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Sleep is usually associated with complex animals with brains and nervous systems. But new research suggests it may be far older and far more basic than previously thought.
Scientists at Bar Ilan University in Israel studying jellyfish and sea anemones have found evidence of sleep-like states in animals without brains, helping explain why sleep may have evolved in the first place.
Unlike humans and other mammals, jellyfish have no brain and no eyes. Instead, it has a simple nerve net, a loose network of neurons distributed throughout its body. Despite this basic anatomy, scientists have found that jellyfish follow regular cycles of activity and rest that meet the biological criteria used to define sleep.
To study this behavior, researchers monitored jellyfish and sea anemones continuously using infrared video, allowing them to observe movement during both day and night without disturbing natural light conditions.
“The key finding is that you have a very basic organism, doesn’t have a brain or eyes. They still can sleep,” says Oren Levy, a life sciences professor at Bar Ilan University. The researchers found that jellyfish sleep for roughly eight hours a day, mostly at night, with short rest periods around midday. Sea anemones, another simple marine animal, also showed clear sleep patterns, though their rest occurred mainly during the day.
To confirm these were true sleep states rather than simple inactivity, scientists tested how the animals responded to stimulation. The most significant findings came when researchers examined what was happening inside the animals’ nerve cells.
Using specialized imaging techniques, they measured DNA damage within neurons during wakefulness and sleep. DNA carries the genetic instructions that allow cells to function. Damage to DNA can occur as a result of normal metabolic activity, environmental stress, or prolonged cellular activity. If that damage is not repaired, it can impair cell function or lead to cell death.
In both jellyfish and sea anemones, DNA damage in neurons increased while the animals were awake. During sleep, levels of damage decreased, suggesting that sleep allows time for cellular repair processes to take place.
This article was provided by The Associated Press.