sleep in humans were also able to induce similar behaviors in Hydra vulgaris.
"Based on our findings and previous reports regarding jellyfish, we can say that sleep evolution is independent of brain evolution," Itoh said. "Many questions still remain regarding how sleep emerged in animals, but hydras provide an easy-to-handle creature for further investigating the detailed mechanisms producing sleep in brainless animals to help possibly one day answer these questions."
This work also revealed that whether organisms have a brain or not, there are molecular and genetic patterns of sleep regulation in cells. For example, if hydras are exposed to a common, natural sleep aid called melatonin, the amount of frequency of sleep-like behaviors increased. In addition, an inhibitory neurotransmitter linked to sleep called GABA had a significant impact on hydra and greatly increased their sleep activity. Exposure to dopamine, a neurotransmitter linked to arousal, also had a sleep-promoting impact on the hydras.
"While some sleep mechanisms appear to have been conserved, others may have switched function during evolution of the brain," suggested Itoh.
When vibrations and changes in temperature were used to disturb the patterns of sleep in hydras, the researchers induced signs of sleep deprivation. The hydras also spent more time 'sleeping' the next day. Sleep deprivation was found to alter the expression of 212 genes.
When the researchers disrupted fruit fly genes that have evolutionary parallels to hydra genes that were altered by sleep deprivation, sleep duration changed in fruit flies.
"Taken all together, these experiments provide strong evidence that animals acquired sleep-related mechanisms before the evolutional development of the central nervous system and that many of these mechanisms were conserved as brains evolved," said Itoh.