Artificial light Is keeping reef fish awake, and the effects may ripple across coral reefs

Artificial light spilling into coastal waters from cities, ports, roads, and hotels is disrupting sleep in coral reef fish and is associated with changes in markers linked to brain health, according to a new study from Bar-Ilan University.

The study, published in Current Biology, shows that even low levels of nighttime illumination can significantly alter the behavior and physiology of reef fish. Fish exposed to artificial light slept less, showed more fragmented sleep, became more aggressive, and fed at unusual hours, effectively behaving as if night had turned into day.

"Artificial light at night is rapidly expanding across coastal environments worldwide," said Prof. Oren Levy of Bar-Ilan University's Faculty of Life Sciences and the H. Steinitz Marine Biology Laboratory in Eilat, who co-led the study with Prof. Lior Appelbaum and doctoral student Shachaf Ben-Ezra, from the Bar-Ilan Faculty of Life Sciences. "We found that even relatively low levels of illumination can disrupt natural sleep patterns and are associated with changes in markers of neuronal health."

The researchers studied the blue-green damselfish (Chromis viridis), a common reef fish that feeds above coral during the day and shelters in branching corals at night.  Using infrared video, machine-learning tracking, lab experiments, and in situ reef studies in the Gulf of Aqaba/Eilat, Israel, they first confirmed that these fish exhibit clear sleep-like states, including inactivity, characteristic posture, and reduced responsiveness.

When exposed to ecologically relevant artificial light at night (ALAN) levels, however, these patterns broke down. Fish no longer remained confined to their usual nighttime territories within the coral, instead expanding their activity range, feeding at unusual hours, interacting more aggressively, and sleeping significantly less.

To assess biological effects, the team examined neurons in a brain region involved in sleep-dependent brain functions. Fish exposed to nighttime light pollution showed elevated levels of markers associated with DNA damage compared with fish under natural dark conditions. While the study does not demonstrate that light directly damages DNA, the findings suggest that sleep disruption may interfere with essential nighttime maintenance and repair processes in the brain. These changes appeared after only a few nights of exposure and persisted during a five-month field experiment conducted directly on a coral reef, suggesting that chronic exposure to artificial light may have lasting biological consequences.

"Sleep is a critical period for biological repair," said Prof. Appelbaum. "Our findings suggest that disrupting sleep with artificial light may have measurable consequences even in wild marine animals."

Approximately 22% of the world's coastal regions and 35% of marine protected areas are already affected by ALAN. In the Gulf of Eilat, where this study was conducted, nighttime light levels near developed coastal areas can reach up to 60 times the brightness of natural starlight due to urban and port development. Previous work from Levy's lab showed that artificial light can affect coral physiology, disrupt the symbiotic relationship between corals and algae, and interfere with the synchronization of coral spawning. The new findings indicate that fish living within those ecosystems are also affected, pointing to broader ecosystem-level consequences.

"Coral reefs depend on tightly connected biological interactions," Levy said. "If artificial light is affecting both corals and the fish that depend on them, the consequences could ripple throughout the reef ecosystem."

The light levels used in the study match those measured near developed coastal areas, underscoring the real-world relevance of the findings. The researchers call for improved coastal lighting practices, including reducing unnecessary nighttime illumination, directing light away from the shoreline and water, adopting smart lighting technologies, and developing guidelines for wavelengths that minimize ecological disruption.

Future research will examine whether these effects are reversible and how widespread they are across marine species.

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