Light can be bad for you
- Green Fingers
- Dec 18, 2025
- 20 min read
Standing on my land in the Lake District on a clear night, and I do that as much as I can, it is still possible to see the Milky Way. The skylines of the surrounding fells are dark, the only lights being a possible farmhouse in the valley, the brilliant glow from Ambleside, or a scatter of vehicle headlights moving along the region’s roads. Yet even here, the night is no longer what it once was. Across Europe, a near 99% of people now live under light-polluted skies. A global atlas of artificial night sky brightness sadly shows that true darkness has become a rarity.[i] Meanwhile, the area of the Earth’s surface that is artificially lit continues to increase by more than 2% every year.[ii]
National mapping by the Campaign to Protect Rural England (CPRE) has shown that, although protected landscapes such as national parks retain the darkest skies in England, light is spilling ever further into the countryside.[iii] In Cumbria, Friends of the Lake District describe how increasing light pollution threatens both the beauty of the night sky and the lives of nocturnal wildlife.[iv] The Lake District National Park Authority has highlighted that central Keswick already records the highest luminosity, while much of the rest of the Park still remains very dark.[v] My own land, just outside Ambleside, lies somewhere in between. It may be blessed with a comparatively low background glow but is slowly being encircled by the spread of artificial light at night.
Too much light is bad for you, in more ways than one. That much is clear, and yet mankind generally prefers not to notice.
Light sources
Apart from my head torch, which I try hard not to use at all, on the ground, light pollution rarely feels abstract. It comes from specific sources. Looking outward from my land, the upward glow from nearby Ambleside and a somewhat further Windermere, adds to the regional skyglow easily visible from outer space. There are security lights on neighbouring farm buildings, sometimes left on all night, while passing vehicles throw their beams along boundary walls and across the open fell. Seasonal tourism can be a personal nightmare, as it brings a wave of headlights and campsite lighting into the valley. The townies arrive and must have their light. Darkness is not a common feature of urban living.
For a human observer, modern lighting may mean slightly more safety and convenience. For the species that share my Lake District, however, it means something more profound. Artificial light at night changes the basic rhythm of light and dark that has structured their lives for aeons.[vi],[vii]
Insects over the bracken
Around me are midges, crane flies, moths, beetles and many other invertebrates moving through the air above the bracken, grasses, and tree canopy. Science now shows that insects are among the most consistently affected groups when artificial light intrudes into the night. The impact of artificial light at night on nocturnal insects results in at least five issues for invertebrates - attraction to lights, repulsion, disrupted movement, altered reproduction and increased predation. It has been argued that light pollution may be one of the drivers of global insect declines, because so many invertebrates respond to night-time light.
On a Lake District hillside, these mechanisms are easy to imagine. A bright, unshielded light on a nearby farm draws moths away from the bracken and woodland where they would normally feed, mate, and lay eggs. Some insects will collide with the light source, many will circle until exhaustion, while all become more vulnerable to bats and other predators that learn to hunt around the light. Meanwhile, a line of road lights along the valley beneath me acts as a barrier that insects are discouraged from crossing. The result is a fragmented population between one side of the valley and the other. This is not theory. It actually happens.
Pollination is also a problem, as shown by fieldwork in the Swiss Alps.[viii] Streetlamp-style light-sources were placed over grassland plots and demonstrated that nocturnal flower visits from invertebrates fell by nearly 60%. Visits by daytime pollinators did not make up the difference. Translated into findings on my own land, this means that any strong lighting near flower-rich corners of woodland edge could quietly reduce seed production in wild plants, especially those that rely on moths and other nocturnal visitors for their pollination. This is not good.
Even without research lamps on my land, and fortunately there are none, the increased background glow from neighbouring valleys changes the balance between night and day for insects, just as it can for humans. Even if the sky appears dark to me, to an insect’s more sensitive visual system, a subtle brightening of the horizon can mask starlight, moonlight, and the contrast cues insects use to navigate.
