Even brainless animals like jellyfish have sleeping habits similar to humans, which indicates that sleep evolved much earlier than previously thought.
Sleep may be one of the most fundamental biological processes on Earth, according to new research that shows that even jellyfish, which lack a brain and anus, spend a significant part of your life sleeping.
A study by researchers at Bar-Ilan University found that jellyfish sleep for about a third of each daya pattern surprisingly similar to that of humans.
The findings, in the journal Nature Communications, suggest that sleep evolved much earlier than expectedpossibly more than a billion years ago. Humans and jellyfish diverged from a common ancestor around this time, but both appear to rely on sleep to maintain healthy nervous systems, says .
Unlike humans and most animals, jellyfish belong to the phylum Cnidaria, which does not have a centralized brain. Instead, they have simple nerve networks distributed throughout the body. Despite this rudimentary neural organization, researchers observed clearly sleep-like behaviors: prolonged periods of immobility, decreased responsiveness to stimuli, and predictable daily rhythms of rest and activity.
The team studied two species of cnidarians — the inverted jellyfish (Cassiopea andromeda) and the star sea anemone (Nematostella vectensis) — in laboratory and natural conditions. It was found that jellyfish sleep mainly at night and take naps during the day, while sea anemones showed an opposite pattern, sleeping mainly during the day. These contrasting schedules suggest that sleep timing is shaped by ecological needs, but that the fundamental need for sleep is universal.
Sleep carries clear risks, including reduced attention span and increased vulnerability to predators. However, researchers have found compelling evidence of its benefits. When deprived of sleep, both jellyfish and sea anemones showed a increased DNA damage in your neurons. Exposure to environmental stressors, such as ultraviolet radiation or chemical mutagens, also increased neuronal DNA damage and, notably, led the animals to sleep more.
The study points to sleep as a crucial mechanism for cell repair. During wakefulness, DNA damage accumulates in neurons, while sleep provides a consolidated period for repair and maintenance. When researchers treated the animals with melatonin, a hormone known to regulate sleep in humans, the cnidarians slept longer and showed a reduction in DNA damage. This suggests that even these simple animals can use a melatonin-based system to synchronize sleep with day-night cycles.
The authors argue that the need to protect neurons from genetic damage may have driven the evolution of sleep long before the emergence of complex brains. “These results suggest that DNA damage and cellular stress in simple nerve networks may have boosted the evolution of sleep“, the researchers conclude.
