Featured Image source: Peter Schuchert (https://commons.wikimedia.org/wiki/File:Hydra_viridissima2.jpg)
Imagine a very simple organism lacking any developed brain, an organism that just consists of two openings-one for the mouth and one for the foot. Ring any bells? That’s hydra for you. The most amazing fact about this organism is its urge to sleep.
A joint study conducted in Japan and Korea revealed the idea of rest as a prerequisite for sleep. Years of research link sleep with brain activity which was thought to be the main driver. However, this recent development has changed the idea that molecules formed outside the brain could influence sleep.
A similar phenomenon of sleep-like behavior was found by Irene Tobler in cockroaches and other species. Then in 2000, Amita Sehgal published a study on fruit flies about this aspect of sleeping which was further confirmed by Paul Shaw’s study. In 2008, David Raizen and colleagues reported this sleep phenomenon in Caenorhabditis elegans (C. elegans). It exhibited short spurts of sleep after stressful conditions. Even so, jellyfish with its minimal nervous system also demonstrates sleep which gave the notion of its evolution at least one billion years ago.
In theory, sleep was only projected as a requirement for the brain to shut off so as to provide rest since it undertakes a lot of important processes essential for keeping the organism alive. However, studies over the years have been pointing towards a different explanation. Apparently, sleep also helps to carry out certain biochemical reactions which would not be able to take place during waking hours. For instance, C. elegans performs the function of body growth while sleeping, and hydra is able to proliferate cells during sleep. The signal to sleep can be obtained from molecules generated outside the brain. These molecules are mostly known to be sleep-promoting. But sometimes they can also have the opposite effect. For example, the above-mentioned same joint study also revealed dopamine, known for causing awakened behavior, acted stimulatory in making hydra sleep. Differential expression of genes associated with sleep regulation was also observed in hydra. It essentially does prove the idea of sleep evolving much before the evolution of the brain and the phenomenon has been present for a very long time than what scientists had thought.