Image credit: NASA's Goddard Space Flight Center  
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Summary: Knowledge is power, but only if it's accessible. The "Artificial Light At Night: State Of The Science" report improves access to the knowledge in thousands of papers about light pollution and its effects. This post explains how the report was written and is updated each year, and it presents some research highlights from 2023.

Accessible scientific information about light pollution and its consequences is key to combating this growing environmental problem. But until recently there were few accessible summaries about artificial light at night (ALAN).

This prompted DarkSky International to publish the first "Artificial Light At Night: State Of The Science" (SOTS) report in 2022. SOTS considers the contents of thousands of published papers, theses and reports. It summarizes the current scientific consensus view on various aspects of ALAN. Written in non-technical language, it includes a comprehensive bibliography to support its content. 

​Each year, DarkSky International updates this report. Such regular updates help keep track of the hundreds of new studies published each year. We are excited to share State Of The Science 2024, which includes the most recent studies and papers published in 2023. 

SOTS is in many respects a 'summary of summaries'. The report authors considered almost 5,000 pieces of scientific literature listed in publicly available Artificial Light At Night Research Literature Database, or ‘ALANDB’.

Scientific literature about ALAN is growing quickly. Rising attention to the problem of light pollution has led to a rapid increase in the number of papers published on this topic. The chart below shows the number of papers published each year for the past 20 years that were included in ALANDB. In recent years, the database contents have grown by an average rate of 15 percent per year.

ALANDB contains a variety of kinds of literature, from journal papers to government reports. SOTS prefers peer-reviewed studies as sources. The scientific community holds this kind of material in the highest academic regard. But where appropriate, SOTS includes references to white papers, student theses, and other sources.

Each source added to ALANDB receives one or more of 22 top-level keywords describing the contents of the paper. These keywords help group papers into a set of seven broad categories: the night sky; human health; wildlife and ecology; public safety and crime; energy and climate; and social and environmental justice. In recent years, SOTS included papers about “space light pollution” from swarms of satellites orbiting the Earth. As the database entries are sorted, quality checks weed out sources that have methodology problems . Sources with the most significant results are highlighted for inclusion in the resulting report.

Using ALANDB as source material, the process of composing SOTS is like completing a jigsaw puzzle. Relationships among the topics become clearer as one looks at the 'big picture'. Drawing connections among topics, and sources within a topic, helps reveal underlying trends. We then better understand the state of light pollution science. But we also begin to see where the missing pieces are. Updating the report each year requires only looking at the “new” pieces and seeing where they may fit.

SOTS has several target audiences. One of them is the scientific research community. New researches to this field, in particular, enjoy an accessible introduction. Issue advocates become more knowledgeable about the facts supporting their cause. Policy makers better understand the issues they must decide. And the public finds an entry point to learn about a major social and environmental challenge.

2023 was an eventful year for light pollution research. In July, the journal Science for the first timefeatured light pollution on its cover, exactly 50 years afterthe first mention in its pages. The special issue featured five comprehensive review papers. Also, the Royal Society's Philosophical Transactions B publisheda 'themed' issue on ecological light pollution. Its papers "investigate light pollution ecology at various environments and scales, from single processes to whole communities, to better understand the relationship between light pollution, ecological balance, and human influence."

​​The 504 ALANDB sources published in 2023 that were considered for SOTS report break down by major keyword as shown in the pie chart below.

Fully 75% of the papers are about wildlife, remote sensing (of ALAN), or human health. They have been the leading topics for the past few years.

Certain key points emerge from the survey of papers published in 2023:

• Few studies break completely new ground, but scientific conclusions about light pollution grew stronger. Most studies simply add weight to previous conclusions.
• New remote sensing tools and techniques give unprecedented views of the Earth at night. This provides more granular data about the distribution of ALAN on land and through time.
• Understanding how light pollution affects global biodiversity is a top priority. In particular, the role of ALAN as it interacts with influences like climate change is becoming clear.
• Health researchers sounded the alarm about ways in which ALAN exposure affects older people. In particular, they found connections to neurodegenerative diseases like dementia and Alzheimer's.
• The nature of the relationship between ALAN and crime remains very unsettled. Past research on ALAN and crime incidence yielded ambiguous results. But newer work focuses on users of outdoor spaces and how lighting influences their perceptions of safety.
• Interest in how light pollution may influence air quality is rising. Some studies found further evidence that ALAN alters the nighttime chemistry of air, linking to poor air quality over cities. In turn, certain kinds of air pollution affect the formation of skyglow.
• The impacts of satellite 'megaconstellations' on the night sky are still poorly understood. Researchers now look at factors beyond only the brightness of individual satellites.

