​[Note: the following information does not constitute legal advice.]

Nearly all of us have been there before. A neighbor installs a bright outdoor light fixture aimed squarely at our home. Oblivious to the effect, the light owner blasts light toward us all night long. We may have to put up window coverings to keep it out. We may lose literal sleep when it lights up our bedrooms. And we feel helpless to do anything about it.

The social anxiety around neighbor disputes is real. Some people are hesitant or even fearful to confront neighbors over such situations. Resentment simmers. Tempers are liable to flare up. Disputes can come out in the open, tainting relationships.

​This month we look into the phenomenon of "light trespass" as a matter of nuisance and what levers there are on the problem. That includes why municipal code enforcement often hits a dead end in such cases. But we'll soon see that not all hope is lost.

Most legal systems around the world consider what happens when one party does something that another simply finds annoying. It may not constitute an injury under the law. But it does constitute a situation demanding some kind of remedy.

In so-called "common law" jurisdictions, courts almost universally treat light invasion as a private nuisance. That's different than a proper 'trespass', which requires physical entry of something tangible. Light, noise and odor, however, are intangible in nature.

That said, they arguably interfere with the "use and enjoyment" of property. They consider this a 'nuisance'. This is a word that comes to us from a Latin root nocere ("to hurt"). It is anything that causes offense, annoyance, trouble or injury. It further distinguishes public ("common") and private varieties.

Situations causing one party an injury are known as torts. There are many doctrines around this ancient legal tradition, one of which deals with nuisance. Under this doctrine, a private nuisance claim requires proving that the action causes substantial or unreasonable interference. Courts often test these claims by what a "reasonable person" would think or do about a situation. One presumes the reasonable person (a kind of legal fiction) is not unusually sensitive.

​At the same time, courts have to make decisions in these cases. They have to balance the harm against the utility of whatever causes the nuisance. For light, that harm might range from sleep deprivation to the inability to use one's back yard or garden. The utility could be something like ensuring outdoor safety at night. So, for example, mitigating the nuisance of an unshielded floodlight might be simple. A shield would cut down on the glare, and the tendency toward trespass. But it wouldn't undercut the recognized utility of the lighting equipment.

Sometimes jurisdictions have explicit laws that regulate outdoor lighting. They may even state that among their purposes is reducing the likelihood of nuisance situations developing. But often they have weaker laws or none at all.

Even where local laws prohibit light trespass, they may not define enforceable standards. That puts law enforcement staff at a distinct disadvantage. Establishing violations of a law requires being able to clearly discern what the law allows versus what it does not allow. A prohibition without a standard results in a weak legal claim. Law enforcement is often hesitant to enforce laws vigorously in that case.

Even where enforcement is usually robust, there may be practical obstacles in the way. For example, law enforcement officers typically work daytime hours. They may also lack the kind of equipment needed to objectively establish violations.

One's local jurisdiction may have no law about light trespass at all. Or it may have a law that is unenforced (or unenforceable). But for those on the receiving end of unwanted outdoor light, there are other routes to achieving relief.

​So what should one do in a light trespass situation? We recommend the following, in this order:

The first action taken should always be direct. The majority of neighbor disputes result because of lack of, or a breakdown in, communication. Many of us no longer know our neighbors, like earlier generations did. We don't start from a position of familiarity or even trust. Instead, we view neighbors as strangers.

There are strategies for making initial contact without triggering defensive reactions. Don't assume your neighbors are simply ignorant. Rather, present an opportunity to learn more about the issue and how it affects you. Consider offering advice on hardware fixes that can mitigate the problem. If it's financially possible, offer to pay for inexpensive fixes like aftermarket shields for lighting equipment. Invite them to come to your property after night falls to see for themselves what their lighting looks like from your home.

To ensure more effective communication, consider writing down your thoughts before making the approach. Think about the words used and whether they invite a solution or stoke tensions. These notes can turn into scripts for a gentle initial phone call or written note.

It is often the case that light trespass results from a lack of mindfulness about lighting. Owners of lighting equipment often install it during the daytime. They may not come back to see the effect at night. They may underestimate how bright their lighting is as seen from adjacent properties. Many people are ultimately reasonable. When asked to do something, and provided with the right information, quite often they will comply.

​That said, not everyone is suitably agreeable about hearing complaints. If the neighborly approach doesn't work — or you're simply uncomfortable about it — move on.

Before taking administrative or legal steps, get your facts together. Document the trespass as best you can. This can include nighttime photos and/or videos. Bear in mind that the camera is not an unimpeachable witness! Use fixed settings for your images, not "auto" or "night" mode that may exaggerate the situation.

If you have access to basic measurement equipment like light meter apps or an illuminance ('lux') meter, take some readings. These should vary in position from the property line to interior surfaces where light is observed.

Make a simple log of your observations. Note dates, hours of illumination, and any personal impacts. These can include sleep disruption and headaches or other physical symptoms. Note the duration of the trespass, especially if it can be traced to installation or modification of lighting equipment.

​Once you have assembled your case, try an escalating series of remedies.

Look at simple fixes short of formal legal action. Is your home part of a property owners' association? These organizations sometimes put broad "architectural review" or "nuisance" clauses into covenants and other legal documents. These are often much easier to enforce privately than city codes.

