Sleep is when biological repair happens. Disrupt it, and you disrupt the body's ability to maintain itself.
Along the illuminated coastlines of the modern world, darkness has become a diminishing resource — and for the reef fish of the Gulf of Eilat, its loss is no small matter. A study from Bar-Ilan University reveals that artificial nighttime light is not merely inconveniencing these creatures but disrupting the deep biological rhythms that govern sleep, repair, and survival. In the long human story of reshaping nature to suit our waking hours, this research adds a quieter chapter: the cost paid by those who cannot turn off the lights.
- Artificial light near developed coastlines reaches sixty times the brightness of natural starlight, effectively erasing night for fish that evolved to sleep in darkness.
- Exposed reef fish sleep less, fragment their rest, grow more aggressive, and feed at wrong hours — their bodies convinced that night has become day.
- More alarmingly, neurons in the brains of light-exposed fish show elevated DNA damage markers, suggesting that lost sleep is also lost biological repair time.
- These changes appeared within days and persisted across a five-month field experiment, signaling that chronic light pollution may leave lasting biological scars.
- With 22% of coastal regions and 35% of marine protected areas already affected, researchers are urging smarter, dimmer, redirected coastal lighting before ecosystem-wide ripple effects compound.
Every night in the Gulf of Eilat, as the sun sets over the coral, something else rises: the glow of cities, ports, and hotels bleeding into the water at levels reaching sixty times natural starlight. For the blue-green damselfish — a small reef fish whose life is organized around a simple rhythm of daytime feeding and nighttime shelter — this light is a profound disruption. A new study from Bar-Ilan University, led by Profs. Oren Levy and Lior Appelbaum, shows the damage runs far deeper than changed behavior.
Using infrared video, machine-learning tracking, and experiments both in the lab and on the reef itself, the researchers first mapped what healthy sleep looks like in these fish: stillness, a characteristic resting posture, reduced responsiveness. Then they introduced artificial light at levels matching real coastal conditions. The fish stopped behaving like themselves — sleeping less, fragmenting their rest, roaming beyond their usual territories, growing aggressive, and feeding at hours meant for rest. Their bodies had received a false signal: night had become day.
Beneath the behavioral surface, something more troubling emerged. Neurons in brain regions tied to sleep-dependent functions showed elevated markers associated with DNA damage in light-exposed fish — suggesting that disrupted sleep is also disrupted repair. These changes appeared within days and held through a five-month field experiment, pointing toward lasting biological consequences.
The problem is not confined to one species. Some 22% of the world's coastal regions are already touched by artificial nighttime light, as are 35% of marine protected areas. Earlier work from Levy's lab had shown that the same light pollution damages corals directly — harming their symbiotic algae and disrupting synchronized spawning. Fish and corals are not separate concerns; they are threads in the same web. Harm both, and the effects cascade through predator-prey dynamics, reproduction, and the intricate dependencies that hold a reef together.
The researchers are calling for concrete responses: reducing unnecessary coastal illumination, directing light away from water, adopting smart lighting technologies, and establishing guidelines around less ecologically disruptive wavelengths. Whether the will exists to act at the necessary scale remains uncertain. What is no longer uncertain is that the light spilling into coastal waters at night is quietly remaking the biology of the creatures living there — one stolen dark hour at a time.
The reef fish in the Gulf of Eilat have a problem that arrives every night with the sun's departure. As darkness falls over the coral, artificial light from nearby cities, ports, and hotels bleeds into the water—so much light that in some places it reaches sixty times the brightness of natural starlight. For fish that have evolved to sleep when darkness comes, this constant illumination is a kind of permanent disruption, and a new study from Bar-Ilan University shows the consequences run deeper than simple behavioral change.
Researchers led by Prof. Oren Levy and Prof. Lior Appelbaum studied the blue-green damselfish, a small reef fish with a straightforward daily rhythm: it feeds above coral during daylight hours and retreats into branching corals to shelter and sleep at night. Using infrared video, machine-learning tracking, and experiments both in the lab and directly on the reef, the team first established what healthy sleep looks like in these fish—inactivity, a characteristic resting posture, reduced responsiveness to stimuli. Then they exposed them to artificial light at levels that match what actually exists near developed coastlines.