Recent research achievements mark a clear advance in our knowledge about light pollution. But many questions remain. SOTS concludes with a list of some of these questions, such as:

• How does ALAN contribute to species population decline or extinction?
• To what extent is outdoor ALAN harming human health and wellbeing?
• Of known light pollution mitigations, especially policy solutions, which are most effective?
• Which kinds of legal mechanisms may help solve the satellite problem?

We expect that researchers will wrestle with these and many other questions in 2024 and beyond. New kinds of light-sensing devices are becoming available, as well as new platforms on which to place them. Computer models of skyglow grow more sophisticated and their outputs more realistic. Possibilities for engaging citizen scientists in research abound. And a new generation of young scientists is enriching the field.

Yet it remains difficult in most cases to get adequate support for light pollution studies. It's a multidisciplinary subject, and many funding agencies don't know where to place it. Full funding of large-scale, complex projects is still unusual. This often leads to 'siloed' knowledge that may hold major advances back. But as the field gains mainstream recognition, it becomes more attractive to potential funders. Expect new breakthroughs as access to resources improves.

Want to learn more about the SOTS 2024 report? Have a look at this presentation from our own John Barentine to the DarkSky International advocate community introducing the report and reviewing its highlights:

"Water is life." Anyone who lives in an arid climate has heard this phrase. As the global climate crisis intensifies, more people are hearing it. It signals the value of a crucial natural resource threatened by overconsumption and pollution. Many years of water conservation efforts have shown the need for a unified approach. And careful collective management of water resources is a proven way of sustaining access to clean water supplies.

​What lessons can we learn from this kind of resource management that we may apply to conserving nighttime darkness? In this post, we dig into the idea of light pollution reduction through the management of "lightsheds". This approach has great untapped potential to surpass current shortcomings in the way we protect the night around the world.

We have previous written here about aspects of outdoor lighting pollution. In "Good/Better/Best Outdoor Lighting Policies" we examined the main elements of such policies. And in "Toward a 'Clean Night Skies Act': a roadmap for a U.S. national light pollution policy​" we presented a vision of a national outdoor lighting management strategy.

We find there are two main regulatory approaches: "supply side" and "demand side". Supply side limits the kinds of lighting that can be sold, installed, and/or operated in a jurisdiction. (Make bad lighting difficult to get and people mostly won't use it.) The demand side aims to change behavior by incentivizing the use of some kinds of lighting  through, e.g., more allowances. (Encourage the development of a market for better lighting products catering to consumer preferences.) Some policy approaches are a combination of these two that apply in "carrot-and-stick" fashion. A hybrid policy might restrict some popular kinds of lighting applications to reduce light pollution while permitting others. 

Another dichotomy in lighting policy involves when enforcement happens: "before the fact" and "after the fact". The former takes place before operation of the lighting. Examples include planning permission, comparison with standards, obtaining building permits, and post-construction inspection. Policies are enforced after the fact through citations issued to property owners whose lighting does not follow the law. 

Most lighting policies thus tend to prescriptive and/or proscriptive: they say what is and isn't allowed, and if lighting is allowed, how it is to be designed, installed and operated. But in many countries, as a practical matter, this is all still matter of local enforcement. Local authorities make decisions about how to interpret lighting policies and how (or even whether) to enforce the law. 

But often that doesn't happen. The public doesn't like being told what to do. It also dislikes state regulation of individual behaviors. The result is that property owners may simply disregard the law. Jurisdictions with laws on the books may be loathe to enforce the law, citing factors like inadequate staffing.

From time to time, other ideas about how to regulate outdoor lighting are tried. Some countries have very strong top-down policies: national legislation decrees artificial light at night (ALAN) to be environmental pollution subject to regulation and tasking lower jurisdictions with enforcement. 

An example of this is Mexico, which recently amended its General Law of Ecological Balance and Environmental Protection of 1988, also known as LGEEPA. The move attached light pollution to the existing LGEEPA, which makes its control obligatory to municipalities.

