😴 This discovery in jellyfish upends our understanding of sleep

The jellyfish, like the sea anemone, although lacking a brain, experience deep rest phases that meet all the criteria of sleep. This discovery provides valuable insights into this state that occupies nearly a third of our existence.

The work, published in Nature Communications, challenges the timeline of sleep evolution. Contrary to what was previously assumed, they suggest that the emergence of sleep is in fact much earlier than the development of a centralized nervous system. By observing these animals, researchers have traced the roots of a universal behavior. To do this, they analyzed cycles of activity and rest, using infrared lights or applying controlled disturbances.


Sleep in these invertebrates could thus be defined by analyzing prolonged and reversible decreases in motor activity, the reaction threshold to stimuli, and also a surprising phenomenon observed after sleep deprivation. For instance, they found that the jellyfish Cassiopea andromeda and the anemone Nematostella vectensis devote about eight hours a day to this rest.

Sleep, a cellular necessity before being a cerebral one

The regulation of this cycle involves distinct mechanisms, showing an evolutionary convergence. In the anemone, an internal circadian clock seems to drive the sleep-wake alternation. The jellyfish, in symbiosis with photosynthetic algae, is more sensitive to the direct alternation of light. This difference shows that different pathways can lead to the same functional state. The administration of melatonin, a key sleep hormone in vertebrates, also induces drowsiness in these animals.

The most compelling evidence lies in the response to deprivation. Indeed, when researchers disturb the jellyfish's sleep by agitating the aquarium water, they exhibit a rest duration increased by half the next day. This need indicates that sleep fulfills a non-negotiable physiological function, essential for survival.

DNA repair, the original function of rest

The importance of this maintenance is clearly apparent at the cellular level. Neurons, due to their intense electrical activity and high metabolism, are particularly prone to DNA damage during wakefulness. To visualize these breaks in the animals' nervous network, the researchers used a fluorescent marker. The results are unequivocal: genomic lesions accumulate during active phases and decrease significantly after a period of sleep.

To test the causal link, the team directly induced DNA damage. Exposing jellyfish to ultraviolet light or treating anemones with a chemical mutagen consistently leads to prolonged sleep. Conversely, promoting rest via melatonin reduces damage levels. This dialogue establishes a direct relationship between the integrity of the neuronal genome and the need to sleep.

This discovery reframes sleep within a fundamental evolutionary perspective. Before serving memory or cognition, it may have emerged as a mandatory period of cellular maintenance for the first animals with neurons. Its apparent risk (immobility and thus vulnerability) would therefore be compensated by a vital benefit: preserving the genetic capital of nerve cells that, for the most part, do not renew themselves. This primordial function would have been conserved throughout the ages, from the ocean depths to our own species.

Article author: Cédric DEPOND