The results were stark. Fish under artificial nighttime light stopped behaving like themselves. They slept less and in fragmented bursts. They expanded their activity range beyond their usual nighttime territories. They became more aggressive toward one another. They fed at hours when they normally would be resting. In essence, their bodies were receiving a signal that night had become day, and they responded by abandoning the rhythms that had shaped their evolution.
But the behavioral changes were only the visible part. When the researchers examined neurons in a brain region responsible for sleep-dependent functions, they found something more troubling: elevated markers associated with DNA damage in fish exposed to artificial light compared to those kept under natural darkness. The study does not prove that light directly damages DNA, but the pattern suggests something important—that sleep disruption interferes with the essential maintenance and repair work that happens in the brain during rest. These changes appeared within just a few nights of exposure and persisted through a five-month field experiment conducted on an actual coral reef, indicating that chronic light pollution may leave lasting biological marks.
The scope of the problem extends far beyond a single species or a single reef. About twenty-two percent of the world's coastal regions are already affected by artificial nighttime light, as are thirty-five percent of marine protected areas. In the Gulf of Eilat, where this research took place, the transformation is particularly acute. What was once a dark seascape is now lit by urban sprawl and port infrastructure.
Previous research from Levy's lab had already shown that artificial light disrupts coral physiology itself, damaging the symbiotic relationship between corals and the algae they depend on, and interfering with the synchronized spawning that allows corals to reproduce. Now the evidence suggests that the fish living within those ecosystems are suffering parallel damage. Corals and fish are not separate problems but interconnected parts of a single system. If light pollution is harming both, the consequences will ripple outward through the entire reef—affecting predator-prey relationships, feeding patterns, reproduction, and the intricate web of dependencies that holds a reef ecosystem together.
Appelbaum framed the core issue plainly: sleep is when biological repair happens. Disrupt sleep with artificial light, and you disrupt the body's ability to maintain itself. For wild marine animals living in ecosystems already stressed by warming waters and other human pressures, this adds another burden they did not evolve to bear.
The researchers are calling for practical changes: reducing unnecessary nighttime illumination in coastal areas, directing light away from shorelines and water, adopting smart lighting technologies that can be dimmed or turned off when not needed, and developing guidelines around light wavelengths that cause less ecological disruption. Whether these measures will be adopted widely enough to matter remains an open question. What is clear is that the light spilling into coastal waters at night is not neutral. It is remaking the behavior and biology of the animals that live there, one dark hour at a time.
Notable Quotes
Sleep is a critical period for biological repair. Our findings suggest that disrupting sleep with artificial light may have measurable consequences even in wild marine animals.— Prof. Lior Appelbaum, Bar-Ilan University
Coral reefs depend on tightly connected biological interactions. If artificial light is affecting both corals and the fish that depend on them, the consequences could ripple throughout the reef ecosystem.— Prof. Oren Levy, Bar-Ilan University
The Hearth Conversation Another angle on the story
So the fish are just staying awake longer because of the light. That seems like it should be reversible—turn off the lights, they go back to normal?
That's the intuitive assumption, but the study suggests it's more complicated. The DNA damage markers appeared within days and persisted through months of field exposure. Sleep disruption isn't just about lost rest—it's about lost repair time. The brain does maintenance work at night that doesn't happen during the day.
But they didn't prove the light actually damages DNA, right? They just found markers associated with damage.
Correct. What they're saying is that the markers suggest sleep disruption is interfering with the brain's ability to repair itself. It's like if you never got to do maintenance on your house—eventually things break down, even if you can't point to the exact moment the damage occurred.
How widespread is this actually? Is it just the Gulf of Eilat, or are we talking about most reefs?
Twenty-two percent of the world's coastal regions are already affected. That's not most reefs yet, but it's growing. And in places like Eilat, the light pollution is extreme—sixty times brighter than natural starlight. Other developed coastlines are heading in the same direction.
The study mentions corals are also affected by artificial light. So it's not just the fish being disrupted—it's the whole ecosystem?
Exactly. Corals are damaged, their reproduction is disrupted, the algae they depend on are affected. Now we know the fish living in those corals are also being harmed. When both the habitat and the animals living in it are under stress from the same source, the ripple effects compound.
What would actually fix this? Turning off all the lights?
Not all lights, but smarter ones. Reducing unnecessary illumination, directing light away from water, using wavelengths that disrupt marine life less, adopting technologies that can be dimmed or turned off. The researchers are being practical about it—they're not asking for darkness, just for less careless light.