Some cities and counties offer free or low-cost, neutral dispute resolution programs. These "community mediation services" are often designed specifically for resolving neighbor disputes. While it sometimes works, mediation requires the voluntary participation of both parties in a dispute.

A so-called "demand letter" from a local real estate or land-use attorney to the offending property owner can sometimes bring relief. That is especially true if the letter details private nuisance liability and property devaluation.

Lastly, consider filing a formal health/nuisance complaint with the local public health authority or city manager. This can be more effective than submitting a standard law enforcement complaint. (Note that public health departments vary widely by jurisdiction in whether they accept light trespass complaints. Readers should check local administrative authority scope before filing.)

​If none of these angles yields success, it may be time to make a formal legal issue of your complaint.

Court proceedings can be lengthy, protracted and expensive. While they can bring enforceable relief to light trespass situations, they often fail due to various legal weaknesses. Before launching a legal effort, it is important to understand all the potential consequences. It is also essential to seek professional legal counsel.

First, consider carefully whether your case is truly an example of nuisance. Would a "reasonable person" conclude that the light substantially interferes with the "quiet enjoyment" of your home? Is the offending light out of character with lighting use in your neighborhood? Are you sure that the problem with your neighbor isn't deeper and possibly unrelated to lighting altogether? These are all considerations that courts weigh in adjudicating nuisance claims.

It is also helpful to note any legal precedents in your jurisdiction. Have local courts granted relief in similar cases? If so, what is the typical burden of proof? While an inability to find such precedents isn't fatal to one's case, it does not build a strong, supportive base.

​Bear in mind, too, the value of a formal demand letter from an attorney (Step 3). It serves as a powerful bridge between informal discussions and actual court filings. It can also serve as evidence the recipient of the light trespass made a good faith effort to resolve the dispute outside of court.

Residents suffering light trespass aren't completely powerless. That applies as much in communities that have outdoor lighting laws as in those that don't. Successful mitigation of trespass can demand concerted and sustained effort. And it remains true that parties in many such situations settle matters without taking any legal action.

The steps above establish a workflow: Observe → Document → Collaborate → Mediate → Escalate. One could add an extra step, early on: (self-)Educate. That is to say, educate yourself about light trespass, nuisance, your community's laws, and relevant past legal precedents. If your light-trespassing neighbor is amenable, educate them, too. Show them that light trespass isn't an inevitable consequence of their desire for security, safety, or amenity at night.

Sometimes neighbors are happy to comply if given the knowledge of how to comply. A great place to start is by showing them DarkSky International's "Five Principles for Responsible Outdoor Lighting". These are simple, functional, cost-effective ideas. Properly implemented, they can defeat light trespass.

​The neighbor's entryway light coming on each night at dusk can be a source of dread. But following the old adage that knowledge is power, learning one's rights (and responsibilities) can be a source of inspiration. Moving from despair to enlightenment can be the first step on the road to reclaiming the night.

During the spring and fall bird migration seasons, volunteers with the Fatal Light Awareness Program (FLAP) fan out across Canadian cities. They collect the remains of birds that have died in collisions with buildings. For each bird found, they meticulously record information about the circumstances of its death. The birds are preserved for long-term study. Then, once a year, the volunteers meticulously lay out the bodies in public view. The Annual Bird Layout is FLAP's main awareness-raising event.

The results are sobering. Hundreds or even thousands of birds will die during one night in a single city. Bright urban lighting draws them away from their migratory routes. Some see light reflected in building windows and fly into what they think is clear air. Others circle until they drop dead from exhaustion. FLAP aims to document them all.

The space above and near cities is an invisible ecological superhighway. Billions of animals pass through this space each night. The Moon guides some of them, the stars others. City lights create a virtual deathtrap for these species. And it piles on other environmental stressors like climate change and habitat loss.

​One solution is to dim lights in cities during these mass migrations. According to this idea, dimmer cities draw fewer birds and lead to decreased carnage. Does the theory hold up in the face of evidence? Do seasonal light-dimming programs measurably save birds, or are they a feel-good form of "slacktivism"? This month we dig into the question and check the evidence.

Migrating species like birds make connections between distant biomes. They move biomass, disperse plant seeds, and enable reproductive success. They also provide what ecologists refer to as "ecosystem services". These are the positive benefits that wildlife or ecosystems provide to people. They also connect is intimately to the natural world. Examples of ecosystem services provided by birds include agricultural pest control, maintaining natural plant diversity, and transporting essential organic nutrients. Humans can't do these things with any efficiency.

Artificial light at night (ALAN) interferes with this process. We previously wrote about how this adds to the sense of natural nighttime darkness as an object of conservation. The idea is that the value of these objects can elevate the level of attention society adds to them.

For example, much has been made of the notion of an ongoing, global "insect apocalypse". Declining pollinating insect populations could wreak havoc on food production worldwide. ALAN draws pollinating insects away from their target plants. Humans can't effectively replace their natural pollinating actions. In this way, ALAN becomes an issue of food security and even Indigenous sovereignty.

Light pollution is a primary predictor of where migrating birds land to rest. It unintentionally draws them into high-risk urban environments. There they find secondary threats like predators, vehicle strikes, and collisions with window glazing.