Where there is strong public support, this method can work well. But it is difficult to enact this kind of legislation in contentious political environments. And implementing jurisdictions may well ignore the central government's mandate. As a result, these efforts often produce only symbolic outcomes. 
Another idea stems from the fact that light pollution isn't restricted to any particular jurisdiction. It does not respect boundaries between municipalities, regions or nations. As a global problem, it might benefit from a global solution. This could involve a new international treaty that binds signatories to meet pollution reduction targets. 

Such was the goal of the spectacularly successful Montreal Protocol on Substances that Deplete the Ozone Layer (1987). 198 countries that ratified it agreed to phase out the production of industrial chemicals that historically caused a 'hole' in the Earth's ozone layer. Since it took legal effect, compliance with the Montreal Protocol has been high, and the ozone layer is repairing itself. 

This example proves that treaties can achieve important and meaningful results. But such solutions need buy-in from many countries. Developing economies may balk, making it difficult to achieve consensus. And the process of drafting and bringing a new treaty into force is often painfully slow.
These challenges prompt some to ask: what if we tried something new and thus far untested? One such new way of looking at outdoor lighting regulation turns many of the existing ideas on their heads. 

A lightshed is the territory around a given point containing all the sources that send light at night to that place. Some of that light is directly emitted from sources on the ground near the point, while other sources contribute indirect light scattered in the atmosphere.  Managing a lightshed to reduce light pollution in that place targets the source of the pollution in an outcome-based fashion.

Lightsheds are analogous to watersheds. The U.S. National Oceanic and Atmospheric Administration (NOAA) defines a watershed as "a land area that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually to outflow points such as reservoirs, bays, and the ocean". It's built on the water cycle that carries water through the terrestrial environment from and back to large accumulation basins through an interface with the land. 

The management of watersheds identifies water as a shared resource that all entities and jurisdictions in the watershed must protect in order that they themselves — as well as others — can continue to enjoy it. The intent is to make the resource indefinitely renewable by ensuring that the "cash flow" in the system is always greater than zero. It attempts to avoid a so-called "tragedy of the commons" in which groups within the watershed consume all of the resource.

The analogy between water and light isn't perfect, of course. There are plenty of sources of light in the nighttime environment but also plenty of sinks. There isn't a "light cycle" that works like the more or less closed loop of the water cycle. And while there are both natural and artificial sources of LAN, but there's nothing like "artificial water". (An imperfect comparison is how humans draw water out of the watershed, depleting it.) In a watershed the conservation object is the resource (water). But in a lightshed the object is natural nighttime darkness. 

Lightshed management focuses on places where nighttime darkness still exists (at least to begin with). Managers determine the sources of ALAN arriving at a given location through computer simulations that use satellite data as model inputs. Associations of governments at different jurisdictional levels pledge to protect darkness in the target location through policy changes. The policies aim to limit and ultimately reduce light emissions in their territories. This could involve a strategy like "cap-and-trade": the influence of newly installed lighting is offset by the removal of unnecessary lighting elsewhere. Over time, the rate at which ALAN reaches the target area slows to zero and then turns negative.  

Another way to look at this concept is to think of light pollution is the byproduct of consumption of light at night for useful purposes. Dividing the light emissions in a region by its population yields a metric like "lumens per capita". The number in a certain area depends on many social variables. But in that area it provides a point of comparison as time passes under a lightshed management plan. 

​By reducing waste and improving outdoor lighting, an area's lumens per capita consumption can drop. Even as its population grows, light pollution can continue decreasing. Acting on the entire system rather than its components, as lighting policy typically does now, might turn out to be the winning strategy. Instead of blanket regulations and ill-defined targets, lightshed management targets the underlying sources of the problem for remediation. 

To be clear, there are many ways that lightshed management may fail. No one has (yet) tried lightshed management as a policy lever on the problem of light pollution, at least not at any meaningful scale. As with new international treaties, it may be very difficult to sign up all the actors in a lightshed to take part in the project. And if even one major light polluter doesn't sign up, the integrity of the project may be compromised. 

Still, conditions are ripe for a small-scale experiment. We have glimpses of what this could look like already. Large parks and protected areas participating in the International Dark Sky Places Program are trying something a lot like lightshed management. Large and complex land arrangements accredited by the program must devise Lighting Management Plans. This often involves obtaining participation from many stakeholders. When it works, we see real reductions in light pollution; for examples, see here and here.

