Turns out jellyfish and sea anemones – among the ancient creatures with a nervous system instead of a brain – have a very similar sleeping routine to our own. A new study published in Nature Communications reveals that these animals not only sleep but do so for the same portion of their day as humans, spending about one-third of the day sleeping. The findings also suggest that sleep evolved long before the brain to help maintain cells under stress.
"Jellyfish and sea anemones really do sleep, which wasn’t entirely clear before," Lior Appelbaum, a neurobiologist at Bar-Ilan University, told New Atlas in an interview. "Sleep is important even for animals without a brain. It helps them recover from cellular stress and carry out cellular maintenance."
Why animals sleep is a big question that has long perplexed researchers. While sleep is seen across the animal kingdom, its purpose is still debated. What adds another layer of enigma is that when sleeping, animals become less aware of their surroundings and more vulnerable to predators. This apparent evolutionary trade-off prompted Appelbaum to study why we sleep.
To understand the evolution of sleep, Appelbaum and his colleagues focused on upside-down jellyfish (Cassiopea andromeda) and the starlet sea anemone (Nematostella vectensis). The team observed the active and inactive periods of these cnidarians using infrared cameras and movement-tracking software. The standard 24-hour cycle was used to mimic a natural day – i.e., 12 hours of light and 12 hours of darkness.
Researchers found that the jellyfish, which host photosynthetic algae, are active during the day, sleep mainly at night, and even take midday naps, much like some humans do after lunch. In contrast, the sea anemones are more active at night and sleep more in the early morning hours.
But how did the team confirm that they were truly sleeping, and not merely resting?
They measured how quickly the animals responded to stimuli such as light or food during periods of inactivity. Using this response, Appelbaum defined a sleep-like state in jellyfish as pulsing fewer than 37 times per minute for over three minutes, and eight minutes or more of stillness in sea anemones.
"What was surprising was that both animals sleep approximately eight hours, which is interesting because we also sleep approximately one-third of our lives, and they have more or less the same part," Appelbaum told us.
Crucially, the study also explored what drives this sleep. Earlier work has shown that an increase in DNA damage in nerve cells promotes sleep in zebrafish. Building on this research, the team exposed the animals to ultraviolet (UV) radiation or specific chemicals known to damage DNA. They also deprived them of sleep with subtle disruptions like water pulses or gentle flow.
Appelbaum says that the DNA damage increased during the wake periods and cleared away during nighttime sleep. Jellyfish and sea anemones responded to this DNA damage by sleeping more, suggesting that sleep is crucial for performing necessary cellular repairs.
"Sleep evolved way before the brain – it’s essential for cells, cellular maintenance, and neurons," says Appelbaum.
The new study was published in Nature Communications.