​One study identified skyglow as a top predictor of bird migration stopover density in over 70% of seasonal models. Another found that skyglow increased stopover density at regional scales near illuminated urban corridors. It creates ecological "traps" by altering the natural habitat selection of birds.

​Reducing ALAN to lower its impacts on species sounds promising. Research using weather radar finds that ALAN actively lures migratory songbirds into dense urban centers. Turning off ground lighting breaks the disorienting "light trap". This allows birds to disperse within minutes and resume safe flight paths. Radar tracking during New York City's annual Tribute in Light demonstrates this best. Turning off high-intensity lights for 20 minutes allows thousands of trapped, circling songbirds to fly away.  

Scientific data confirms that "Lights Out" initiatives yield immediate, quantifiable ecological benefits. They can also serve as gateways to essential municipal policy reforms.

Consider the results of even one such study. Analysis of 40 years of collision data at Chicago's McCormick Place convention center found that extinguishing window lighting reduced fatal bird collisions by about 60%. Halving the illuminated window area among buildings decreased collision counts by nearly 11 times. It didn't require any costly changes to buildings. It only asked building owners to turn out unnecessary lights.

​The data confirm that "Lights Out" programs are not mere "slacktivism". Turning off lights yields immediate, quantifiable reductions in avian mortality alongside energy savings. Although not a silver-bullet solution, campaigns can become an entry point for comprehensive urban conservation legislation.

But there are also some limitations to these efforts. The American Bird Conservancy points out that while dimming lights prevents night disorientation, it does not stop daytime glass collisions. Once light at night draws birds into a city, reflective glass remains the main threat the next morning regardless of night lighting. Policy advocates complain that voluntary programs suffer from inconsistent compliance. And building managers and municipal authorities sometimes resist the programs on public safety grounds.

Yet campaigners can push back on these objections. They can address the concerns by promoting the use of warmer-colored lights and adaptive lighting controls rather than complete blackouts. Reframing the issue away from "either/or" dichotomies respects the interests of diverse stakeholders.

​Critics say that "flipping a switch" is a passive solution. But it doesn't have to be. There are low-friction corporate and civic entry points for all "Lights Out" programs. Over time, these can build momentum for binding legislation and bird-friendly building codes. 

The available science suggests that "Lights Out" programs work as advertised. Bird mortality declines without any obvious, negative effect on business operations or public safety. They can be paired with bird-friendly glass treatments for a more holistic approach, both day and night. Better nighttime lighting practices that align with the Five Principles for Responsible Outdoor Lighting can further reduce harm to birds.

Over time, these changes can be codified into law. Local lighting ordinances can enforce curfews, whether during migration season only or year-round. Bird-friendly lighting practices can be made part of local building codes. If carefully managed to avoid the phenomenon of "greenwashing", these programs can fit within broader ESG (Environmental, Social, and Governance) and sustainability frameworks. What's good for birds can be good for business, too.

​By preventing birds from becoming trapped in urban sinks, "Lights Out" initiatives preserve the healthy bird populations necessary to sustain essential ecosystem services. While not a panacea, they can be an important part of bird conservation programs. For now, campaigners are focusing on obtaining more voluntary cooperation as the pathway of least resistance. But as we learn more scientifically about the issue, the cause for making these actions obligatory may well only grow stronger.

Solid-state lighting (SSL) began replacing legacy streetlights across the globe almost two decades ago. Light pollution researchers and dark-sky advocates viewed the transition with a mix of optimism and anxiety. On paper, light-emitting diodes (LEDs) offered incredible energy efficiency, long lifespans, and precise optical control. But in practice, early retrofits often backfired. "Cheap lumens" led to over-lighting, an example of the classic "rebound effect" in economics. Blue-rich white LEDs flooded urban night skies with light that scatters strongly in the atmosphere. That intensified skyglow and disturbed biological rhythms.

Yet the narrative that LED retrofits inevitably harm the night sky is incomplete. Evidence emerged in recent years that careful municipal lighting design can slow the growth of skyglow. In some cases it can even reverse it, making the night sky marginally darker. Such outcomes demand thinking differently about how we light our cities. Such large-scale changes must be considered holistically rather than one lamp at a time.
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Three world cities — Tucson, USA; Vienna, Austria; and Metz, France — tried a different approach. Each faced a different set of starting conditions. Each implemented a different combination of levers on the problem. Together, their experiences offer a comprehensive playbook for dark-sky friendly urban lighting.

Tucson's municipal LED lighting conversion (2016-2017) was the first of these efforts. It provides an important benchmark for what happens when a city starts with a well-shielded lighting system. Tucson's pre-retrofit lighting system consisted of fully shielded high-pressure sodium (HPS) luminaires. These emit no direct light above the horizontal plane. In industry terms, they had an Upward Light Output Ratio, or "ULOR," of zero. Although the luminaires we well shielded, they were too bright. By international standards, the system emitted much more light than was necessary. The primary problem was not (wasted) direct uplight, but sheer volume.

When the City of Tucson began planning the retrofit of its nearly 20,000 municipal street lights, it made a bold promise. The retrofit would not imperil astronomy and space science, which contribute over half a billion dollars a year to the local economy. To ensure this, total light emissions had to decrease in the transition to modern LED luminaires.