The next big test is to identify a lightshed and set a quantitative goal for light pollution reduction. For the test to be realistic, it should encompass a large landscape with diverse land uses, including urban centers. Both public and private landowners should be involved in the experiment. 

If it is successful, the same stakeholders should take part in planning for how to maintain the improved conditions. They have a strong interest in doing do, whether for conserving local nocturnal wildlife, furthering astrotourism development, or for many other reasons.

Big, challenging problems call for bold new solutions. Light pollution is readily reversible, and when steps are taken to reduce it we see clear social and economic benefits. Now is the time for society to take a more measured and deliberate approach to preserving nighttime darkness. Lightshed management may be the solution that brings the meaningful global change that the problem deserves.

When people hear the word "pollution," they may think of many different things. One that comes to mind immediately is air pollution, which is easy to perceive because it is often visible to the eye. The familiar brown cloud over some cities where air pollution is prevalent is its major manifestation. But much of the problem of air pollution is invisible, as it constituents don't interact with light in ways that the human eye can see them.

Despite its name, light pollution has some similarities with air pollution in that sense. One of them is that many of its hazards are not immediately obvious to people. Not all its affects are visible to the eye in a direct way. And many people don't notice it at all, given that it is most evident when few people are awake to see it.

We often frame light pollution as an environmental problem in that it is a problem of the environment. In other words, it is a way in which human activity interacts with the environment to cause harm. But it is also a problem for the environment, in that it negatively affects the environment in particular ways.

We have previously written here about how light pollution relates to climate change. It does so through electricity consumption and carbon emission. But artificial light at night (ALAN) also interacts with the air itself, changing its chemistry. Those changes can reduce air quality and impact public health.

​In this post, we dissect the two ways in which light pollution and air pollution interact. And we close with an argument for why dealing with one problem can help solve the other (and vice versa).

Scientists realized many years ago that poor air quality, especially in cities, can lead to adverse health outcomes. In particular, small particles suspended in the air, called aerosols, have harmful effects on human health.

Very fine particles known as "PM2.5" are so small that they can travel deep into the human respiratory tract. There they reach the lungs, embedding themselves in tissue and causing both short- and long-term health effects. The dominant source of PM2.5 in cities is exhaust from the internal combustion engines in motor vehicles. But they also come from burning wood, natural gas and other carbon fuels, and fires.

We also know that aerosol air pollution has a strong interaction with local weather conditions. When air over cities becomes stagnant and does not move, the pollution can settle in for many days at a time. This is especially true in winter when temperature inversions in the air may occur. Only when the winds change and blow the pollution away does the air quality tend to improve.

Efforts to write laws intended to reduce air pollution began in the 1950s. Where governments implemented such policies, they have been spectacularly successful. Despite these outcomes, air quality measures still have many opponents. The main objection they raise has to do with the cost of the regulations. As a result, one-third of the world's countries still have no laws regulating air quality. Over eight million people worldwide die each year from complications due to air pollution exposure. [1]

​Beyond reducing this public health burden, there are other reasons to reduce air pollution. One of them is that doing so would also reduce light pollution.

Skyglow is a phenomenon involving ALAN emitted on the ground. It travels upward into the sky, where it interacts with molecules and aerosols in the atmosphere. These interactions direct some light rays back down toward the ground. Observers looking toward the sky see some of this light. It competes with the light of the stars beyond, making it difficult to see them. This is the familiar experience for anyone who has lived in or visited a city where there are a few stars in the night sky.

The same aerosols responsible for air pollution also affect this light. The smaller particles tend to interact with light more strongly than larger particles. The effect is stronger for light that is more blue compared to other colors. This is the same reason why the daytime sky is blue and not some other color.

Scientific research has shown that the interaction between ground light and aerosols can make the night sky brighter. In places where air pollution is prevalent, this effect is stronger. [2] The connection between them is solid. Researchers now use nighttime light measurements from space to estimate how much PM2.5 is in the air. [3]

In 2021, we hypothesized that reducing air pollution over cities would also reduce night-sky brightness. We expected that the effect would persist even when all other influences on skyglow were fixed. To test this idea, we measured the sky brightness over a city before and after the passage of a weather front. These moving airmasses tend to clear out air pollutants where they collect under usual conditions.

