Coastal City Lights Disrupt Sharks' "Sleep Hormone"

A recent study by the University of Miami reveals that artificial light at night (ALAN) significantly reduces melatonin levels in sharks, particularly in coastal urban areas. Nurse sharks exposed to bright lighting showed reduced melatonin, while mobile blacktip sharks were unaffected, highlighting species-specific responses to light pollution.

A recent investigation conducted by the University of Miami has found that artificial light at night (ALAN) substantially lowers melatonin concentrations in certain shark species. While the effects of nighttime lighting have been documented in terrestrial animals and other marine species, including sea turtles, this study represents the first examination of ALAN’s influence specifically on sharks.

Research Focus

The study targeted two shark species: nurse sharks and blacktip sharks. According to lead researcher Abigail M. Tinari, the primary objective was to establish baseline data on how shark melatonin levels respond to artificial lighting. This foundational information is intended to support subsequent research on ALAN effects in different marine regions.

Artificial Light and Wildlife

ALAN is known to disrupt wildlife behavior across multiple groups. Migratory birds can become disoriented during navigation, nocturnal predators may experience interference with hunting activities, and marine species often exhibit altered reproductive and foraging cues. For instance, hatchling sea turtles rely on dim horizon light to orient toward the ocean, with bright coastal illumination causing misdirection, inland movement, and higher mortality. Other species, such as Atlantic salmon and tropical damselfish, also show melatonin suppression when exposed to ALAN. Coral reproductive cycles can be disrupted by artificial light, resulting in premature spawning and decreased reproductive success, which negatively affects reef ecosystems.

Effects of ALAN on Sharks

The University of Miami study, conducted by Abigail M. Tinari, Danielle McDonald, Steven J. Cook, Austin J. Gallagher, and Neil Hammerschlag, demonstrated that ALAN can suppress melatonin levels in wild sharks. The results indicate that bright coastal lighting alters nighttime hormone levels, but the extent varies by species. Published in Science of the Total Environment on January 20, 2026, the study found that sharks inhabiting heavily illuminated urban coastlines, such as Miami, had reduced melatonin levels compared to those in darker coastal regions. Nurse sharks, which tend to remain in a single location for extended periods, exhibited clear melatonin reductions. Conversely, blacktip sharks, which are more mobile, showed no significant hormonal changes regardless of light exposure.

Methodology

The fieldwork, carried out by the Shark Research and Conservation Program at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, took place along the Miami coastline. Researchers compared melatonin levels in sharks from urban waters to those in less illuminated areas.

• Species examined: Nurse sharks (resident) and Blacktip sharks (mobile)
• Sampling: Short-duration drumlines deployed at night to reduce stress
• Equipment: 100-foot, 600-pound monofilament lines with circle hooks for safe handling
• Data collection: Blood samples analyzed for melatonin; water depth, temperature, and light intensity recorded
• Lighting precautions: Low-impact red lights used to prevent interference with shark vision

Fieldwork and data analysis were conducted over approximately one year. The study showed that nurse sharks in illuminated zones had significantly lower melatonin, while blacktip sharks were unaffected. These findings suggest that species with restricted movement are more susceptible to artificial light pollution.

Significance of the Findings

This research provides the first documented measurements of blood melatonin in wild sharks. Understanding how environmental stressors, such as coastal lighting, affect top predators is essential. Neil Hammerschlag, senior author, highlighted that changes in shark physiology could produce cascading impacts on marine ecosystems. Danielle McDonald noted that variations in melatonin receptors among shark species could inform future studies on therapeutic applications or drug development related to melatonin regulation.

Future Directions

The researchers hope that these results will stimulate further studies on how artificial light influences shark behavior, reproductive activity, and physiological processes in various coastal regions. Establishing baseline melatonin levels provides a framework for ongoing monitoring and comparisons among shark populations exposed to differing light conditions. By documenting the link between ALAN and melatonin suppression, the study emphasizes the importance of addressing light pollution as an emerging environmental stressor in marine ecosystems.

Sources:

• How Coastal City Lights Are Robbing Sharks of Their “Sleep Hormone”
• Urban light pollution alters nighttime hormones in sharks, study shows
• City Lights Are Messing With Sharks’ Internal Clocks
• How Coastal City Lights are Robbing Sharks of Their “Sleep Hormone”

Author: Maya Henderson
Maya Henderson is a journalist focused on technology and startups. She has experience in leading IT publications and excels at conducting interviews and creating visual content.