​The new 3000K white LED luminaires were less bright than the HPS sources they replaced. The new lighting system reduced light output by almost 63% while keeping illumination above minimum safe levels. The City further reduced emissions by implementing an overnight dimming program. At midnight most streetlights dim from 90% to 60% of their full power. This not only reduces skyglow but lowers energy costs and improves the field lifetime of the lighting products.

Our 2018 study of Tucson’s municipal LED conversion confirmed the strategy's success. Satellite observations of upward-directed light radiating from Tucson showed a 7% reduction. Ground-based measurement revealed even steeper decreases. At remote astronomical observatory sites, night sky brightness decreased by as much as 20%.
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The Takeaway: When starting with well-shielded luminaires, further reductions in skyglow come from lumen reductions. Cities must turn down the overall "volume knob" by capping total light emissions and implementing curfew dimming.

While Tucson’s challenge was total lumen volume, Vienna faced a classic optical shielding challenge. The city's municipal lighting department manages approximately 133,000 public streetlights. Before the current retrofit program, legacy luminaires allowed considerable upward light "spill" into the night sky. Vienna began modernizing its municipal lighting stock in the mid-2010s. The program replaced old fixtures with fully shielded (ULOR=0) luminaires. By late 2024, Vienna updated over 108,000 fixtures — roughly 80% of the network.
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Researchers established the “Licht über Wien” monitoring program to check the real-world impact of this conversion. The team deployed a network of high-precision LightMeter sensors at three key locations. They represented central urban (Wien Zentrum at Stubenring), suburban (Kuffner Observatory) conditions. A third site served as a dark-sky baseline reference (Großmugl, ~33 km away).

​They isolated the specific contribution of streetlights by analyzing sudden step-changes (Lichtstufen) in sky brightness. These occur each night during scheduled dimming events (Teilnachtschaltungen). In 2015, this dimming occurred at 11 P.M. local time. Sensors then recorded a distinct ~5% drop in horizontal irradiance in the city center. At the observatory, the drop was almost 10%.

By 2024, Vienna achieved the conversion of 80% of its street lights. Measurements showed that skyglow was then one-quarter of the 2015 baseline level. The study projects that once the city completes the retrofit project, it will fall below the 1% detection threshold of the instruments.

At first glance, a streetlight contribution below 1% might seem counterintuitive for a major European city. Wouldn't light reflecting off streets and buildings still create substantial skyglow? The answer depends on what the meters actually measure: the hourly modulation signal. By adopting a zero ULOR standard, Vienna mitigated the component of light emissions escaping directly into the night sky. The remaining skyglow attributable to the street lights comes from unavoidable reflections from the ground and building facades. Such reflections are very diffuse and uniform. The sudden step change during late-night dimming disappears into the noisy 'background' from light sources other than street light.
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The Takeaway: Eliminating direct upward waste light delivers enormous skyglow dividends. For cities with legacy unshielded fixtures, shielding is by far the single most effective intervention.

Tucson demonstrates the power of lumen reduction. Vienna proves the necessity of strict optical shielding. The French city of Metz illustrates how to combine both tactics. But it goes a step further with innovative spectral and smart lighting management.
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According to the Metz 2023/2024 Sustainable Development Report, the city is executing a €15 million public lighting transformation across its 16,500 municipal luminaires. LED adoption surged from 21% in 2022 to 60% by late 2024, replacing 2,500 to 3,000 fixtures per year. A top priority was the aggressive removal of unshielded, 360-degree globe lights (luminaires boules). The city replaced them with flat-glass downward directional LED units.

Metz’s results are striking. Total municipal lighting energy use dropped by 25.3% in a single year. Synchronizing late-night light reductions with the operating schedules of the local public transit network further saves electricity and reduces carbon emissions. Rather than over-lighting roads, new luminaires use precise optical beam shaping. These designs meet minimum illuminance standards without going overboard by emitting too much light.

But where Metz truly excels is in its commitment to ecological lighting design. That is particularly important in its sensitive urban ecosystem. Along the banks of the Moselle River (Île du Saulcy), Metz installed motion-activated streetlights. These products have an ultra-warm 1000 K (amber/red) light appearance. Removing the blue-component of white LED almost completely reduces the atmospheric scattering causing skyglow. It also protects sensitive nocturnal wildlife in the river and on land.

Metz experimented further. In its La Grange-aux-Bois neighborhood, the city deployed smart radar sensors. These sensors can distinguish pedestrians and cyclists from motor vehicles. The system uses this information to dynamically increase light levels when it detects human activity. When activity ceases, the lights extinguish. But it didn't stop there. It paired the infrastructure upgrades with community accountability. Volunteer teams known as Sentinelles de la Nuit ("Night Sentinels") audit Metz's commercial corridors after 1am each night. They assess illuminated storefront display compliance with France's national light pollution regulations.

The Takeaway: Modern retrofits can extend beyond standard white LED installations. Integrating ultra-warm lighting ecologically sensitive zones protects local wildlife. Using dynamic sensor controls allows cities to significantly reduce both energy consumption and nighttime light emissions. 