​The experiment validated our hypothesis. [4] We saw reductions in the brightness of the night sky by up to a factor of 2.5 as the frontal boundary passed. No other explanation best accounts for the measurements than clearing the air of aerosol pollutants. The result also matches observations from other measurement sites where air pollution is significant. From this result we argued that, after improving outdoor lighting, reducing air pollution could be the next-best way to lower skyglow. 

That would be the end of the story if we were only concerned about skyglow. But there is another way in which air and light pollution are intimately related. If on the one hand we talk about tackling light pollution by clearing the air, on the other it may be that reducing light pollution can itself clear the air.
Some air pollution results from the direct action of burning fuels. For example, sometimes motor vehicles will emit visibly dark clouds from their tailpipes. Many people have in mind that this kind of scene is what air quality laws have in mind to prevent.

But some materials emitted by vehicles and through other activities do not immediately become "pollution". Rather, they dwell in the atmosphere for hours to years. Some of these chemicals react with sunlight or other compounds in the atmosphere to form visible hazes. Natural processes in the air can "clean" certain pollutants by reacting with them chemically.

One of those is the nitrate radical (NO3). Nitrate reacts with volatile organic materials that would otherwise become ozone molecules and smog. But it only works at night because ultraviolet light from the Sun in the daytime destroys it. In 2010, Harald Stark and his collaborators wondered whether ALAN could play a similar role. They set up a clever experiment to find out.

In Stark's experiment, the researchers outfitted a small airplane with sensors capable of detecting both nitrate radicals and ozone, as well as ALAN. They flew the plane over Los Angeles, California, at night. Then they flew the plane over the Pacific Ocean and compared the measurements with those from the city. The nitrate concentrations showed distinctly different patterns over the city versus the ocean.

From this the team concluded that city lights were interfering with the nitrate radical's nighttime work. [5] Although the effect was small — only 4% — the reliability of the measurements was high. Since then, other research has pointed toward ALAN's influence on harmful ozone [6] and nitrogen dioxide [7] levels.

​It's important to note the limitations of these studies. They have so far been carried out on relatively small scales, and they find small effects. But as city lights get brighter around the world, the significance of the issue is expected to grow. So it makes sense to consider which solutions are most effective.

In 2021 we wrote that "cleaner air not only has obvious public health benefits, but it could further reduce diffuse artificial light in the night sky and improve astronomical viewing after the utility of other methods such as lighting changes has been exhausted." [4] Connecting the two in this way might be a means for dealing with both issues at once.

Although cleaner air is the goal that resonates most with the public, "clean" night skies have some appeal. Dealing with the main sources of air pollution is difficult, but we may find it easier to score some wins around the edges of the challenge.

The best policy approaches for reducing ALAN's influence on air pollution have to do with timing. Much of the chemical mechanism involved in scrubbing pollutants from the air works during the overnight hours. Limiting the times in which outdoor lighting is used through curfews is an important tool. Another is insisting on full shielding of all lighting. This ensures that the direct emission of light into the night sky is limited or eliminated. The third leg of the stool is limiting total light emissions. Given that much of the light pollution we see in the world results from over-lighting relative to needs, we expect these methods to work well.

​Light pollution is perhaps the only environmental problem of our time that is fully reversible. What's more, we know that the solutions are cost-effective and produce the anticipated results when implemented correctly and consistently. Solving the problem might also have the added benefit of tackling air pollution in and near urban areas. This not only adds to the quality of life in cities, but it does so without any obvious downsides. It's an easy win that the world desperately needs.

Summary: ​Natural cycles of light and dark are needed for ideal wellbeing. Artificial light at night disrupts this rhythm, but does it lead to ill health? This post explores the surprisingly complicated relationship between light at night and human health, and what we still need to learn to find the optimal balance between them.

Light at night (and light pollution) are in the news these days for a reason that may surprise some readers. Consider some recent headlines:

"Bright outdoor light at night may increase stroke risk"
"Does More Outdoor Light at Night Help Cause Macular Degeneration?"
"The light you see may be as important to your health as the food you eat"
"Getting Too Much Bright Light at Night May Increase Your Cancer Risks"

The popular media are notorious for running stories about human health that seem contradictory. What's good for people one week seems harmful to them the next. Some of this results from the ways the media report news about science in general. But more often it reveals something about the often messy process by which we collect and judge evidence.
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The same is true of questions about light and health. What do we really know about this subject? While some conclusions drawn by researchers are concerning, we're still a long way from definitive answers. In this post, we'll dig into the evidence and learn whether there's a real 'dark side' to light at night.

