The Origin of Sleep
Studies on sleep mostly treat it as a neurobiological phenomenon. However, new evidence from simple animals suggests that it evolved long before brains evolved.
Freshwater polyps are simple creatures. Less than half an inch tall, their tubular bodies have a foot on one end and a mouth on the other. The foot attaches itself to an underwater surface-such as a plant or rock-and the tentacled mouth catches passing water fleas. The animals have neither a brain nor any other significant nervous system. And yet, as new research shows, they sleep. Studies by a team in South Korea and Japan showed that freshwater polyps periodically fall into a dormant state that meets the essential criteria for sleep.
At first glance, this may seem unlikely. For more than a century, researchers studying sleep have searched the brain for its purpose. They have explored the links between the slumber state and memory and learning. They have captured the neural circuits that take us to and from dreamland. They've recorded the changes in the brainwaves that mark our journey through the different stages of sleep and tried to understand what causes them. Countless research results and life experience show that human sleep is closely connected to the brain.
But recently a counterpoint to this view has emerged. Researchers found that molecules produced by muscles and some other tissues outside of the nervous system can regulate sleep. The nocturnal break affects the metabolism throughout the body, which is why its influence is probably not exclusively neurobiological. A number of other works have shown that some simple organisms spend a lot of time in something very similar to sleep. Sometimes experts put their behavior in the drawer "sleep-like". But the more details come to light, the less necessary this distinction seems…