Jellyfish and Sea Anemones Sleep Like Humans Despite Lacking Brains

Lead

Researchers report that the upside-down jellyfish Cassiopea andromeda and the starlet sea anemone Nematostella vectensis enter prolonged, regular sleep-like states despite lacking centralized brains. Field observations in Key Largo, Florida, and controlled laboratory tests show Cassiopea rests roughly eight hours per day, mostly at night with a short midday nap, while Nematostella spends about one-third of each day in rest concentrated around dawn. The study, published in Nature Communications, examines behavioral criteria for sleep in these cnidarians and explores a cellular hypothesis: that sleep helps protect and repair individual neurons. The findings reinforce the idea that sleep, or sleep-like rest, may be an ancient trait linked to the maintenance of neuronal integrity.

Key Takeaways

• Cassiopea andromeda exhibits about eight hours of sleep-like behavior per 24-hour period, with most rest at night and a brief midday nap observed in the wild and lab.
• Nematostella vectensis spends roughly one-third of a 24-hour cycle resting, with rest concentrated around dawn in laboratory conditions.
• The research appears in Nature Communications and combines field recordings from Key Largo, Florida, with controlled laboratory assays to quantify rest patterns.
• Authors report behavioral hallmarks of sleep—reversible inactivity, reduced responsiveness, and homeostatic rebound—applied to animals that lack a central brain.
• Team members propose a cellular function for sleep: protecting DNA and repairing damage in long-lived neurons that do not divide, which could explain evolutionary conservation across animals with nervous systems.
• The study expands the list of species shown to meet operational definitions of sleep, strengthening the hypothesis that core sleep functions predate centralized brains.

Background

Sleep is widespread across animals with nervous systems, yet its origins and essential functions remain debated. Sleep carries clear costs—reduced vigilance, lost foraging time and increased predation risk—so evolutionary conservation implies a fundamental benefit. A 2017 study first documented sleep-like states in jellyfish, suggesting elements of sleep could exist without a centralized brain. That work shifted attention toward whether sleep emerged alongside neurons themselves, rather than with brains, and whether cellular maintenance might be a primary function.

Cnidarians such as jellyfish and sea anemones possess distributed nerve nets rather than centralized brains; their neurons are often long-lived and do not divide in adult animals. Because neuronal DNA and cellular components accumulate damage during wakeful activity, some researchers hypothesize that periods of behavioral quiescence evolved to enable repair. The current study builds on prior behavioral criteria—reversible inactivity, increased arousal threshold, and compensatory increase after deprivation—to test sleep-like rest in two cnidarian species across laboratory and field settings.

Main Event

Appelbaum and colleagues monitored Cassiopea andromeda in shallow mangrove lagoons of Key Largo and in laboratory tanks, recording pulsing activity, responsiveness to stimuli, and recovery after enforced wakefulness. They quantified daily cycles and found consistent nocturnal rest spanning about eight hours, plus brief midday reductions in activity that resemble naps. In parallel, experiments with Nematostella vectensis in the lab showed prolonged dawn-centered rest amounting to roughly one-third of the day, meeting the authors’ behavioral benchmarks for sleep.

Across both species, the researchers documented three operational features of sleep: spontaneous and reversible behavioral quiescence, elevated arousal thresholds during quiescence, and a homeostatic rebound—animals deprived of rest later increased their time spent inactive. The paper also reports molecular and cellular assays that suggest restorative processes occur during rest, pointing toward DNA maintenance and cellular repair pathways as candidate mechanisms. Observations were consistent across naturalistic and controlled environments, strengthening the claim that these rest states are neither artifact nor solely context-dependent.

Field data were crucial for Cassiopea, where natural light cycles and environmental cues were preserved; the midday nap was more apparent in situ than in standard laboratory lighting. For Nematostella, lab-based entrainment to light-dark cycles revealed dawn-focused rest that may relate to the species’ tidal and photic ecology. The combined approach allowed the team to link ecological timing with conserved behavioral features of sleep-like states in animals without brains.

