Jellyfish Sleep Like Humans—Even Without a Brain

Sleep has long been considered a hallmark of complex brains. Humans, mammals, birds, and even insects all display clear sleep cycles tied to brain activity. But a new study published in Nature Communications is forcing scientists to rethink this assumption. Researchers have found that jellyfish and sea anemones—animals without brains—enter sleep states that closely resemble human sleep patterns, both behaviorally and biologically (Appelbaum et al.).

The findings suggest that sleep may have emerged far earlier in evolution than previously believed, possibly as a protective mechanism for individual nerve cells rather than as a function of a centralized brain.

Sleeping Without a Brain

Jellyfish and sea anemones belong to one of the most ancient animal groups on Earth. Unlike humans, they lack a brain and instead rely on a decentralized nerve net to sense and respond to their environment. Despite this simplicity, scientists observed that these creatures regularly enter prolonged periods of reduced activity that meet all scientific criteria for sleep.

These criteria include decreased responsiveness to stimuli, a characteristic posture or behavior during rest, and a compensatory “rebound” period following sleep deprivation—features also seen in human sleep cycles.

This discovery builds on earlier research showing that jellyfish exhibit sleep-like behavior, but the new study is the first to document and analyze sleep patterns in sea anemones in detail.

Why Sleep Is a Risky Yet Essential State

From an evolutionary standpoint, sleep is dangerous. Sleeping animals are more vulnerable to predators and environmental threats, and time spent resting could otherwise be used for feeding or reproduction. The fact that sleep persists across nearly all animals with nervous systems suggests it serves a critical biological purpose.

Scientists increasingly believe that sleep evolved to support cellular maintenance—particularly in neurons. Unlike skin or blood cells, neurons do not regenerate easily. Damage that accumulates while an organism is awake must be repaired to maintain long-term function.

“Neurons are incredibly valuable,” explains molecular neuroscientist Lior Appelbaum of Bar-Ilan University, one of the study’s authors. Because these cells do not readily divide, maintaining their integrity is essential for survival (Appelbaum et al.).

Jellyfish Sleep Patterns Mirror Human Habits

To test how closely jellyfish sleep resembles human sleep, researchers studied the upside-down jellyfish (Cassiopea andromeda) both in laboratory conditions and in its natural habitat near Key Largo, Florida. Surprisingly, the jellyfish slept for approximately eight hours per day—almost identical to the average human sleep duration.

Most of this rest occurred during nighttime hours, with a shorter rest period around midday, similar to a nap. When researchers disrupted these rest periods, the jellyfish compensated by sleeping longer afterward, a classic hallmark of true sleep behavior.

The team conducted similar experiments on the starlet sea anemone (Nematostella vectensis), documenting sleep behavior in the species for the first time. The anemones also spent about one-third of their day resting, with sleep concentrated around dawn.

Sleep May Protect DNA, Not Just the Brain

One of the most intriguing implications of the study is its support for a growing theory: sleep’s original evolutionary role may have been to protect DNA inside nerve cells.

During waking hours, neurons are exposed to metabolic stress, environmental damage, and chemical byproducts that can harm DNA. Sleep appears to activate repair processes that help restore cellular health. Even without a brain, jellyfish neurons may rely on sleep to prevent long-term damage.

This finding aligns with previous neuroscience research showing increased DNA repair activity during sleep in mammals (Zada et al.).

Rethinking the Evolution of Sleep

The idea that sleep predates centralized brains gained traction in 2017, when researchers first demonstrated sleep-like behavior in jellyfish. The current study strengthens that hypothesis by showing that sleep is not just present, but highly structured, even in animals with the simplest nervous systems.

According to neuroscientist Ravi Nath of Stanford University, who was not directly involved in the new research, sleep likely evolved alongside neurons themselves. While the core function may be universal, each species appears to have adapted sleep to suit its own ecological needs.

Chiara Cirelli, a sleep researcher at the University of Wisconsin–Madison, notes that every new species added to the list of sleeping animals helps scientists better understand why sleep is so deeply embedded in biology.

Why This Matters for Human Health

Understanding why sleep exists—even in brainless organisms—has profound implications for human health. It reinforces the idea that sleep is not optional or merely restorative at a psychological level, but a fundamental biological requirement tied to cellular survival.

As sleep deprivation continues to rise globally, insights from simple organisms may help researchers uncover new ways to protect neurons, slow neurodegeneration, and improve brain resilience over a lifetime.

References

1. Appelbaum, Lior, et al. “Sleep-like States in Cnidarians Reveal Ancient Origins of Sleep.” Nature Communications, vol. 15, 2024, pp. 1–12.

2. Cirelli, Chiara, and Giulio Tononi. “Is Sleep Essential?” PLoS Biology, vol. 6, no. 8, 2008, e216.

3. Nath, Ravi, et al. “The Jellyfish Cassiopea Exhibits a Sleep-like State.” Current Biology, vol. 27, no. 19, 2017, pp. 2984–2990.

4. Zada, Dan, et al. “Parp1 Promotes Sleep, Which Enhances DNA Repair in Neurons.” Molecular Cell, vol. 81, no. 3, 2021, pp. 497–510.

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