Kamal Ghorui
Sleep appears to have emerged far earlier in animal evolution than previously thought, according to new research examining two brainless marine species. A study of the upside-down jellyfish Cassiopea andromeda and the sea anemone Nematostella vectensis finds that both exhibit sleep-like states despite lacking a centralised brain. Researchers observed periods of reduced movement, lowered responsiveness, and compensatory rest following sleep deprivation, meeting behavioural criteria commonly used to define sleep. Although the two species follow different daily activity patterns, each spends roughly one-third of the day in a sleep state. Cellular analysis suggests a link between this rest and reduced DNA damage in neurones.
Jellyfish sleep without brains and it may explain why sleep exists at all
The research “DNA damage modulates sleep drive in basal cnidarians with divergent chronotypes” says both animals lack a brain. What they have instead is a simple nerve net spread through soft tissue with no central control. Even so, they rest. Their movements are slow. Responses to light or touch take longer. When this state is disrupted, it returns later, stretched out and deeper, as if something unfinished needs to be completed. The pattern is not random. In controlled conditions and in the wild, these animals show repeated cycles of reduced activity that meet established behavioural definitions of sleep. The absence of a brain does not remove the need for rest. It only changes its shape, suggesting sleep emerged before complex nervous systems existed.
Light-driven sleep in Cassiopea jellyfish
In Cassiopea, sleep follows light. Activity drops at night, rises during the day, and briefly dips again around midday. There is no strong internal clock keeping time in the dark. Instead, light itself seems to do most of the work. When the jellyfish are kept awake at night, they sleep more the next day, suggesting a basic homeostatic drive layered on top of light control.
Circadian control of sleep in Nematostella
Nematostella behaves differently. It is most active around dusk and dawn, and its sleep is shaped not just by light but by an internal circadian clock. When that clock is genetically disrupted, the timing of sleep falls apart, though the total amount does not. Across species, despite different schedules and patterns, sleep still adds up to roughly one-third of the day.
Sleep reduces DNA damage in early animals
When the researchers looked inside the animals’ neurones, they found something striking. DNA damage accumulated during wakefulness and eased during sleep. This happened in both species, even though one sleeps mostly at night and the other closer to dawn. When sleep was artificially prevented, DNA damage increased. When sleep resumed, damage declined.
Environmental stress increases sleep pressure
The effect was not subtle. Exposure to ultraviolet radiation or chemical mutagens sharply increased DNA damage, and soon after, the animals slept more. Sleep, in turn, reduced that damage. Even melatonin, added externally, pushed the animals into sleep and lowered markers of genomic stress, regardless of whether they were normally active at that time.
Sleep as cellular maintenance , not luxury
The picture that emerges is not of sleep as a luxury or even primarily as a tool for learning or memory. Instead, sleep looks like maintenance. A pause that allows fragile, irreplaceable neurones to cope with daily molecular wear and tear. In animals exposed for hundreds of millions of years to sunlight, oxidative stress, and environmental damage, that pause may have been essential.
