Artificial Light at Night, or ALAN, refers to the human-generated light that illuminates the nocturnal environment. This phenomenon originates from a wide array of sources, including streetlights, commercial and industrial facilities, residential buildings, and illuminated advertisements. As urbanisation intensifies globally, this form of pollution continues to spread, creating a luminous veil known as skyglow over inhabited regions. While often associated with safety and modernity, a substantial and growing body of scientific research confirms that ALAN is a potent environmental stressor with demonstrable, far-reaching ecological consequences.
What Constitutes ALAN?
ALAN encompasses a spectrum of light, ranging from the amber glow of older high-pressure sodium streetlamps to the intense, blue-rich light emitted by modern LEDs (Light Emitting Diodes). The key characteristics of this light – its intensity, duration, and spectral composition – are the primary determinants of its environmental impact. This artificial illumination disrupts the planet’s natural and ancient cycle of light and dark, a fundamental rhythm that has regulated the physiology and behaviour of organisms for millennia. The persistent presence of artificial light fundamentally alters an environment’s natural light cycle, which can be particularly damaging to nocturnal species that have evolved to thrive in darkness.
Widespread Ecological Effects
The ecological impacts of ALAN are profound and diverse, affecting individual organisms, entire populations, and broader ecosystem functions. Specific studies have pinpointed these effects across various biological taxa and processes:
1. Disruption of Wildlife Behaviour and Physiology:
Perhaps the most extensively documented impact is on wildlife. Artificial lighting is known to fatally lure sea turtle hatchlings away from the sea and disorient migratory birds, leading to exhaustion and increased vulnerability to predation or collisions. Early work by researchers such as Witherington and Bjorndal (1991) and Salmon et al. (1992, 1995) highlighted how ALAN disrupts sea-finding behaviour in sea turtle hatchlings. Deeper physiological impacts have also been recorded. For instance, in certain mammals, exposure to even low levels of ALAN at night is sufficient to suppress the production of melatonin, a key hormone that regulates circadian rhythms, sleep cycles, and reproductive timing. This phenomenon has been explored by researchers such as Stanton and Cowart (2024), who review the effects of ALAN on the circadian biology of marine animals, including melatonin suppression. Research has also shown that some mammals, such as deer, are less likely to use vital passages for movement if these areas are illuminated, effectively fragmenting their habitat and isolating populations. Studies evaluating road networks and wildlife-vehicle collisions, such as that by Shilling et al. (2021), indirectly touch upon factors that contribute to habitat fragmentation for deer.
2. Contribution to Insect Population Decline:
ALAN is recognised as a significant driver of the global decline in insect populations, a phenomenon with cascading effects throughout ecosystems. The fatal attraction of nocturnal insects to light sources is a well-known problem, resulting in direct mortality. Scientific investigations have provided stark evidence of its impact on pollination, revealing that in areas with artificial lighting, visits from nocturnal pollinators to plants can be reduced by over 60%. Researchers such as Hölker et al. (2010a, 2010b) and Kyba et al. (2017) have contributed significantly to understanding the broader impact of ALAN on insect populations and communities. Furthermore, ALAN can interfere with the mating signals of insects like fireflies, as the artificial brightness makes it harder for males to respond to female flashes, thereby impairing their reproductive success. Research by A.L. Swartz and others (2018) specifically demonstrated how short- and mid-wavelength artificial light influences the flash signals of Aquatica ficta fireflies.
3. Altered Plant Physiology:
Plants rely on precise natural light cues to regulate their growth, flowering, and seasonal cycles, a process known as photoperiodism. It is now confirmed that ALAN disrupts these processes. Street lighting, for example, can cause trees to bud earlier in spring and retain their leaves for longer in autumn. This desynchronises the trees from the natural seasons, making them more vulnerable to damage from early frosts or late thaws and altering their crucial interactions with insects and birds that depend on them for food, shelter, or breeding sites at specific times of the year. This desynchronisation can lead to trophic mismatches, where the availability of resources (e.g., insect larvae) does not align with the needs of dependent species (e.g., nesting birds). Reviews by researchers such as Grubisic et al. (2019) and Dominoni et al. (2020) have synthesised significant findings on how ALAN disrupts circadian rhythms and photoperiodic responses in plants.
4. The Unseen Impact: Soil Microbial Ecosystems
More recently, scientific inquiry has delved into an even more subtle impact: the effect of ALAN on the microscopic world within the soil, which forms the foundation of terrestrial ecosystems. This emerging field has produced critical new insights. Research has provided direct evidence that ALAN is a significant factor in altering the community structures of soil bacteria and fungi in urban areas. These microorganisms are essential for decomposition, nutrient cycling, and maintaining soil health. Crucially, nocturnal light has been found to significantly reduce the abundance of key functional genes involved in vital nutrient cycling processes, such as nitrogen fixation and carbon sequestration. This interference with fundamental soil processes can reduce soil fertility, negatively affect plant health, and potentially impact carbon dynamics, revealing that the consequences of light pollution extend deep into the foundational ecosystems beneath our feet. A key study in this area was conducted by Liang et al. (2023), who investigated how artificial light at night triggers negative impacts on nutrient cycling and plant health regulated by the soil microbiome in urban ecosystems.
Conclusion
The pervasive glow of artificial light at night, while often perceived as harmless or beneficial for human activities such as safety and navigation, represents a significant and escalating form of environmental pollution. Its far-reaching ecological consequences, from direct impacts on wildlife behaviour and physiology to subtle yet profound alterations in plant cycles and soil microbial communities, underscore the urgent need for a more considered approach to urban and rural illumination. The cumulative research by numerous scientists, including those highlighted above, continually deepens our understanding of these complex interactions.
Mitigating the adverse effects of ALAN requires a multifaceted strategy. This includes adopting dark-sky friendly lighting designs that minimise upward light emission and spectral frequencies harmful to wildlife, such as reducing the use of blue-rich light. Promoting the use of adaptive lighting systems that dim or switch off lights when not needed can significantly reduce light spill. Furthermore, increased public awareness and education are crucial to foster a collective understanding of light pollution’s ecological costs. Organizations like the International Dark-Sky Association (IDA) and researchers such as Dr. Franz Hölker and Dr. Thomas Davies are at the forefront of advocating for and researching solutions to light pollution. By implementing responsible lighting policies and practices, guided by ongoing scientific research, we can strive to restore the natural nocturnal environment, benefiting both biodiversity and human well-being, and ultimately ensuring the long-term health of our planet’s delicate ecosystems.