Scientists study artificial light at night (ALAN) and its effects on human health along three lines of enquiry:

• Laboratory studies in controlled conditions
• "Real-world" cohort studies
• Correlation studies that match remote measurements of light with data on disease incidence

These are standard research approaches and none of them is especially problematic. Of course, there are always details of study design that are better or worse in different regards. But these basic methods are well understood, as are the limitations of their results. To get the "big picture" of what ALAN may do to our health and wellbeing requires using all the tools we have.

Laboratory studies are usually small, involving a few volunteers exposed to ALAN. Researchers measure their responses to light with various tests. While the controlled exposures mean that the "dose" of light is definite, the sample sizes are small. Information they yield can be compared against results from cohort studies. These look at effects in larger groups of people ("cohorts") in situations where the exposure isn't controlled as well. The idea there is that the larger sizes of cohorts make it easier to find the individuals who are not very representative of the group. If the presumed health effect in a cohort study is real and prevalent, it should stand out in the results.

Both laboratory and cohort studies of ALAN and human health tend to consider only indoor light exposures. And those studies show that ALAN affects people in ways that harm them. (For a comprehensive review of what we know about ALAN and health, see DarkSky International's "Artificial Light at Night: State of the Science 2023" report.) Effects reported in scientific papers include:

• Sleep problems
• Increased inflammation throughout the body
• Temporary changes at the genetic level
• Metabolic disorders (obesity, diabetes, etc.)
• Higher risk for cardiovascular disease and certain kinds of cancer
• Problems with mental health including mood disorders

​These observed changes largely stem from the tendency of ALAN to disrupt the body's "circadian rhythm". This is a roughly 24-hour cycle of biological activities tied to the rising and setting of the Sun. Over billions of years, living things became sensitive to the reliable sequence of light-dark-light-dark in nature. ALAN mimics certain qualities of sunlight, which confuses organisms. And because ALAN is so new in the environment, they have not yet evolved adaptations to it.

All this marks ALAN as an emerging "lifestyle risk" for people. We use light at night for many reasons associated with modernity and progress. Science is telling us that there can be too much of a good thing in that regard. But so far we have only considered evidence linked specifically to indoor light at night exposure. Should outdoor exposures also concern us?

That's where the picture gets murky.

The third kind of study mentioned above attempts to answer the question by looking down from overhead. The world's cities shine at night with an intensity that makes them visible to astronauts aboard the International Space Station. Since the 1970s, satellites orbiting the Earth have measured "nighttime lights" (NTL) on our planet.

The signals they sense represent only a fraction of total light emissions. While some light emitted upward escapes the atmosphere completely, other light scatters back down to the ground. Simple models of the situation imply some relationship between light levels on the ground and the intensity of NTL seen from orbit. Reality is more complicated, and the actual relationship can be quite complex.
Potential trouble results when researchers use NTL measurements in human health studies. The idea seems reasonable enough: match up NTL data with information about the spatial distribution of various maladies. Of course, there are many other social and environmental factors that influence human diseases. Once we account for and remove those factors in the data, only the effects of ALAN should remain. But do they?

On the surface, the answer appears to be "yes". Studies find correlations — that is, mutual relationships or connections between two or more variables. People who live in areas with higher NTL levels do seem to suffer more from various illnesses than people who live in darker places. That conclusion persists even after eliminating other possible explanations for the results.

It is said, famously, that "correlation is not causation". While correlation is the appearance of a relationship between variables, it says nothing about why they are related. Causation is the idea that cause precedes effect, and that for every effect there is a cause. Nature and society are rife with spurious correlations, in which there is no plausible cause-and-effect relationship.

It's fair to ask whether there's an underlying cause for the observed correlation between NTL intensity and health problems. Do the laboratory studies sufficiently establish a causal link? We don't know. They reveal a clear connection between ALAN exposure and ill health in both individuals and groups. But they don't (yet) tell us what the threshold of harm is.

