Artificial light at night disrupts melatonin in Miami sharks

As the sun falls beneath the horizon, the ocean should become dark. For most of Earth’s past, night created a dependable cycle of illumination and darkness that influenced how marine animals ate, rested and travelled.

Yet genuine darkness is increasingly uncommon along modern coastlines.

Artificial light from urban areas, ports and seafront development now reaches well beyond the shore, making shallow coastal waters continuously illuminated environments.

Although this glow may seem ordinary – or even reassuring – to people, it is a major alteration for animals that evolved with natural day-and-night cycles.

New University of Miami research indicates that sharks, among the ocean’s most ancient predators, could be subtly affected by this change.

By examining how urban lighting changes a hormone responsible for regulating biological clocks, scientists are starting to reveal how contemporary coastlines are altering life below the water’s surface.

City lights invade oceans

Artificial light at night is produced by streetlights, buildings, ports and homes along the beachfront. It does not remain on land: in shallow coastal waters, light can spread considerable distances, changing formerly dark habitats into illuminated areas.

Night-time lighting can feel practical and secure for people. For marine creatures, however, it can discreetly disrupt natural bodily functions.

Researchers have already established that artificial light can interfere with sleep-related hormones in birds, insects and bony fish. Sharks, by contrast, had remained an unknown.

Measuring sharks’ night hormones

To investigate this concealed effect, scientists from the Shark Research and Conservation Program conducted an innovative study.

The research was carried out off Miami, Florida, one of the United States’ brightest coastal cities. Sharks in these urban waters were compared with sharks inhabiting nearby coastal locations with less light.

It was the first study in which scientists measured blood melatonin concentrations in wild sharks. That accomplishment itself provided a new insight into shark physiology and health.

Movement shapes light exposure

The study examined two shark species with distinctly different movement habits. Nurse sharks generally remain within one location for extended periods.

Their movement is restricted, while familiar resting sites can become long-term homes. Blacktip sharks, on the other hand, have a very different lifestyle. Their everyday behaviour involves frequent travel across broad coastal areas.

This difference enabled the researchers to test a key proposition. If artificial light influences melatonin, sharks staying in brightly lit places could experience greater effects than those moving between light and dark waters.

The role of melatonin

Melatonin is essential to the regulation of internal clocks. Across many animals, its levels increase at night before declining during daylight hours. This cycle helps to support rest, metabolism and general health.

For humans, disturbed melatonin is associated with sleep disorders and metabolic issues. Comparable disruption in wildlife can influence feeding, movement and survival.

Before this research, the role of melatonin in sharks had received little attention. Sharks first evolved more than 400 million years ago, well before artificial lighting was present. Their extensive evolutionary past makes any contemporary disturbance particularly worrying.

Resident sharks show disruption

The findings identified a distinct contrast between the two species. Nurse sharks exposed to strong artificial light had substantially lower night-time melatonin levels than nurse sharks in darker settings.

Blacktip sharks did not display any significant difference between light and dark locations.

“These findings suggest that exposure to artificial light at night can suppress melatonin levels in wild sharks, but vulnerability depends on behavior,” said Abigail Tinari, the lead author of the study.

“Species that are highly resident in light-polluted areas appear more susceptible than species that regularly move between illuminated and darker habitats.”

The result underlines the way lifestyle determines risk. Staying in the same bright place night after night raises both exposure and its effects.

Sharks react like humans

Sharks began inhabiting Earth’s oceans over 400 million years ago. Their potential response to disrupted light in a manner resembling humans demonstrates just how vital – and evolutionarily conserved – this biological process is.

The discovery is especially notable given that lengthy evolutionary history, as it illustrates the extent to which shared biological systems endure among highly different species.

“It also reinforces concerns doctors have about LED lighting, screens, and urban light pollution as contributors to illness and chronic disease,” said study co-author Danielle McDonald, a professor in the Department of Marine Biology and Ecology at the University of Miami.

“Furthermore, researchers might now look to other aspects of shark melatonin physiology to explore whether there are differences in their melatonin receptors that may inform new therapies or targets for drug development for melatonin-related disorders.”

This relationship between shark biology and human health gives the research an unexpected additional value.

Collecting samples at night

The study was conducted over approximately one year. At night, sharks were caught using short-duration research drumlines: floating lines fitted with baited hooks and deployed briefly to catch sharks safely while minimising stress.

Researchers took blood samples straight after capture to retain accurate hormone measurements. They also carefully recorded environmental variables including light intensity, water depth and temperature.

The night-time work used low-impact red lighting to prevent disruption to natural light perception. This method enabled scientists to make a direct connection between urban light exposure and physiological changes in sharks.

Effects ripple through ecosystems

“Sharks play a key role in maintaining balanced marine ecosystems, and physiological changes in top predators could have cascading effects throughout the food web,” added Neil Hammerschlag, the senior author of the study.

Since sharks occupy the upper levels of marine food chains, even minor biological shifts can extend throughout entire ocean ecosystems.

The results indicate that light pollution needs to be regarded as a serious environmental stressor, alongside better-recognised threats including habitat loss and chemical pollution.

Growing cities, brighter seas

The research establishes important baseline melatonin values for nurse sharks and blacktip sharks. Future studies can now monitor how expanding coastal development affects shark biology over time.

As urban areas grow and coastal defences become more brightly illuminated, it is increasingly urgent to understand the unseen cost of artificial light.

From the shore, the ocean at night may appear peaceful. But for sharks living beneath persistent illumination, darkness has become scarce, and its consequences are only starting to emerge.