A study published in Nature Communications indicates that jellyfish and sea anemones exhibit sleep-like states, a phenomenon observed in animals without a centralized brain. These findings contribute to the theory that sleep may have evolved partly to facilitate DNA repair in individual nerve cells, addressing damage accumulated during periods of wakefulness.
Lior Appelbaum, a molecular neuroscientist and co-author of the study at Bar-Ilan University, stated that neurons do not divide and require maintenance.
Background on Sleep Research
Previous research, including work by Ravi Nath in 2017, established the presence of a sleep-like state in jellyfish. This current study marks the first detailed characterization of sleep patterns in both jellyfish and sea anemones. Chiara Cirelli, a sleep researcher at the University of Wisconsin–Madison, commented on the significance of expanding the list of species known to sleep for the field.
Sleep is recognized as a biologically fundamental function, preserved across animal species with nervous systems studied to date, despite the associated risks such as increased vulnerability to predators and reduced time for essential activities like foraging or mating. The hypothesis that sleep predates the evolution of a centralized nervous system gained support with Nath's 2017 findings. Nath has indicated that sleep likely emerged with neurons, suggesting a core function adaptable to species-specific needs.
Observed Sleep Patterns
The recent study provides a more precise definition of sleep and investigates its potential cellular effects in animals possessing neurons but lacking a brain. Researchers observed the upside-down jellyfish (Cassiopea andromeda) sleeping for approximately eight hours daily, primarily at night, with a brief nap during midday. This observation was consistent in both laboratory settings and in a natural habitat in Key Largo, Florida.
Additionally, the starlet sea anemone (Nematostella vectensis) was studied in the laboratory, with its sleep patterns characterized for the first time. This species also slept for approximately one-third of the day, with its rest periods concentrated around dawn.