Researchers refer to the "dose-response" relationship between cause and effect. So far, we don't have a strong handle on that even under controlled conditions. In the case of outdoor lighting, we know even less about individual exposures. And there's reason to believe that the majority of light exposure most people get is from distinctly indoor sources. (Think about watching television or staring at the screen of your phone late at night, long past your usual bedtime.)
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The bottom line is that the effect of outdoor lighting isn't zero, but the amount of indoor light exposure most people get probably dwarfs it. Most studies claiming one health effect or another from ALAN rely on NTL data as an estimate of actual exposures. But therein lies the problem: we don't know what those exposures really are. So we can't take extreme positions like "streetlights give people cancer". The evidence just doesn't support that.

So what can we conclude from all this? Is it hopelessly confused?

For the benefit of preserving health and wellbeing we should try to limit all ALAN exposure, whether from indoor or outdoor sources. There is little scientific doubt that the observed connection between light and health is real. The hypothesis about why, with its strong biological underpinnings, is sound.
Some of it has to do with timing. During the daytime, we need exposure to bright light, especially that rich in short wavelengths (i.e., blue). This helps calibrate the internal 'clock' that maintains the circadian rhythm. Inadequate daytime light exposure is almost as harmful as too much ALAN exposure. While humans have some resilience to seasonal changes to the length of the day, we have no natural defense against the influence of ALAN.

What can we do with the information we have in hand, knowing it is incomplete? There is a case for taking steps to limit ALAN exposure, especially in sleeping rooms at night. Those spaces should be completely dark to avoid disrupting the circadian rhythm. Sleeping with lights on at night is not advised. Good "sleep hygiene" includes avoiding bright light exposure in the hours before going to bed at night.

Heavy window coverings may be needed to prevent the intrusion of outdoor lighting into sleeping rooms. While we know that exposure to bright indoor sources of light at night is bad for people, we know much less about how very dim outdoor lighting coming in through windows affects us. As one prominent light/health researcher once put it, "Photons are photons, and the body and brain don't care where they come from."
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The owners of outdoor lighting can take care to ensure light is not shining into people's homes. Public policies governing lighting can assist by making this "light trespass" against the law. They can also assist by limiting the intensity, color, and hours of operation of outdoor lighting. That approach also benefits nocturnal wildlife, because plants and animals can't go indoors and draw the blinds to avoid ALAN exposure.

When practiced well, science is a remarkable self-correcting process. New information suggests new explanations that replace old ones. That is clearly true for health research in general, and more so for efforts to understand how ALAN affects our wellbeing. Although we have learned much, there are still significant unanswered questions. These include:

• Does measured exposure to ALAN in outdoor space affected human health?
• Does outdoor light at night entering indoor spaces affect sleep and health?
• Are the observed correlations between outdoor ALAN and health problems the result of cause and effect?

These questions suggest directions for future research. We need more compact and reliable light sensors small enough that study participants can wear them. Such "wearable dosimeters", especially those that provide color information in addition to light intensity, are a potential game-changer. They may be able to better discriminate the indoor and outdoor components of ALAN exposure to disentangle their effects. That would, in turn, make light and health research less dependent on NTL measurements as proxies for real exposure data.

Of course, that is not to say that we should stop relying on satellite measurements altogether. But the available nighttime satellite data isn't optimized to address health research questions. It often doesn't allow for detailed identification of outdoor light sources in relation to where people may be seeing them. Satellites may pass over a given location at night at irregular intervals or at times of night when little human light exposure is expected. And some don't detect all the lights we know exist on the ground.

Researchers are supplementing existing NTL data sources with new ones. Some have experimented with platforms closer to the ground, such as high-altitude balloons and unmanned aerial vehicle (also known as 'drones'). These get much closer to light sources and can measure them in more detail. They are coming up with new ways to ground-truth the remote measurements by collecting information about lighting on the ground. As scientists pair these new data about outdoor lighting with better measurements of the 'dose' it yields, the quality of health research results will improve.

​When electric lighting was introduced to the world in the late nineteenth century, it was celebrated for its myriad benefits to society. Almost 150 years on, we now understand it to represent both promise and peril. As our knowledge increases, we may yet find ways to balance risk and reward that cater to social needs for ALAN while avoiding poor health outcomes.

Summary: Although subject to widely held folk beliefs, the truth about how outdoor lighting and crime interact is very unsettled scientifically. This post examines the evidence, identifies shortcomings in current research and practice, and suggests the factors and considerations that really matter.