Comparing these three cities reveals that successful public lighting modernizations rely on balancing three core variables. These are optical shielding, lumen caps, and spectral/dynamic controls.
From these empirical case studies, three key rules emerge for municipal decision-makers:

1. Zero upward light (ULOR = 0) is essential. Direct upward emissions scatter far more efficiently than reflected light from the ground. Vienna proved that eliminating direct uplight yields immediate, measurable reductions in urban skyglow.
2. Efficiency gains must not fuel over-lighting. Converting to LED without reducing total lumens or establishing curfews risks worsening skyglow. Tucson demonstrated that cutting total lumen output by over 60% maintained safe, effective lighting while lowering skyglow.
3. Spectrum and controls are location-sensitive. Blanket lighting policies across an entire city are obsolete. As Metz showed, deploying ultra-warm 1000 K amber/red LEDs in sensitive places mitigates ecological harm. And it did so without compromising public safety in transit hubs.

For decades, dark-sky advocates and municipal lighting managers were often viewed as opposing sides. One side advocated for nighttime preservation and the other for urban safety. Decision makers came to believe that a city could have one or the other, but not both. The experiences of Tucson, Vienna, and Metz prove that this binary choice is false.
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These cities employed tailored combinations of shielding, lumen caps, dimming and spectral management. They also reduced light pollution and energy consumption. Yet they also tended to the needs of their residents. The outcome is win-win all the way around. As more cities prepare to convert or upgrade their street lighting infrastructure, these case studies stand as clear proof that smart design can restore the night sky without leaving our streets in the dark.

Light pollution is now pervasive in our world. Experts and governments more often recognize it as a form of environmental pollution. The tide of public awareness continues to rise. Both light pollution and dark skies are topics of media stories, television shows and podcast episodes. One finds in them clear messages: artificial light at night (ALAN) threatens the natural world.

Yet stories and presentations tend to consider the matter at only a shallow depth. Detailed 'deep dives' are few. It can be difficult to understand the topic very well by only consuming those media. At the same time, access to accurate and reliable information is more important than ever. And both decision makers and the public need that information now more than ever.

This month we look at the latest global view of light pollution and ALAN science. We highlight some key research findings published in 2025 and consider the most important unanswered questions. 

In 2022 DarkSky International released the first report called "Artificial Light at Night: State of the Science". It aimed to present the best of what we know about light pollution in a way that the public can understand. It surveyed a landscape of almost 4,000 published papers and studies in the ALAN Research Literature Database.

The text distilled decades of scientific research into a few pages of understandable language. Hundreds of source citations supported its conclusions. The organization published the report under a Creative Commons license to enable its re-use.

Each year DarkSky International updates the report with the latest results from scientific studies of ALAN. It culls through over 500 new papers published on average each year. In these sources it looks for the common threads tying new results to a growing body of knowledge. The search sometimes turns up unexpected results worth highlighting. It also updates existing knowledge with new source citations.
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The annual update ensures that DarkSky International and its dark-sky advocates stay current on new scientific developments. The report is a valuable resource for the advocates in communicating with both the public and policy makers around the world. It also adds credibility to their advocacy.

The report for 2026, released last month, finds light pollution a bigger social and environmental concern than ever. Studies published in 2025 both build on previous knowledge and reveal some surprises. Here we highlight some notable examples.

Our understanding how ALAN lights up the night sky is now very sophisticated. Miro Kocifaj (Slovak Academy of Sciences and Comenius University, Slovakia) and co-workers published the first model of night sky brightness that accounts for various cloud types and coverage. Their model "can be used to explain observational data collected by light pollution monitoring networks, particularly at sites where the combination of amplifying and darkening effects of clouds generates ambiguous brightness outcomes."

Light pollution has distinct synergies with other pollutants. Yongbin Wang (Xinxiang Medical University, China) led a team that explored links between ALAN and air pollution. They found that the combined effects elevated the risks of high blood pressure, heart and liver disease. They conclude that "policy interventions targeting light pollution reduction and air quality improvement are urgently needed to mitigate environmental health risks."

ALAN is causing ecological changes with profound effects, especially in cities. Lvlv Wang (Wuhan University, China) and coauthors examined the competing effects of light pollution and urban heating on plants. Plants exposed to ALAN experienced a longer growing season compared to those affected only by warm nighttime temperatures. They argued that "ALAN is a critical driver of vegetation dynamics in cities, one we should consider during urban management and development."

ALAN may affect entire ecosystems. It was long believed that light pollution had local-to-regional effects on segments of natural systems. But a Nature Climate Change paper by Alice Johnston, Jiyoung Kim and Jim Harris (Cranfield University, UK) found something very different. ALAN seems to influence the cycle that governs the exchange of carbon between the living and non-biological worlds. This has serious implications for a planet that continues to rapidly warm.

Public acceptance of outdoor lighting policies finds its roots in perceptions of darkness. Two papers provided new social science data assessing the public's attitudes toward nighttime darkness. Richard Jedon (Eindhoven University of Technology, Netherlands) and colleagues showed that increased anxiety experienced by pedestrians in dark areas of cities "made participants less willing to allow lower levels of streetlighting".

Meanwhile, Solène Guenat and Nicole Bauer (Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Switzerland) demonstrated that study participants "who lived or grew up in high-sky brightness regions were more likely to feel unsafe". They also have a more favorable view of ALAN as having social benefits, decreasing its recognition as a pollutant.