Bats commuting along walls
Bats are important predators of insects, and several species use tree lines, stream corridors, and drystone walls as commuting routes between their roosts and feeding areas. Research has shown that many bats, especially light-averse species such as the rare horseshoe bats, are reluctant to fly through illuminated sections of their commuting routes. When high-pressure sodium streetlights were installed along hedgerows, one threatened species dramatically reduced its use of those routes compared with unlit controls.[ix]
Additional work with light-emitting diode streetlights reinforced the message that even modest levels of artificial light at night can fragment bat movements. In practical terms, if a poorly informed local authority decides to upgrade their roadside lighting, something a less-educated box-ticker might be tempted to do, it can easily become a barrier to bats that would otherwise cross the landscape freely. The animals might still forage over nearby woodland and fell, but their access routes can lengthen, and their exposure to predators and bad weather increases. Street lighting is not good for bats.
The same applies on smaller scales. A bright security light mounted on a barn wall can discourage light-sensitive bats from following that wall line, even if they are otherwise tolerant of nearby human activity. The result is subtle. There is not necessarily an immediate disappearance of bats, but a slow reduction in the quality and connectivity of their habitat.
Trees, understorey and seasons
My trees are entirely broadleaves. Hazel, birch, rowan, and oak are very common. These trees also have an understorey, in addition to copious bracken all around. The seasonal timing of budburst, flowering, and leaf fall is closely tied to the daily pattern of light and dark. Artificial light at night is sufficiently bright to trigger physiological responses in plants and can bring about earlier budburst, delayed leaf senescence, and altered growth in wild species near light sources.[x]
In woodland, or along lanes with night-time lighting, leaves may remain green later into autumn and emerge earlier in spring than in darker surrounding areas. That shift can have several knock-on effects. Early budburst increases the risk of frost damage. Extended leaf retention in autumn changes the timing of leaf litter input to the soil, and can affect fungi, invertebrates and nutrient cycling. Increased light on a woodland edge may favour some plant species over others and gradually alter the composition of ground flora. With Nature, everything depends on everything else.
There are interactions with insects as well. If trees in brighter spots bear leaves for longer or develop tougher foliage, as they can, herbivorous insects find them less palatable. This has recently been shown for street trees exposed to artificial illumination. Combined with the direct effects of light on insect behaviour, this chips away at the food base for insect-eating birds and bats.
The water world
My land includes water in several forms - streams, wet flushes on the fell, in addition to several small pools. Just ask my soggy feet if you ever have doubt. It is rare for me to return home with dry toes. A wet flush on a fell is a marshy or boggy area of vegetation where groundwater seeps or flows to the surface, creating a wetland habitat. The water table in these areas is close to or above the ground for most of the year.
These freshwater ecosystems are also shaped by daily light cycles. Many zooplankton species carry out diel vertical migration, hiding in deeper water by day and rising to the surface at night to feed. Zooplankton are tiny, drifting animals in water bodies such as oceans or lakes that form a crucial part of the aquatic food web. They feed on phytoplankton (microscopic plants) and other small organisms. Meanwhile, diel vertical migration is the daily, synchronised movement of marine organisms, such as zooplankton and fish, from deeper waters up to the surface at night to feed, and then back to deeper waters before dawn. It is the largest migration on Earth, driven by the need to find food while avoiding daytime visual predators. The migration is influenced by environmental cues such as light and plays a significant role in the ocean's carbon cycle. Research has shown that urban light pollution reduces both the amplitude and proportion of Daphnia migrating in a suburban lake. Fewer individuals move upwards, and those that do, travel a shorter vertical distance.[xi]
In a Lake District beck or pool, any increase in artificial light can flatten these movements. This allows visual predators such as fish to feed for longer at night and reduce the grazing pressure that zooplankton exert on algae. Over time, this can change water clarity and nutrient dynamics. In very dark upland pools, these effects may be negligible. But in lower, more accessible stretches of water, the glow from settlements and roads becomes more obvious and can influence vertebrate and invertebrate behaviour.
Birds, mammals and night
Anyone who has slept near a streetlight will know that birds begin to sing earlier in areas with artificial lighting. I see this in London almost whenever I go. Studies in urban Europe have found that species such as blackbirds and robins advance their dawn chorus in response to night lighting, which can influence their energy use and breeding timing. Incremental increases in light from its various sources can shift the cue that birds use to measure morning.
