In my frequent visits to a native village named Moutorh in Purulia district, (West Bengal) to promote science education through practical demonstration, among the students from V to IX, many had experienced the problem of pain in the eye while reading continuously for 3-4 hours at night. On enquiry from their parents, I came to know that it would be a great excuse not to read but to play on their mobiles. But the way students were stating the fact convinced me that it was real as I had little experience on the impact of artificial light at night (ALAN). Light pollution may be the cause of this suffering.

Another incident can be stated that there is not a single crow in my village. Further probing I realised that ALAN, particularly during marriage and other social functions as well as cutting the trees forced the crow leaving the places as they were living close to the village houses. Along with crows, the population of other birds and insects severely declined. Many old people in this villager have agreed that this is the evidence that light pollution has profound and serious impacts on ecosystems. Today around a quarter of the Earth's surface is polluted by ALAN originating from industry, residential areas and transportation networks. In India, New Delhi, Telangana, Maharashtra, Karnataka and Uttar Pradesh experienced an increase in very high light pollution intensity. The increase in ALAN merits immediate attention to create awareness among the people.

Upon devising light bulbs in September 1878, Thomas Edison wrote in his laboratory notes: "With the process, I have just discovered, I can produce a thousand – aye, ten thousand – from one machine. Indeed, the number may be said to be inﬁnite". It is true because recent satellite data estimates over 80 per cent of the world's human population experience artiﬁcial light at night, with both the extent and brightness of lit areas increasing at a rate of 2.2 per cent per year between 2012 and 2016. Arguably, the light bulb is the most transformative invention humans have introduced to this planet. Working hours have remarkably increased both in-office and at home. Obviously, ALAN has some clear beneﬁts for humankind, such as increasing opportunities for economically productive activities, leisure and recreational activities. But, light bulbs have a dark side because they have stolen the night.

Light in nature mainly originates from extra-terrestrial sources, such as the sun, moon, and stars, or emission from the upper atmosphere like airglow or aurora. The illuminance (measured in "Lux" i.e "lx") that reaches on Earth's surface is a maximum of about 120,000 lx during the day and decreases to about 800 lx at sunset. At night, the maximum illuminance reaches about 0.3 lx on a full-moon night, which decreases to about 0.001 lx on a moonless clear night and even further for cloudy conditions. In urban areas, direct ALAN can reach light levels up to 150 lx, which is 1000-fold brighter than a clear full-moon night and of markedly different spectral signature than natural light. Indirect light pollution originating from light is scattered within the atmosphere and occurs as skyglow that is visible over large distances. Skyglow changes with atmospheric and weather conditions, potentially resulting in night-sky brightness (luminance) levels hundreds of times brighter than natural and surface illuminance levels brighter than a full moon. Skyglow can also mask the blue peak present during twilight which plays an important role in the circadian entrainment. In some extremely light-polluted places, the sky is so filled with light that 99.5 per cent of all stars are completely invisible without optical aid.

The widespread introduction of high-intensity white LEDs, which are cheap, bright, highly efﬁcient and low energy consumption exacerbates the problem due to light emissions with "bluer" and more polluting light spectra compared to more yellow light emitted by previous lighting technologies, such as incandescent and low-pressure sodium lights. Most importantly, LEDs are rapidly becoming one of the world's most important light sources and are increasingly being used for lighting in both residential and commercial areas as well as the transport routes between them and thereby more short-wavelength (commonly called blue) light, is introduced into the night environment and simultaneously this ALAN is endangering ecosystems by harming animals whose life cycles depend on dark.

One possible reason for not feeling sleepy as reported by the students is the suppression of the production of melatonin because of exposure to excessive ALAN, though this problem was faced by only a few students. Melatonin is responsible for unleashing a cascade of reactions that regulates sleep-wake cycles, lowers body temperature, slows metabolism, and increases leptin, a hormone that reduces appetite that reduces our hunger at night. The melatonin normally begins rising at sundown and peaks around midnight. Low melatonin levels due to exposure to electric light (computer/mobile screen indoor or outdoor) and circadian disruption also play a role in heart disease, diabetes, depression, and cancer.

Light pollution has a signiﬁcant impact on the organismal functions of birds, insects and other animals because it disrupts their internal clock and circadian rhythm even at the intensity of 0.05lx. According to the scientists, the melatonin levels in birds decreases with the increase of ALAN intensity and causes sleep deprivation and stress response in birds and therefore change their rest patterns. The American Bird Conservatory estimated that more than four million migratory birds perish each year in the United States by colliding with brightly illuminated towers and buildings. Lights are well known to disorient migration of sea turtles. Among other navigational aids, sea turtles hatch at dark and hatchlings use moonlight over the water to return to the ocean. But bright ALAN originating from high rises, resorts, bars, malls, restaurants, and homes along the coastlines create countless false moons and alluringly bright horizons and the tiny turtles get disoriented and wander into roads in huge numbers. As a result, millions of sea turtles die every year in Florida. Light pollution can also meddle with aquatic life in lakes. Zooplankton normally dwells deep below the water in the day and ascend to the surface at night to feed on algae. But ALAN impedes zooplanktons from consuming surface algae which leads to algal blooms that may disrupt plant life and lower the water quality.

Habitats that are exposed to increased ALAN due to anthropogenic activities face a variety of other manmade abiotic stressors including noise, chemical contamination, or other effects related to changes in land-use practices. An estimate of the effects of street lamps in Germany suggested that the light could wipe out more than 60 billion insects over a single summer. Street lighting prevents moths pollinating as they end up flying into glare instead, exhausting themselves and leaving them vulnerable to predators. Therefore there is an urgent need to put emphasis on the importance of keeping any disruption to the natural system to a minimum. Ecologists all over the world face challenges such as measuring light accurately and assessing how multiple species behave in response. But early results suggest that light at night is exerting pervasive, long-term stress on ecosystems, from coasts to farmland to urban waterways, many of which are already suffering from other, more well-known forms of pollution. But still, the study that has been carried out to address the climate change, has not been done so extensively for a clear understanding of light pollution and its impact on the whole ecosystem.

Best solutions to light pollution include turning off unnecessary lights to save money and environment and putting shields on streetlights to direct beams downward to prevent skyglow. Keeping the lights off between midnight and dawn is enough to prevent the disturbance to animal behaviour that they trigger. An adequate understanding of light pollution and a holistic approach involving stakeholder and community participation is necessary to minimise its consequences.

Dr Debapriya Mukherjee is a former Senior Scientist, Central Pollution Control Board. Views expressed are strictly personal