The modern world has slowly begun to redefine the concept of night. What used to be a time associated with darkness, moonlight, and the appearance of the stars is now an extension of the daytime. The roads are still lit even when there is no more traffic; commercial buildings are lit into the early hours of the morning, and the residential complexes hardly ever encounter total darkness. While this might symbolize advancement and convenience for humans, it could mean something else entirely for nocturnal creatures.
The problem is caused by the fact that artificial light not only brightens the night but also has its negative effects. Nighttime is essential for the natural system of how organisms know what time it is. Birds that migrate, a moth looking for food, a bat preparing to migrate and an insect reacting to changes of daylight do not view the night as something that separates the two days. Instead, they use the light-dark cycles in their organisms’ bodies to organize the sleeping, breeding, migrating and resting. Therefore, when artificial light affects the natural cycles, it breaks biological timing, which leads to negative consequences for living organisms.
This leads to an issue which is easy to overlook in the bright environment of the modern world: what will happen if the calendar of nature differs from the signals received by animals from the environment?
In order to understand the answer to this question, it is necessary to understand how animals perceive the changing of seasons.
For most animals, the key factor is the photoperiod, which is the ratio between the day and night hours within one twenty-four hours. Seasonal changes affect the proportion of day and night hours in a certain way. They allow predicting the environmental changes instead of responding to them immediately. An animal should not wait until the food has been eaten up and start making adjustments to its behaviour; it should begin this process beforehand. Light cycles of nature give an information that regulates such actions.
The environmental cues that synchronize biological clock are called zeitgebers or “time-givers”. The light, temperature and other regular environmental factors play such role. Light pollution can disturb these cues both on daily level and seasonal one. It is described by research about nocturnal insects.
It should be noted that the main thing we can understand as a result of this distinction is that the effect of artificial light depends on a variety of factors, including the species, type and intensity of light, the time in which exposure occurred, and sensitivity of the organism being illuminated. For instance, artificial lighting near midnight will have different results than that witnessed in dusk. Some species rely on day length while other focus on temperatures and moonlight; therefore the result cannot be a simple uniform response, but rather diverse changes in biological timing.
The problem becomes even more serious when it comes to migratory animals because they can change the timing of their migrations due to artificial light exposure.
For instance, let us take into account the purple martin. It migrates from South America to North America, which means that it has to arrive in different breeding zones at the right time, which implies having the right weather conditions and food at that time. A study conducted in 2021 made use of light loggers to observe the effects of artificial nighttime illumination on purple martins at their wintering grounds.Among the 155 birds included in the study, 48 birds, or roughly 31 percent, showed signs of having experienced artificial light for at least one night during the thirty days leading to spring migration. On average, birds that were exposed to artificial light for ten days or more departed around eight days earlier as well as arrived at the breeding ground about eight days earlier compared to birds whose exposure to artificial light was not recorded.
This finding is important because it shows a consequence of artificial light that may not be noticed by someone observing a brightly lit city. The problem is not that the bird cannot make it to its destination; rather, it may be reaching the destination at a wrong time.
Early arrival can be helpful when it allows a bird to possess a desirable breeding territory. However, not every early arrival is good. Unless the birds arrive when their food resources are sufficiently available, they may experience difficulties in feeding themselves and their young. Depending on the weather, birds may find that their efforts to migrate early were in vain. If conditions are not favourable, the energy expended in making the trip may not be very useful. The research concludes that changes in migration timing may lead to an imbalance between birds and their environment.
It is referred to as a phenological mismatch where the timing of one biological event has been disturbed in relation to another biological event. It applies to a situation where insects appear before the chicks of birds need them as food, or flowers appear out of season for the pollination process to occur. In the case of a changing climate where temperature changes affect the seasons of nature, artificial light could serve to make life more difficult for the already disrupted seasons.
Another way that bird migration provides evidence of the problems posed by artificial light is through the locations that the artificial light affects. In a study published in the journal Nature Communications in 2023, wherein ten million remote sensing and radar observations were used to examine migratory bird stopovers in the contiguous United States. They find that skyglow, which is the brightness in the sky due to artificial illumination, was a very important positive predictor of stopover density in more than 70 per cent of their models. They stated that light pollution zones around urban settings could constitute a massive ecological trap, luring birds into an area where their existence is threatened.
This is one example of a very telling paradox. A landscape that is lit up could be appealing to migrating birds, but appealing does not mean that it would be good for them. Birds that flock to bright lighting systems can run into glass houses, traffic, and broken-up vegetation as well as fewer places to rest safely. Previous studies conducted using weather radar revealed that the stopover density of migrants became higher in brighter regions but lower within several kilometers from the brightest spots.
In other words, light pollution can impact the seasonal clock and the pathway by which it impacts an organism.
The implications are not just for birds. Nocturnal insects play an important role in food chains, serving as food for various organisms like bats, birds, amphibians and many more. They also help in plant pollination. But there are various responses shown by these nocturnal insects to artificial light, whereby some are attracted to it while others get disoriented. In a review of the effects of artificial light on nocturnal insects, three pathways are identified: time disorientation, space disorientation, and attraction to light. The review also stresses the impact of artificial light on fireflies and their signals for mating.