Artificial light at night does not merely alter daily patterns but can also disrupt circadian and seasonal rhythms. A review of laboratory and field studies concluded that night-time light exposure in mammals and birds interferes with their normal behaviour, including foraging, predator avoidance and reproduction, in ways likely to reduce fitness and modify ecosystems.[xii] For small mammals, a sudden bright light in a formerly dark corner may force them to change routes or restrict feeding periods to avoid predators. For ground-feeding birds, increased light can extend their feeding time but at the cost of a higher predation risk.
These changes may seem distant from the Lake District fells, but they do exist. A brighter car park at the head of a valley, additional lighting at a farm shop, or a new holiday development on an approach road, all form part of the night-time sound and lightscape where more activity spills into hours that were once dark and still.
A changing night
Artificial light at night is now recognised as a genuine environmental driver, alongside climate change and the modification of land use. There are many reasons why light should be treated in this way, including its rapid global expansion, the breadth of species affected, and the fact that light can be reduced or redesigned more quickly than many other pollutants.[xiii] Light is certainly a pollutant. Artificial night lighting has been shown to produce strong biological responses across physiological traits, activity patterns and characteristics of life history, in many different groups of organisms.[xiv]
The Lake District, and my land within it, is not isolated from these trends. Light levels in rural England have risen measurably over recent decades, so that only a minority of people in the United Kingdom now experience truly dark, starry skies.[xv] The broader ecological literature suggests that even small increases in light can have disproportionate effects on behaviour and interactions. There is no single threshold at which a piece of land suddenly flips from healthy to damaged. Instead, there is a spectrum, where each new light at the edge of a field, each slightly brighter car park or tourism development, gradually tips the balance.
What can I do?
The advantage of light pollution, if I look favourably at an unfavourable situation, is that it can be reduced rapidly if there is the will to do so. The scientific literature and local dark sky initiatives converge on a set of straightforward principles.
First, artificial light should only be used when it is genuinely needed. I spend much of my time in darkness and am now accustomed to it. Not all can say the same. For some, they see darkness as a threat. I was once a soldier, and so much took place in darkness that it almost became a way of life. It was when I became civilianised that head torches appeared, and all the problems they might create.
Second, if lighting must be used, its direction should be controlled. Fully shielded fittings that direct light downwards and avoid sideways or upward spill will reduce skyglow. They will also prevent dazzling along any walls and hedges that serve as wildlife corridors. Many different types of animals follow the lines of streams and walls when travelling from one point to another on a Lake District fell.
Third, and again, if you feel a need to use artificial light, try to minimise its intensity and duration. Many tasks on a farm or woodland holding can be undertaken safely with lower light levels and with motion sensors or timers rather than all-night lighting. This also reduces energy costs, which is no bad thing.
Fourth, choose the light spectrum carefully. Warmer, coloured lights with reduced blue content are generally less disruptive than cold, blue-rich sources. In practical terms, this means avoiding very cool white lamps and using warmer designs instead. Preferably, do not use a light at all.
Finally, work with neighbours and local bodies. Planning authorities increasingly recognise the value of dark skies. Mind you, please do not get me started on planning departments, as I have some material differences with them and sometimes wonder if their hearts really are in the same place as mine. The Lake District National Park promotes dark sky tourism and identifies particularly dark valleys, such as Langdale and Ennerdale, as assets to be protected. Local campaigns in Cumbria aim to secure Dark Sky Reserve status by raising awareness and reducing unnecessary lighting.
As a landowner, I can add my voice to those efforts, sharing the scientific evidence that light pollution is not only an aesthetic issue but one that affects bats, insects, plants, streams, and so much more, in multiple and different ways.
Over the past two decades, evidence has accumulated that light pollution is not only an ecological problem but also an important determinant of human health. Experimental and epidemiological studies link nocturnal light exposure to disruption of circadian rhythms, suppression of melatonin, impaired sleep, metabolic disturbance, adverse mental health and possibly even an increased cancer risk.[xvi] I was once a medic, so forgive me if I now become carried away.
