Jellyfish and Sea Anemones Exhibit Sleep Patterns Similar to Humans

A recent study reveals that jellyfish and sea anemones have sleep patterns strikingly similar to humans, sleeping for about one-third of their day. Published in the journal Nature Communications, the research indicates that these ancient creatures, which possess a nervous system but lack a brain, engage in sleep for cellular recovery and maintenance.

Neurobiologist Lior Appelbaum from Bar-Ilan University led the study, which showed that jellyfish, particularly the upside-down jellyfish known as Cassiopea andromeda, and the starlet sea anemone, Nematostella vectensis, undergo sleep-like states. Appelbaum stated, “Sleep is important even for animals without a brain. It helps them recover from cellular stress and carry out cellular maintenance.”

Understanding the reasons behind sleep remains a significant challenge for scientists. While sleep is observed across many animal species, its evolutionary purpose continues to be debated. The vulnerability of animals during sleep raises questions about its benefits versus risks.

To investigate the evolutionary origins of sleep, Appelbaum’s team monitored the activity patterns of both jellyfish and sea anemones using infrared cameras and movement-tracking software. The study followed a typical 24-hour cycle, encompassing 12 hours of light and 12 hours of darkness.

Findings showed that jellyfish are primarily active during daylight, sleep at night, and even take midday naps, mirroring human behavior. Conversely, sea anemones are more active at night, resting in the early morning. Appelbaum noted, “What was surprising was that both animals sleep approximately eight hours, which is interesting because we also sleep approximately one-third of our lives.”

To confirm sleep rather than mere rest, the researchers assessed the animals’ responses to stimuli such as light or food during inactive periods. Appelbaum classified a sleep-like state in jellyfish as exhibiting fewer than 37 pulses per minute for more than three minutes, while sea anemones demonstrated stillness for eight minutes or more.

The study delves deeper into the underlying mechanisms driving sleep. Previous research suggested that increased DNA damage in nerve cells could prompt sleep in zebrafish. Extending this concept, the team exposed jellyfish and sea anemones to ultraviolet (UV) radiation and other chemicals known to cause DNA damage. They also introduced subtle disruptions, such as water pulses, to deprive the animals of sleep.

Appelbaum observed that DNA damage escalated during wakefulness and diminished during sleep, indicating that both jellyfish and sea anemones responded to DNA damage by increasing their sleep duration. This suggests that sleep plays a crucial role in cellular repair processes. Appelbaum concluded, “Sleep evolved way before the brain – it’s essential for cells, cellular maintenance, and neurons.”

This groundbreaking research not only sheds light on the sleep behaviors of these ancient marine creatures but also raises important questions about the fundamental nature of sleep across the animal kingdom.