(Study shows that even hydra with a primitive nervous system demonstrate sleep like behavior)
January 01, 2023
Author: Manish Verma
Editor: Dr. Jitendra Kumar Sinha
The hydra is a simple creature. Less than half an inch long, its tubular body has a foot at one end and a mouth at the other. The foot clings to a surface underwater — a plant or a rock— and the mouth, ringed with tentacles, ensnares passing water fleas. It does not have a brain, or even much of a nervous system. Jellyfish, a relative of hydras that also lack a brain, have also demonstrated sleeplike behavior. But the new study by a team in South Korea and Japan showed that the hydra periodically drops into a rest state that meets the essential criteria for sleep.
On the face of it, that might seem improbable. For more than a century, researchers who study sleep have looked for its purpose and structure in the brain. They have explored sleep’s connections to memory and learning. They have numbered the neural circuits that push us down into oblivious slumber and pull us back out of it. They have recorded the telltale changes in brain waves that mark our passage through various stages of sleep and tried to understand what drives them. Mountains of research and people’s daily experiences attest to human sleep’s connection to the brain.
But a counterpoint to this brain-centric view of sleep has emerged. Researchers have noticed that molecules produced by muscles and some other tissues outside the nervous system can regulate sleep. Sleep affects metabolism pervasively in the body, suggesting that its influence is not exclusively neurological. And a body of work that has been quietly but consistently growing for decades has shown that simple organisms with less and less brain spend considerable time doing something that appears to be sleep. Sometimes their behavior has been pigeonholed as only “sleeplike,” but as more details are uncovered, it has become less and less clear why that distinction is necessary.
It appears that simple creatures—including, now, the brainless hydra — can sleep. And the intriguing implication of that finding is that sleep’s original role, buried billions of years back in life’s history, may have been quite different from the standard human conception of it. If sleep does not require a brain, then it may be a profoundly broader phenomenon than we thought. The insights regarding sleep in hydras take sleep research to a new level. The hydra has an even more primitive anatomy and neurological system than Cassiopea (jellyfish).
Nonetheless, as proved by scientists at Kyushu University in Japan and Ulsan National Institute of Science and Technology in South Korea in 2020. Once a hydra reached a rest state, a pulse of light would awaken it, and it, too, slept longer with repeated deprivation. In hydras deprived of sleep, the cell divisions that constitute a normal aspect of life halt. Similar changes have been observed in the brains of rats and fruit flies deprived of sleep. It is possible that sleep plays a crucial role in regulating the distribution of energy.
Sleeping is different for hydras. For example, dopamine, which usually makes animals sleep less, made the hydra motionless. Also, there is no evidence that the hydra follows a 24-hour sleep cycle; rather, it appears to sleep for about an hour every four hours. In addition, the hydras were also exposed to substances implicated in sleep regulation in humans, such as melatonin and the neurotransmitter GABA. Exposure to both of these drugs enhanced hydra sleep activity. These results imply that whereas certain sleep pathways seem to have been conserved, some may have changed their function over brain evolution.
With so much focus on the most basic of sleepers, it’s natural to wonder what the earliest sleeping creature would have been. Whatever this first sleeper was, it became extinct over a billion years ago. If the hydra and human ancestor shared neurons and muscle-like tissue, then the cessation of movement would have been indicative of its form of sleep, satisfying its unique requirements.
References:
- Kanaya, H. J., Park, S., Kim, J. H., Kusumi, J., Krenenou, S., Sawatari, E., Sato, A., Lee, J., Bang, H., Kobayakawa, Y., Lim, C., & Itoh, T. Q. (2020). A sleep-like state in Hydra unravels conserved sleep mechanisms during the evolutionary development of the central nervous system. Science advances, 6(41), eabb9415. https://doi.org/10.1126/sciadv.abb9415
- Nath, R. D., Bedbrook, C. N., Abrams, M. J., Basinger, T., Bois, J. S., Prober, D. A., Sternberg, P. W., Gradinaru, V., & Goentoro, L. (2017). The Jellyfish Cassiopea Exhibits a Sleep-like State. Current biology : CB, 27(19), 2984–2990.e3. https://doi.org/10.1016/j.cub.2017.08.014
- Siegel J. M. (2022). Sleep function: an evolutionary perspective. The Lancet. Neurology, 21(10), 937–946. https://doi.org/10.1016/S1474-4422(22)00210-1