Space light pollution continues to rise. As the orbital space around the Earth fills with tens of thousands of new satellites, researchers expect the night sky to get brighter. Rockets and satellites shedding debris in space are especially concerning. In a study of low-Earth orbit space, Miro Kocifaj , František Kundracik and Stefan Wallner reported projections "an increase in night sky background brightness of 5 to 11 per cent above natural levels" by 2035. In some parts of the world, this increase could exceed the amount of ground-based light pollution.

"Artificial Light at Night: State of the Science 2026" closes with a summary of the most pressing research questions of our time. New additions this year include:

• Which lighting policy interventions are most effective in reducing skyglow? Are lighting policies effective? If not, why?
• Are organisms evolving responses to light pollution?
• Can any reliable ‘dose-response’ relationship be defined between outdoor ALAN and safety and/or security?
• How are some measures of ALAN such as skyglow specifically related to a suite of undesired outcomes (e.g., adverse ecological, health, or astronomical outcomes)?
• Is it possible to define an overall efficiency metric for outdoor lighting that considers both its energy efficiency and unintended, negative environmental/social consequences?
• How will novel uses of outer space, such as the deployment of orbital sunlight reflectors, change nighttime conditions on Earth?

The expanding body of artificial light at night (ALAN) research published in 2025 represents a turning point. Our understanding of the profound ecological, physiological, and cultural impacts of light pollution is deepening. We recognize that the gap between scientific consensus and public policy must close.

The task of science now is to address the remaining unanswered questions. Its findings can help translate data into actionable local, national and international protections. In turn, we can begin to reverse the tide of global skyglow and safeguard the nocturnal commons for generations to come.

For as much as we know about light pollution, there is still much we don't. Scientists all over the world struggle to understand the details. Although we frame our work as an effort to preserve "dark skies", there is much more to it than that. On the surface it seems like a matter of whether we can see the stars at night. Below that surface, it's about managing a massive public resource that affects everyone.

Every other year, researchers gather to discuss the latest results and find ways forward. These include the social and public policy dimensions of our work. The recent Light Pollution: Theory, Modelling and Measurement (LPTMM) conference attracted astronomers, lighting designers, ecologists, public officials and others. This year's edition of LPTMM focused on standardizing how we measure light pollution. It's clear that lacking a common vocabulary is holding back scientific discovery.

​This month we look at the case for standardization in light pollution research and why it matters. It's much more than a dry, pedantic matter of academic bookkeeping. Rather, it's a vivid and logical problem with many practical effects. And it may be the missing key to making our neighborhoods safer and lowering our taxes in the process.

As a form of environmental science, light pollution research has immediate applications. Artificial light at night (ALAN) harms our world in many ways. It is a known hazard to almost all living things including humans. It brightens the night sky, hampering our views of the cosmos. It has some kind of interaction with crime and public safety. Most importantly, it represents a waste of energy and of money.

We didn't start out with that in mind. Early lighting technologies were very inefficient. As our understanding of human vision improved, so did our lighting. But the last quarter-century has seen a revolution in how we light the world at night. Light-emitting diodes (LEDs) have made outdoor lighting cheap to own and operate. As a consequence, there is now much more outdoor lighting than ever before. That has sent light pollution skyrocketing worldwide.

​Many people are afraid of the dark. Various influences conditioned them to believe that a brighter space is always a safer space. The availability of cheap and highly energy-efficient outdoor lighting caters to that perception. But it also validates a fear that people feel viscerally even if it isn't always rational. Any attempt to change the trajectory of light pollution must confront this reality. 

Bright lighting creates harsh shadows where threats can hide. Its glare can be blinding to motorists, pedestrians, bicyclists and others. While intended to make the night more like the day, it can also leave people feeling exposed in outdoor spaces. But smart, well-designed lighting often yields a different response. It can help people feel secure and reassured when outdoors at night — even empowered. And it can achieve that by simply reducing or eliminating waste.

Outdoor light at night is a kind of shared public resource like roads or water. Poor-quality outdoor lighting is a colossal waste of energy and money. It is light paid for by taxpayers that often shines into bedroom windows or escapes into outer space. The goal of understanding light pollution is to design better lighting installations.

​Done well, this protects the fiscal bottom lines of towns and cities. It's also of interest to the world of private enterprise. There are reasons to believe that light pollution influences climate change. As "sustainability" is top of mind for many people nowadays, this matters more than ever. Demonstrating the true sustainability of good lighting design can be a powerful motive for change.

When researchers across disciplines can't understand each other's work, we lose real opportunities. The light-pollution research community experiences this now in an immediate way. To illustrate this, consider for whom we characterize the nighttime environment.

Light pollution research began decades ago in the astronomy community. As the most impacted "early adopters", astronomers considered skyglow a real threat to their profession. Long before there were space telescopes, astronomers built observatories far from cities. Clear, dry air on mountaintop sites was already good for their observations. Moving further from cities isolated their telescopes from interfering city lights.

The human perception of the night sky became centered in astronomers' measurement systems. They used tools tuned to the physiology of the human eye. Their goal was to characterize the night according to how people see it. That was a natural consequence in a science that began with the human eye as its only detector of light.

Of course, that happened long before we began to understand how ALAN affects other organisms. Our human experience of light at night can be very different than that of other animals. A migrating bird, a sea turtle, or a nocturnal insect doesn't care about human vision metrics. They can sense specific colors of light, like blue or ultraviolet, that humans may not register at all.

