Jellyfish Need Beauty Sleep Too: A Look Into the Ancient Origins of Sleep
Even a boneless, gelatinous sack lacking a dedicated anus and brain needs its beauty sleep, a new study by researchers from Bar-Ilan University in Israel finds. Jellyfish, despite their outlandishly different physiology, sleep a third of every day away, just like we do. This suggests that the origin of sleep is extremely ancient, dating back to around a billion years ago when human ancestors diverged from the jellyfish phylum, Cnidaria.
Cnidaria lack a centralized brain but have neural networks lining the length of their bodies. Despite this simple neural arrangement, these water drifters have been observed sleeping, just like animals with nervous systems do. The period of immobility and decreased arousal that comes with sleep poses risks such as reduced awareness of the environment and vulnerability to predation, as explained by chronobiologist Raphaël Aguillon and colleagues in their paper.
Yet, jellyfish tend to sleep through the night like humans and even take naps around midday. Their close relative, the sea anemone, takes the night shift, sleeping during daylight hours. This indicates that there must be a powerful benefit to sleeping that counteracts the risks.
Specimens of upside-down jellyfish and starlet sea anemone suffered an increase in neuronal DNA damage when deprived of sleep, as observed under both laboratory and natural conditions. The researchers also found that if their external environment caused increased neuronal DNA damage, both Cnidaria slept more, suggesting that sleep may have evolved as a way to protect cells from damage.
When treated with melatonin, the animals slept more, and DNA damage was subsequently reduced. The researchers suspect that Cnidarians use a melatonin system to synchronize their sleep cycles to daylight cycles. Sleep deprivation, ultraviolet radiation, and mutagens increased neuronal DNA damage and sleep pressure, while spontaneous and induced sleep facilitated genome stability.
Even simple neural systems require rest to reduce the inevitable DNA damage that accompanies wakefulness. The balance between DNA damage and repair is insufficient during wakefulness, and sleep provides a consolidated period for efficient cellular maintenance in individual neurons. These results suggest that DNA damage and cellular stress in simple nerve nets may have driven the evolution of sleep.
This fascinating research was published in Nature Communications, shedding light on the ancient origins of sleep in creatures as seemingly simple as jellyfish. Sleep, it seems, is a universal need that has deep evolutionary roots, even in organisms without brains or complex nervous systems.
