A groundbreaking study has demonstrated that jellyfish and sea anemones exhibit sleep patterns remarkably similar to those of humans. Research published in Nature Communications reveals that these ancient creatures, which possess a nervous system but lack a brain, spend approximately one-third of their day asleep. This discovery indicates that sleep may have evolved long before the development of the brain, serving a vital role in cellular maintenance during stress.
Lior Appelbaum, a neurobiologist at Bar-Ilan University, stated, “Jellyfish and sea anemones really do sleep, which wasn’t entirely clear before. Sleep is important even for animals without a brain. It helps them recover from cellular stress and carry out cellular maintenance.” The question of why animals sleep has intrigued researchers for years, as sleep is observed across the animal kingdom, yet its purpose remains a subject of debate.
To investigate the evolution of sleep, Appelbaum and his team focused on two specific species: the upside-down jellyfish, Cassiopea andromeda, and the starlet sea anemone, Nematostella vectensis. The researchers monitored the activity levels of these organisms using infrared cameras and movement-tracking software, mimicking a natural 24-hour cycle with 12 hours of light and 12 hours of darkness.
The findings showed that jellyfish, which host photosynthetic algae, are active during daylight hours, primarily sleep at night, and even take midday naps, much like humans. In contrast, sea anemones exhibit more nighttime activity and sleep predominantly in the early morning hours.
To confirm that these animals were indeed sleeping rather than merely resting, the team measured their responses to stimuli such as light or food during inactive periods. Appelbaum defined a sleep-like state in jellyfish as a pulsing rate of fewer than 37 times per minute for over three minutes, while sea anemones demonstrated eight minutes or more of stillness. “What was surprising was that both animals sleep approximately eight hours,” Appelbaum noted. “This is interesting because we also sleep approximately one-third of our lives, and they have more or less the same part.”
The study also examined the mechanisms driving sleep in these marine creatures. Previous research has indicated that increased DNA damage in nerve cells can promote sleep in zebrafish. Building on this, the team exposed jellyfish and sea anemones to ultraviolet (UV) radiation and other chemicals known to damage DNA. Additionally, they disrupted their sleep through subtle disturbances like water pulses or gentle currents.
The results revealed that DNA damage increased during waking periods and decreased during nighttime sleep. Jellyfish and sea anemones responded to this damage by increasing their sleep duration, underscoring the importance of sleep for cellular repair. Appelbaum emphasized, “Sleep evolved way before the brain – it’s essential for cells, cellular maintenance, and neurons.”
This research highlights the significant role of sleep in the lives of these ancient organisms and contributes to the broader understanding of sleep’s evolutionary significance in the animal kingdom.