Lighting scientists and engineers largely adopted the astronomers' human-centric approach. They devised measurement quantities and units tied to the human visual response. Again, this makes sense if the point is to light the world to cater to human needs. But we're not the only ones who inhabit outdoor spaces at night. Biologists studying light pollution found themselves awash in the wrong measurement tools. To this day, they tell other researchers to stop measuring light like (and for) humans.

Astronomers publish their skyglow data using "human-eye units". Biologists can't use that data to figure out if, say, a local ecosystem is in danger due to ALAN. Meanwhile, skyglow researchers don't understand the metrics ecologists throw back at them. This breakdown stalls scientific progress. Biologists can't explain a species' needs to lighting engineers in accessible language. Engineers can't design better street lights that keep neighborhoods safe while protecting wildlife. Everyone becomes stuck in a never-ending loop of guessing. 

In everyday speech, "standard" means "basic" or "normal". But in science and technology, a standard is a kind of superpower. It is an official, universal prescription that everyone agrees to follow. And they don't happen by accident or simple acclamation. Instead, bodies like the International Organization for Standardization (ISO) bring together global experts to write the rules. Then, the public gets a chance to review them before they become official.

Standardizing measurements has a particular goal in mind: creating accountability. If a city or a scientist says "we are following the ISO standard," everyone on Earth knows exactly what that means.
As a kind of a rulebook, technical standards are only useful if people actually use them. Standards that don't get enough buy-in from end users risk being disregarded or even ignored. At the same time, no standard is agreeable to everyone. By design they are re-evaluated every few years. If users see a need for change, a process follows to revise the standard. In each case, the first version is just a jumping-off point.

To do this, technical standards rely on the International System of Units (SI), a kind of modern metric system. The SI is the ultimate global dictionary for measurement. It uses 7 base units and 22 derived units to measure everything in the universe. There's no need to invent strange new units to fix the light pollution language barrier. We need only take care to root our light measurements in a system the global scientific community already trusts.

In turn, standardizing measurements can improve public confidence in scientists and their work. High-profile disagreements among scientists can lead to belief the scientific method is failing. Yet disagreement isn't a sign of weakness. It’s how we test, break, and improve our ideas. In fact, it's science's greatest strength.

​But there's a big difference between arguing over theories and arguing because your rulers are different. Current confusion in light pollution science happens because researchers use different measurement tools. Standardizing our methods and reporting units helps clear up these unnecessary misunderstandings. When scientists speak the same language, it eliminates false disagreements. It also gives the public well-deserved confidence in the safety and resource advice scientists provide.

Standardizing measurements isn’t only neat bookkeeping for scientists in lab coats. It has practical, real-world effects that everyone should care about. If scientists standardize their language, they can give unassailable data to city planners. Decision-makers can in turn write better outdoor lighting policies. Those policies, if implemented well, can save money, reduce pollution, and improve public safety. It's much harder to reach these goals if scientists keep talking past each other.

How do we actually fix this scientific Tower of Babel? We can't expect everyone to wake up tomorrow and agree on a single language. So we need a plan.

An idea that emerges is a simple, two-part strategy. First, we propose forming specialized expert working groups across different sciences. For roughly a year, these teams will draft the basic ground rules for measuring light in their specific fields. Next, everyone will gather at an international pre-standardization congress. Think of this as a global peace summit for science. There, researchers can debate these ideas and hammer out a shared compromise. The result would be ready for submission to international bodies like the ISO to become an official, global standard. It’s a transparent, human process designed to get everyone speaking the same language.

​Ensuring an inclusive process, one that is both flexible and scalable, is crucial. If a durable standard results, the real work begins. We must then lobby for acceptance of the outcome in the research community. We must also socialize the results to encourage uptake and practical use of the standard. We'll know if we're successful by every scientific paper and government report showing adherence.
Achieving this result will take years, and it starts with admitting we need to try something different. The road to a standard for light pollution measurements leads toward a direction in which nights are better, safer, and maybe just a little darker.

Images of the Earth at night from space are an important tool to understand light pollution. They allow us to see the "big picture" across our planet at night and how it is changing. But satellite images aren't perfect by any means. Last year we wrote here about the limits of what those images can tell us. We also sometimes struggle to understand what the images mean in relation to the brightness of the night sky. And their designs and capabilities are far from ideal.

Still, satellite measurements are indispensable method in the researcher's kit. In the past decade they told a compelling story. In the mid-2010s, nighttime light emissions seemed to rise worldwide at about 2% per year. There were reasons to think this was a very low estimate. In 2023 an analysis of visual observations of the night sky showed the rate must be much higher.

​According to new research, the picture is even more complicated than we once believed. This month we dive into a recent paper led by Tian Li (University of Connecticut) that gives an unprecedented look behind the curtain. What the authors found was not entirely surprising.

The workhorse instrument for measuring "nighttime lights" is the Visible Infrared Imaging Radiometer Suite, or VIIRS. One of its cameras, called the Day-Night Band (DNB), has a sensitivity that allows it to measure very faint light. As its name suggests, it works well on both the day and night sides of the Earth. At night, it sees the glow of our cities as well as the light of natural phenomena like wildfires and the aurorae. Scientists used information about the artificial lights to learn about patterns of human activity.

