On a daily and seasonal basis, light has consequences at multiple levels of biological organization, significantly influencing organisms' physiological and behavioural responses. The nocturnal lightscape in densely populated cities has changed significantly in the last few decades as a result of the widespread use of artificial light at night (ALAN). Because of technological advances such as electric lighting, access to light has shifted from a social privilege to a right. Excessive and misdirected artificial light that escapes its functional area becomes a pollutant.
Light pollution results in distinct spectral, spatial, and temporal light patterns that disrupt natural processes that rely on the light-dark cycle, negatively impacting biodiversity and human health. The global transition to energy-saving LED lighting has been accompanied by a rapid increase in light pollution caused by evening-time overexposure to blue light. Many LED lighting sources emit more blue light than earlier lighting technologies, rendering satellite sensors ineffective and causing emission levels to be underestimated. Light smog or light contamination has been estimated to increase up to 270% globally, and up to 400% in highly illuminated areas.
The material culture of light
Light is a relational material whose interactions with other materials allow us to see colour, texture, reflection, and shadows. Culture materializes light; it is the foundation of safe, accessible, and meaningful nightlife. Through its functional and aesthetic contribution to the streetscape, lighting can add value to the actual sense of place.
Like light, the night sky is a cultural heritage. Since time immemorial, it has been a symbol of enlightenment and hope. Until ALAN began to compete with starlight and disrupt our connection with the natural nocturnal environment, stars guided mobility, inspired the arts, and provided therapeutic comfort.
The hidden cost of cities that never sleep
Most of the Earth has only ever been exposed to celestial light at night for thousands of years. Half of the planet was always in complete darkness at any given moment. Cycles of day and night have influenced the development of life on Earth.
Our collective fascination with 24-hour cities has resulted in light trespassing into unintended areas, causing pervasive and long-term stress on ecosystems. Light pollution is rapidly increasing in its spatial coverage and intensity. Both point sources and skyglow, which is the accumulated effect of point source lighting that spreads through the atmosphere, have contributed to over-illumination. Typical ALAN emissions create skyglow over cities that can be seen from hundreds of kilometres away. The spectral content and intensity of ALAN have an impact on individual, community, and ecosystem responses.
In terms of the colour and brightness of street lights and hoardings, the non-homogeneous distribution of ALAN can be distressing to the eye and have an adverse effect on urban aesthetics. The amount of light in an area can influence mood; it can be uplifting and comforting or depressing and damaging to well-being. ALAN also disrupts homeostatic control mechanisms, such as the circadian clock, which regulates daily rhythms in physiological processes. The blue spectrum of light suppresses melatonin while encouraging activity. Fatigue, insomnia and depression have all been linked to ALAN. Circadian rhythm disruption caused by light pollution has also been linked to cancer in humans. Furthermore, different people have different physiological responses to nocturnal light.
Changes in circadian phases caused by light have resulted in shifts in the timing of rest and activity cycles in wildlife, as well as behavioural cycles that are independent of the circadian rhythm. Artificial light can cause night-time species that rely on stars to become disoriented. In species drawn to artificial light, it may crash and result in fatalities. Light can also act as a physical barrier for nocturnal species that steer clear of it. Predators have little chance of spotting prey on cloudy, dark nights. But, the predator-prey dynamic is changed by ALAN. It alters the trophic structure and composition of a community in a variety of ways. There is a lack of knowledge regarding the levels of ALAN that wildlife encounters, despite light pollution being a crucial topic in both research and policy formulation.
Cloudy nights were dark before the invention of artificial lighting, concealing the light from the stars. Soft point light sources provided light and warmth on the ground below. Clouds now reflect the city lights from below, brightening the sky on stormy nights. However, cities can appear frighteningly dark at ground level due to street light glare and light scattered off wet roads and pavement, creating a blinding effect. For safe urban navigation, it is important to take into account the fact that light intensity is not the only factor influencing vision.
The right to access the dark
Access to dark nightscapes in cities is as important for health as clean water, food, and air. Living beings must be able to sleep and rejuvenate, as well as navigate safely at night. ALAN has contributed to the economic and cultural vitality of urban areas by improving safety and aesthetics. To preserve the night economy and culture, however, the opportunities and experiences that exist in the absence of artificial light, as well as the values they provide, must not be lost. Currently, approximately 80% of the world's population is unable to view the Milky Way or undisturbed nocturnal wildlife.
Design for darkness in urban lightscapes
Light is a tricky material to design for use in the built environment because its intensity and colour vary throughout the day. Sensitive light topography can increase a sense of spaciousness and safety by transforming light into a resource for night-time living rather than an environmental stressor.
Light emitted directly from fixtures accounts for 75% of sky glow. A cobra-head streetlight's light emitted directly upwards accounts for over a quarter of the total light emitted. Light fixtures that are shielded prevent light from traveling into the sky and significantly reduce sky glow, while also saving energy by requiring fewer light sources for the same level of lighting.
The objective of responsible lighting is to maintain adequate ground illumination while maintaining a dark sky. Artificial light should be used only when absolutely necessary, at the lowest intensity possible, and as shielded as possible. Warmer, orange-based colours must be prioritized over the harsh white spectrum of light.
Lighting design can benefit from information on the actual and perceived brightness of the night sky. Along with the Bortle scale readings that measure location-specific night sky brightness, data from citizen science initiatives can be used to assess local perspectives on night sky brightness and the effects of ALAN on sleep and animal behaviour.
ALAN is not the villain who stole our starry night sky. In our homes and public spaces, we had forgotten to ‘design for darkness’. Intentionally unlit ‘dark infrastructure’ protects the habitats of nocturnal species. Dark bedrooms promote melatonin production and thus better sleep. Future policies should incorporate more sensitivity in urban lighting to address the world's urgent ecological and health challenges.
From consumptive to reflective nights
A transdisciplinary approach involving experts from ecology, urban design, architecture, human physiology, social psychology, and other fields is necessary to close the current knowledge gap in urban lighting research, practice, and policymaking. Concerted action is required to mitigate light pollution in order to avoid exacerbating the biodiversity and energy crises, and negatively impacting human health.
There is a pressing need to identify, assess, preserve and restore dark infrastructure to ensure that future generations can experience starry skies and a nocturnal culture that is more reflective than consumptive.
(Ann Rochyne Thomas is a bio-climatic spatial planner and founder of the Centre for Climate Resilience - a sustainability and climate change advisory.)