Why do we sleep? And above all, when was sleep “invented”? The answer, according to a study published in Nature Communicationsis “quite a long time ago”: the work in fact analyzes the sleep of jellyfish, which despite being very distant from us humans from an evolutionary point of view, and not even having a central brain, need a few hours of sleep every day just as much as we do. Sleeping is therefore a habit that dates back at least a billion years ago, perhaps more.
The sleep of cnidarians. The study by Bar-Ilan University, in Israel, focused in general on cnidarians, the phylum to which jellyfish belong but also corals and sea anemones. All groups sleep, although in different ways: jellyfish, for example, sleep at night like humans, and take a nap around midday; in total, they spend about a third of their day sleeping. Sea anemones, on the other hand, sleep during the day and are active at night. Opposite methods, in short, but to obtain the same result: rest, and therefore repair the damage caused by the activity.
In fact, deprived of sleep, both jellyfish and anemones suffer evident damage to neuronal DNA: the team verified this fact both in the laboratory and in natural conditions. Not only that: when they are in an environment that causes more damage to neuronal DNA, anemones and jellyfish sleep more – this means, according to the authors, that sleep was born to protect and repair cells damaged by anything done while awake.
The invention of sleep. Like us humans, then, anemones and jellyfish also use a system based on melatonin to regulate their sleep-wake cycles: this is demonstrated by the fact that the specimens that were administered that hormone slept more. In short: although they do not have a central nervous system, cnidarians also need sleep to mitigate the negative effects of being awake.
Considering that humans and cnidarians separated evolutionarily about a billion years ago, sleep is therefore an ancient invention, created to protect cells: it is so effective at counterbalancing the effects of wakefulness that all animals sleep (or at least rest), despite the fact that it is a state of inactivity that has a series of contraindications, including that of making the animal more vulnerable to predators. But when faced with certain benefits, how can you say no to a nap?