The DNB has some limitations. It can only resolve areas on the ground about the size of a city block, nothing smaller. It flies over every part of the Earth at least once a night, but only at certain times. Most important, it does not see light in the blue-green part of the spectrum well (or at all). As modern outdoor lighting emits a lot in these colors, this sense of 'blindness' is significant. In particular, the DNB cannot see much of the light emitted by white light-emitting diode (LED) sources.

Previous studies published in 2016 and 2017 offered some insight despite these flaws. Artificial light at night (ALAN) is widespread across the world. In many respects, it has completely transformed the nighttime environment in some places. And it is growing in brightness and intensity faster than the growth of the human population. Still, until now our view was fragmented in time and space. And an analysis of a longer data series offers fresh insight.

The new paper by Li and coworkers considers the changes of every pixel in every DNB image from 2014-2022. Previous studies dealt only with long-term (monthly or yearly) averages to filter out influences like clouds. Irregular sampling of light data like this tends to "smooth over" short-duration changes. It also makes it difficult to sense whether changes are happening slowly or quickly. Earlier papers thus focused only on long-term changes, almost all appearing as increases.

Li's innovation is a new way of filtering out clouds and other contamination from nightly DNB images. Also, they accounted for the changing angle between the spacecraft and light sources on the ground. This better handles situations like light emitted 'sideways' from illuminated windows of buildings. And thanks to the orbital properties of the spacecraft, the DNB data include virtually all inhabited parts of the world.

What the researchers found is interesting, if not very surprising. Global light emissions increased 34% during the study period. But that surge masks large areas of dimming, which offsets about half of the brightening. The net result? Artificial lighting at night increased worldwide by about 2 percent each year. That is exactly the same rate measured with the same equipment during 2012-2015.

As a global average number, the result doesn't quite capture how dynamic nighttime lights are. According to the Guardian, "night-time light 'surged' in China and northern India along with urban development." Space.com noted volatility in places like Palestine (due to war) and Puerto Rico (due to natural disaster). Big apparent decreases in countries like France may be the result of policies meant to reduce light pollution (although the new research can't say for sure). And not all the change is due to the behavior of electric light. Citing NASA, the Guardian pointed to "intense gas burn-offs, or flaring, over central US regions" of oil and gas production. In short, the picture is complicated at best. 

When considered alongside earlier studies, the emerging picture is clear: they all point to a brightening world. The Nature paper by Li and colleagues doesn't change that broad conclusion. And a brightening world involves many social and environmental consequences.

In understanding this, there is an important analogy to global climate change. An objection to the evidence for climate change one hears sometimes is that certain places seem to be getting colder. And that's true: even in a world that gets warmer on average with time, some places get colder. But others get a lot warmer than the average. This is an expected consequence of adding energy to the Earth's atmosphere, raising the average temperature.

​The main takeaway from the Li paper is that patterns of change in light emissions over time and geography vary, and by a lot. The surprising conclusion is that "both brightening and dimming have markedly intensified over the past decade." That's one thing we didn't expect, nor can we quite explain it. And in many parts of the world, rates of both brightening and dimming are increasing together. More research is needed to understand why this is.

We need to keep beating the drum about ALAN and light pollution. The new data make clear that the world is leaning on the accelerator, not the brakes. There is a growing realization in the scientific community that light pollution is real pollution. It has harmful effects that can be mitigated through simple actions. When we undertake those actions, we know that they produce the expected results.

To the extent we might ever know what is a "safe" dose of ALAN, if such a thing exists, maps like these are our only way of knowing on a global basis which regions are exceeding it. This has meaningful value for everything from public health to ecological conservation.
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It also underscores the value of satellite data in this effort. Being able to say on the local level what's happening is crucial to inform lighting policy development. When we examine the data after the fact, it helps us understand whether policies work, and if so, how well. The images can even catch scofflaws as the ultimate "eye in the sky". 

Analysis methods will continue to improve and grow in complexity. There is every reason to believe that more insights wait to be extracted even from historical measurements. Yet we already have plenty of information in hand to know that it's time to move from study to action. For decades, satellites have helped characterize the problem. In the future they can show us which solutions work best.

But we need a new satellite mission whose design is driven by unanswered scientific questions. It should also have characteristics that make it best for testing our models and checking progress on light pollution-reduction actions. If those actions aren't working as intended, future studies like this may help explain why.

The new results for countries like France suggest how this might work. If the study analysis is right, decreasing light emissions there may mean that policy interventions are working. That, in turn, shows that long-term reduction of outdoor ALAN is possible. It's a pollutant that moves across jurisdictional boundaries with ease. Village-by-village policy making and planning is unlikely to ever get ahead of it.

Master lighting planning on regional scales may be the solution. That could extend to international agreements as well. We don't yet know if that will work, but the chances might be better if coupled to changes in environmental law. Europe is prime territory to test these ideas. One possibility is through the EU Nature Restoration Regulation. This requires EU member states to "stop, reduce or remediate light pollution in all ecosystems” as part of their national restoration planning. Satellite data can be an important part of assessing the status quo and checking up on how well actions are working.  A new European 'night lights' mission has been proposed and now awaits a funding decision.
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However that situation shakes out, it's worth staying tuned to this story. We still have much yet to learn.