Human biology
Human physiology is organised around endogenous circadian rhythms that are synchronised to the day/night cycle, thanks to light detected by the eye. This is primarily through a subset of retinal ganglion cells that contain the photopigment melanopsin and are most sensitive to short-wavelength blue light. Melanopsin plays a key role in certain functions, such as regulating the body's circadian rhythm, controlling pupil size, and suppressing melatonin production. Melatonin is a hormone that the brain produces in darkness to regulate the circadian rhythm. It is melatonin that signals it is time to sleep. Light reduces its production and promotes wakefulness.
It has been known for almost half a century that exposure to bright light at night acutely suppresses melatonin in humans.[xvii] The action spectrum for melatonin suppression shows a peak sensitivity in the blue range around 460–480 nanometres and supports the existence of a dedicated circadian photoreceptor system[xviii].
Melatonin has multiple endocrine, antioxidant, and oncostatic roles, suggesting a link between light pollution and disease. One research study reviewed experimental evidence that light at night, even at low intensity, altered circadian clocks, reduced glucose tolerance, changed the regulation of body weight and impaired both cardiovascular and immune function in animal models[xix]. Meanwhile, in humans, observational and experimental data showed that nocturnal light exposure disrupted circadian timing, suppressed melatonin and was associated with sleep disorders, metabolic disturbance and even cancer[xx].
Sleep
Sleep is the most immediate and well-documented activity affected by light pollution. Evening and nighttime light exposure delays the circadian rhythm and fragments sleep. Experimental studies in healthy volunteers have shown that dim artificial light at night, at levels similar to those in many bedrooms, increases rapid eye movement (REM) sleep and night-time waking[xxi]. Short-wavelength light has been shown to delay the circadian rhythm and increase alertness.
Epidemiological studies using personal light sensors, or satellite-derived outdoor light estimates, consistently report associations between higher night-time light exposure and poorer self-reported sleep quality, shorter sleep duration and greater odds of insomnia[xxii]. Sleep disruption is not merely a symptom, it is also a pathway through which nocturnal light can influence mental and physical health. Chronically insufficient or irregular sleep is a recognised risk factor for metabolic and cardiovascular disease, as well as depression.
Cardiometabolic outcomes
A growing body of research suggests that artificial light at night contributes to obesity and cardiometabolic disorders. Studies in rodents indicate that exposure to light during the usual darkness leads to weight gain, reduced glucose tolerance and altered lipid metabolism, even when food intake is unchanged.
In humans, large cohort studies have reported similar patterns. In a prospective study of more than 43,000 women in the United States, it was found that those who slept with a television or light on in the bedroom had a higher risk of becoming overweight compared with women who slept in dark rooms, even after adjustment for baseline body mass index, diet and physical activity[xxiii]. Similar findings were made in an older population, when higher light levels during sleep were associated with increased odds of obesity, diabetes and hypertension[xxiv].
These findings can be extended to cardiovascular outcomes. A study of a population-based Spanish cohort reported that higher outdoor artificial light at night, estimated from satellite data and modelled at participants’ residences, was associated with an increased incidence of cardiometabolic disease, including myocardial infarction and stroke, even after adjustment for air pollution and socioeconomic factors[xxv].
Children and adolescents are also vulnerable. It is not just adults and the elderly. A study from China found that indoor night-time light in bedrooms was associated with higher odds of being overweight in young people, particularly among those with low levels of physical activity[xxvi]. Taking all these reports together, it seems clear that light pollution is an emerging environmental risk factor throughout life.
Mental health
Circadian disruption is also an important contributor to mental health disorders. One review summarised evidence that misalignment of circadian rhythms with behavioural cycles was associated with a wide range of psychiatric conditions, including major depressive, bipolar, and anxiety disorders[xxvii]. Artificial light at night was a key environmental driver of such disruption.
Another study concluded that higher exposure to nocturnal light, whether outdoor or indoor, was associated with an increased risk of depression, anxiety, and sleep-related problems[xxviii]. More than one study has found this[xxix],[xxx]. It also appears that people living in highly illuminated urban environments may have a higher chance of insomnia and psychological distress. The available evidence thus supports light pollution as being a plausible environmental risk factor for poor mental health.
