Jellyfish Demonstrate Sleep Patterns Similar to Humans

Research has revealed that jellyfish and sea anemones exhibit sleep patterns strikingly similar to those of humans. A study published in March 2024 in the journal Nature Communications indicates that these ancient creatures spend about one-third of their day sleeping. This finding challenges previous assumptions about sleep in animals without a brain, highlighting its significance even for organisms with a simpler nervous system.

The study, led by neurobiologist Lior Appelbaum from Bar-Ilan University, shows that sleep is crucial for recovery from cellular stress. Appelbaum stated, “Sleep is important even for animals without a brain. It helps them recover from cellular stress and carry out cellular maintenance.” Understanding why sleep evolved remains a complex question that scientists have grappled with for years.

To investigate the evolutionary origins 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 employed infrared cameras and movement-tracking software to observe the active and inactive periods of these cnidarians under a controlled 24-hour cycle, simulating natural light and darkness.

The results were revealing. The jellyfish were primarily active during daylight hours, sleeping mainly at night and even indulging in midday naps, akin to human post-lunch slumbers. Conversely, the sea anemones displayed heightened activity during the night, with their sleep predominantly occurring in the early morning hours.

To confirm that the observed inactivity constituted true sleep rather than mere rest, the researchers measured the animals’ responses to environmental stimuli such as light and food. Appelbaum defined a sleep-like state in jellyfish as having a pulse rate of fewer than 37 beats per minute for over three minutes, while sea anemones were considered to be sleeping if they remained still for eight minutes or more.

The study uncovered that both jellyfish and sea anemones sleep for approximately eight hours each day. 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.”

Investigating further, the team explored the factors that trigger sleep in these creatures. Previous studies indicated that an increase in DNA damage in nerve cells promotes sleep in zebrafish. Building upon this knowledge, the researchers exposed the jellyfish and sea anemones to ultraviolet (UV) radiation and chemicals known to cause DNA damage. They also introduced subtle disruptions to their sleep, such as water pulses or gentle flow.

The findings revealed that DNA damage increased during wakefulness and was repaired during sleep. This suggests that jellyfish and sea anemones respond to cellular stress by increasing sleep duration, emphasizing its role in necessary cellular repairs. Appelbaum concluded, “Sleep evolved way before the brain – it’s essential for cells, cellular maintenance, and neurons.”

The implications of this research extend beyond understanding sleep in these simple organisms. The study contributes to the broader dialogue about the evolutionary significance of sleep across the animal kingdom, providing insight into the fundamental biological processes that govern life, even in the most primitive of creatures.