Analysis & Implications

If sleep-like behavior in cnidarians serves to protect individual neurons, it would support a parsimonious evolutionary model: sleep emerged as a cellular maintenance strategy early in animal evolution and was later co-opted and adapted by species with centralized brains. That model explains why sleep is costly yet highly conserved—because preserving non-dividing neurons is essential for long-term organismal function. The study’s behavioral and preliminary cellular data provide convergent support, but they do not yet prove causation between rest and specific molecular repair pathways.

The findings reshape questions about sleep’s adaptive diversity. Rather than a single function, sleep may have a conserved core—cellular upkeep—around which taxa have layered species-specific roles such as memory consolidation, metabolic regulation or predator avoidance strategies. For example, the timing differences between the nocturnal jellyfish rest and the dawn-centred anemone rest hint that ecological pressures sculpt when restorative periods occur, even if the underlying cellular need is shared.

There are also translational implications: understanding ancient, brain-independent sleep mechanisms could inform how neuronal maintenance is regulated at the single-cell level, which may be relevant to neurodegenerative disease research. However, scaling from cnidarian nerve nets to mammalian brains requires caution; differences in complexity, cell types and molecular pathways mean that direct parallels should not be assumed without targeted molecular validation. Future work that links observed behavioral quiescence to measured DNA repair, protein turnover or autophagy during rest will be decisive.

Comparison & Data

The table summarizes the principal behavioral measurements reported: both species meet operational sleep criteria and spend comparable fractions of the day inactive, though timing differs. Field observations revealed behaviors (notably Cassiopea’s midday nap) that were less evident under standard lab lighting, underscoring the value of combining field and lab methods to capture ecological timing and naturalistic rhythms.

Reactions & Quotes

Lead author Lior Appelbaum and colleagues emphasize neuronal vulnerability as a possible driver of sleep-like behavior. Context: Appelbaum framed the result as evidence that preserving intact neurons is critical across animal lineages.

“Neurons are very precious; they don’t divide, so mechanisms to keep them intact are likely ancient and fundamental.”

Lior Appelbaum, Bar-Ilan University (co-author)

External experts welcomed the addition of new species to the sleep catalogue while urging caution on mechanistic claims. Context: Chiara Cirelli highlighted the study’s value for the field but noted that expanding species lists is an incremental step toward deeper understanding.

“Each credible demonstration of sleep in a new species is an important advance for sleep biology.”

Chiara Cirelli, University of Wisconsin–Madison (sleep researcher)

Researchers who previously identified jellyfish rest noted continuity with earlier findings. Context: Ravi Nath reflected on the broader implication that sleep-like states may align with the origin of neurons rather than brains.

“There is good evidence that sleep emerged with neurons; different species then adapted those core rest mechanisms to their own ecologies.”

Ravi Nath, Stanford University (neuroscientist)

Unconfirmed

• That sleep’s primary evolutionary function is DNA repair remains a hypothesis; direct causal linkage between rest and specific repair processes in these species has not been conclusively demonstrated.
• Whether the same molecular repair mechanisms observed in bilaterian neurons operate identically in cnidarian neurons is unverified and requires targeted biochemical and genetic experiments.
• Generality across Cnidaria is uncertain: only two species were characterized here, so the prevalence of sleep-like states across corals, other jellyfish and anemones is not yet established.

Bottom Line

This study strengthens the view that sleep-like rest is an ancient trait associated with neurons themselves, not solely with centralized brains. By documenting consistent behavioral markers of sleep in both Cassiopea and Nematostella, and by linking those markers to the concept of neuronal maintenance, the work moves the field toward a cellular explanation for why sleep persists across diverse animals.

Key next steps are mechanistic: measuring DNA damage, repair activity and other cellular maintenance processes during rest and wake cycles in these species. If such links are confirmed, they would provide a parsimonious explanation for sleep’s deep evolutionary roots and offer new entry points for studying neuronal health across the animal kingdom.

Sources

• Nature (news report) — media coverage summarizing the Nature Communications study and contextual commentary.
• Nature Communications (journal) — primary research publication venue for the reported study (academic journal).
• Bar-Ilan University (academic) — institutional affiliation of lead co-author and lab information.