Cancer
The possibility that chronic exposure to night-time light also increases the risk of cancer has attracted considerable attention. Studies have linked higher levels of outdoor night-time illumination to an increased incidence of breast cancer[xxxi]. There is certainly a higher risk among women with greater ambient light exposure and in those with a history of night-shift work[xxxii].
The question of night-shift work has been looked at by the International Agency for Research on Cancer, which concluded that night-shift work was probably carcinogenic in humans[xxxiii]. This study was based on evidence for breast, prostate and colorectal cancers.
Vulnerable populations
Not all individuals are affected equally by light pollution. Many cohorts are urban with exposures that often correlate with traffic, noise, and socioeconomic disadvantage. Certainly, people living in deprived neighbourhoods are more likely to have bright street lighting outside their windows, live in smaller properties, and have work patterns that increase evening light exposure.
Older adults are especially vulnerable. Age-related changes in the lens and pupil reduce retinal light transmission, which can prompt individuals to use brighter lights, thereby causing a greater circadian disruption. Children and adolescents represent another group of concern, because of the widespread use of electronic devices late into the evening.
These patterns suggest that light pollution can widen health inequalities. That is, those with the least control over their lighting environment can experience the greatest disruption.
Public health implications
From a public health perspective, light pollution is an unusual environmental hazard. Light is essential for vision, safety, mobility, and wellbeing, yet its misuse can be harmful. Unlike many pollutants, light is highly controllable. It can be dimmed, redirected, filtered, or even switched off instantly.
Evidence from research suggests several principles. First, light should be directed only where it is needed. This reduces skyglow and limits light intrusion into bedrooms. Second, spectral composition matters. Lamps with lower short-wavelength content should be chosen for night-time outdoor use, especially in residential areas. This will reduce melatonin suppression and circadian disruption. Third, indoor environments should be arranged so that bedrooms remain as dark as possible at night, using blackout curtains, switching off televisions and unnecessary lights, and avoiding bright screen exposure before sleep.
Artificial light at night should be considered alongside diet, physical activity, and stress as a modifiable determinant of metabolic and mental health[xxxiv].
Conclusion
On a clear winter night, standing among the trees and bracken of my Lake District land, the stars still feel close. But the science of artificial light at night makes it clear that this darkness is fragile. Global analyses show that the lit surface of the planet continues to expand. National mapping reveals tongues of light pushing ever further into rural England. Local conservation groups in Cumbria warn that even here, in one of the darkest regions of the country, the night is under pressure.
For the species that share my land, and there are plenty, the consequences are varied but rarely good. Insects are drawn to distant lamps and car parks and may fail to pollinate the flowers on my slopes. Bats commuting along tree lines and streams, even rivers, may find their routes fragmented by new lights. Trees at the margins of illumination alter their growth and seasonal timing, with effects that ripple through herbivores, invertebrates, and soil life. Zooplankton in nearby waters alter their nocturnal movements when light reaches the surface, and the clarity of lakes and pools can change. Birds and mammals adjust their activity patterns, potentially at the cost of reproduction and survival. None of this is inevitable. Many of the most damaging features of light pollution arise from habit, convenience and a cultural assumption that brighter is always better.
It is sad to acknowledge that light pollution has transformed the human relationship with the night. Artificial illumination has enabled longer working hours, improved safety, and increased social activity after dark, but it has also extended daylight physiology into hours once reserved for rest. A substantial body of research evidence now links artificial light at night with circadian disruption, sleep disturbance, adverse metabolic and cardiovascular outcomes, poorer mental health and possible increases in some cancers. This strength of association is biologically worrying.
Light pollution manifestly constitutes an essential and emerging environmental health issue. Much further research is needed in this area, and is certainly awaited. Meanwhile, it is worth being suitably cautious and respecting the biological importance of darkness.
If you can, please turn off that light. Not after bedtime. Not tomorrow, not the next day, but now. Please move from whatever position you are in, flick that switch, and bring back darkness.
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Hashtags
#LightPollution #DarkSkies #LakeDistrict #NatureAtNight #EnvironmentalHealth #Biodiversity #HumanHealth #InsectsAndBats #NightEcology #TurnOffTheLight
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References
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