For fireflies, the problem lies not only in the fact that the night sky has become brighter, but also that their light may be harder to spot in the illuminated environment. Issues concerning breeding may eventually show up in numbers that represent their population. Consequently, the consequences of simple events can affect the future generations’ existence.
The same applies to relationship between plants and their pollinators, which was proven in 2025 when an experiment was conducted in Switzerland with wildflower strips under lighting. The results showed that the flowering times were changed in six out of ten species, while the germination of the seeds took place sooner in five out of six. Flowering times were changed in 75 percent of the studied sixteen plants, meaning that everything happened on average ten days ahead of the schedule in some cases. These findings imply that, in future, the timing and interactions between flowers and their pollinators could also be changed for the worst.
The reason for the significance of this matter lies not only in the number of species in a certain location called seasonal biodiversity, but also in the timing of their activity. It means that while pollinators and flowers exist separately, the process of pollination could suffer from lack of time synchronization. One can still witness the migration of one type of birds and the emergence of some insects but once again, the timing does not allow for cooperation. Thus, the existence of some species does not guarantee the necessary number of species acting at the right time.
Nevertheless, the debate about the relevance of artificial lighting from the perspective of wildlife can prove to be totally ungrounded. The use of artificial lighting can benefit human beings. For example, street lighting allows walking along the streets, lighting up public areas, and conducting activities in the dark. Hospitals, transportation, factories, and emergency services need a reliable light source. People may be worried about the lack of light, as lowering illumination can compromise their feeling of safety and businesses require sufficient visibility.
The problem is that the dilemma is often framed as choosing between protecting wildlife and safeguarding humans, but what we should really be asking is if we absolutely need lights to be so bright and ubiquitous and continuous. While a street may require lights for late traffic, it is not necessary for all buildings, billboards, parks and parking lots in the area to be lit up brightly until morning light arrives. A pedestrian path may need light focused on it without illuminating trees, wetlands, or river shores. There may also be a need for lighting in the commercial building to improve security while not lighting up its entire façade after the working day is over.
Such distinctions are important because light pollution is defined not only by brightness but also by the angle and duration of light exposure. A study on nocturnal insects states that modern white LEDs carry a significant blue-light component, while the impact of artificial light is also affected by the spectrum of the light used.
The solution, therefore, need not involve returning cities to complete darkness. It involves making illumination more selective.
DarkSky International and Illuminating Engineering Society suggest five guidelines for proper outdoor lighting: using light where it is necessary, illuminating what needs to be illuminated, keeping the illumination level at the minimum necessary, timing the light usage, and having warm colored light wherever possible. Light timers, motion detectors, light shields and light choice can help decrease unnecessary light levels while preserving necessary visibility.
Seasonal regulation could increase their effectiveness. Cities can lower their decorative lighting in bird migration times, preserve dark corridors between forests and wetlands and have more strict regulations of lighting around sensitive areas. Coastal cities can have less light visible on turtle nesting beaches during turtle nesting and hatching periods. Florida’s wildlife authorities explain that sea turtle hatchlings naturally move towards the brighter open horizon over the ocean; artificial lights can overpower these visual cues and lead hatchlings inland instead.
The aforementioned considerations take into account a crucial point: the value of darkness differs depending on both location and time of the year. Some areas may require special protection during certain biological periods, while others might be exposed to the impact of controlled light with fewer implications.
Moreover, it is important to take into consideration that facts may be uncertain in this field. At the moment, scientists do not comprehend the specifics of every species’ reaction to artificial light. One can safely assert that results obtained in studies cannot be automatically used in every ecosystem. Most of the facts are substantiated just by particular phenomena only such as changing of migratory routes, attraction to light sources and blooming period. However, it is difficult to define the impact of light pollution to the populations of species because it is often entangled with habitat destruction, usage of pesticides, global warming and other things. Good policy should combine the results of the research done with monitoring of the situation instead of assuming that one rule of artificial light regulation will solve the problem.
In the end, the pivotal question revolves not around the ability of mankind to light up the night, but the ability to light it without taking other species into account.
It is true that artificial light has expanded useful hours, increased visibility and changed urban life for the better. However, it is also true that some natural conditions that have developed through time have been disturbed. A lamp switched on by human will may lead birds to leave too soon, insects to mate at the wrong time or plants to blossom when it should not. All these are seemingly insignificant changes, while their end result can reach the entire food chain.
One of the most important questions is not how bright the city is at night, but what its brightness prevents nature from recognising.
When the darkness of night is dispelled, will the wildlife still know the change in season?
According to what appears to be the case, in some species this no longer seems certain. Artificial light may not cancel the seasons, but it can distort the signals that animals are using to track the seasons. What is necessary is to create human environments with proper light some of the time, while keeping ecological darkness intact.
The optimal way will not be the one which gets rid of the greatest number of the lights, the optimal way will be the one which knows exactly where light is needed, when it is needed and where the darkness has to remain undisturbed.
Light does not equal the absence of nature. It is something that is part of the environment of nature. It is not stopping the progress but rather allowing it to take into account nature.
The fate of the seasonal biodiversity depends on such a simple action as switching off the unnecessary light.
By: Ajitesh Jain
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