The invention of the light bulb and subsequent development of longer working hours and more time to be awake and interact has truly altered life as we know it. No longer do humans need to go to sleep when the sun goes down or wait until the sun comes up to go to work.
However, humans, mammals, and some bacteria, all must respond to temporal changes in their environment in order to be successful . These are known as circadian rhythms, or essential body processes that entrain to environmental cues called zeitgebers.
In Latin, circa means “about” and dia means “day;” thus, circadian rhythms operate over a period of about 24 hours . Circadian rhythms are responsible for the sleep wake cycle, daily activity, and even metabolism and eating behaviors, and are overall very important for organism fitness. Circadian disruption can lead to detrimental behavioral and physiological consequences for organisms, particularly as a result from increased exposure to artificial light at night (ALAN).
This summary of scientific literature seeks to define light at night, its impacts, and to outline the myriad of consequences it may have.
First, the question of what constitutes light at night must be addressed.
Artificial light is generally referred to as any light source made by humans (e.g. fire, lamps, lightbulbs, etc.) and light pollution is the brightening of the night sky as a result of man-made light sources . As urban development has increased, the need for artificial light sources has also increased dramatically. These sources can be roadways, shopping centers, stadiums, and homes, to name a few .
This light strays and scatters into the atmosphere, brightening the sky more than normal and creating something called urban sky glow . In 2001, approximately 62% of the world’s population lived with a sky brightness higher than 62% .
According to a brief review published in 2010, light at night is increasing at a rate of about 6% per year . It’s also important to note that while humans and many other organisms are diurnal, or active during the day, approximately 30% of global biodiversity is nocturnal, or primarily active at night .
Therefore, light at night has a broad reach and is ever present in most areas where both humans and animals coexist.
There are a few other sources of light at night that exist for humans. The first is something called shift work. This is when individuals work out of phase, or time, with the normal light/dark cycle of the sun and moon. Of the population of workers, approximately 20% are shift or nighttime workers . Yet another source of exposure to light at night is blue light, which often comes in the form of technology usage. For example, in a study conducted by Gradisar et al (2017), 90% of surveyed people reported using a technological device in the bedroom the hour before bed. This increased exposure to light after the sun has already gone down can harmfully impact melatonin production. Melatonin production is important for instructing your body when to sleep, and exposure to even low level LED and blue light can slow production and thus impact sleep timing and quality . A discussion of the harmful effects of poor sleep specifically is beyond the scope of this paper, but can be found in the referenced review .
Of the harmful impacts of exposure to light at night, for humans, the most impactful seems to be shift work. Examples of shift work include meat processing, medical professionals, mechanical plant workers, and more. Most, if not all, studies examining circadian disruption and shift work have found adverse health effects. These effects include increased risk of cancer, sleep disturbances, gastrointestinal issues, impaired metabolism, and cardio-vascular diseases . For women, breast cancer is of great risk in shift workers. A pioneering study in over 70,000 nurses over a 10 year period showed a significant increase in breast cancer contraction with greater number of years as a nurse . In shift workers, glucose tolerance, or the body’s ability to process sugar, was shown to be impaired, leading to a greater risk for obesity . These are just two examples of the harmful impacts that circadian disruption can have on human health and well-being.
After reading this information about deliberate worker exposure to ALAN, one might become more curious about unknowing exposure to ALAN. An interesting study conducted in adolescents discovered those living in more urban areas with greater light intensity actually had a propensity to stay up later than their more rural counterparts . Despite the data about adolescents and shift work, it appears more research needs to be conducted regarding ALAN in urban areas and daily human exposure. Perhaps the literature is limited since in order for light to have an impact, it must reach the photoreceptors in the eye—thus, if one is sleeping, they may not be as effected by ALAN .
Humans are not the only organisms negatively impacted by ALAN. A multitude of plants and animals have been studied in order to examine the harmful impacts light pollution may be having on ecology. Perhaps one of the most notable animals impacted by ALAN are birds. Studies have shown that their migratory patterns can be altered, even drawing them to land near and stay within city limits . Reduction in tree cover in urban areas leaves birds exposed, and in consequence, at greater risk for predation. Interestingly, birds hatched in an urban environment developed their reproductive organs up to one month earlier than those in a rural environment, indicating many body processes can be harmfully impacted by ALAN .
In addition to birds, other animals impacted by ALAN include cougars , crickets , plants , lizards , sea turtles , and even coral . The scope of ALAN is expansive, and even more research needs to be conducted to understand just how far reaching it may be. Perhaps one of the other key examples are the dangers of light pollution for nesting and newborn sea turtles. Nighttime light significantly reduces the number of sea turtle nests on a particular beach . Furthermore, it can cause adult females to disorient and crawl away from the ocean, sometimes even onto busy roadways . This is detrimental to their fitness and can even prevent them from nesting. Efforts to educate the public and even enact certain laws about lights near vital nesting beaches have been successful in some areas, but more action is needed to eliminate these impacts.
Interestingly, ALAN also impacts plants and corals. Some early studies in deciduous trees near roadways discovered that artificial light has an impact on leaf retention, actually delaying leaf loss in the fall . Furthermore, artificial light may cause trees to form buds up to two weeks earlier in the springtime than trees not exposed to altered light sources . A variety of backyard plants and shrubbery have either enhanced or suppressed growth when exposed to differing light conditions . As for corals, light is critical for photosynthesis and calcification, and they have strong circadian behaviors . Furthermore, some corals depend on lunar illumination cues to synchronize biological processes . When coral was placed in a light pollution environment, the circadian associated genes in corals lost their rhythms, causing abnormalities . Additionally, corals showed increased oxidative stress and susceptibility to bleaching . Bleaching is when coral releases the algae living in its tissues; this doesn’t always kill the coral, but heavily impacts its stress response. Considering the reduction in coral reef densities as it is, this could have global impacts for ocean acidification. Ultimately, the harmful impacts of ALAN on plants and other flora need studied in much greater depth.
Finally, ALAN also has the ability to alter between-species interactions. For example, a Utah study discovered that mule deer were less active in areas with high levels of night light, and cougars were in turn more likely to hunt in high night light areas in the wild but not in urban areas . This does not just occur in apex predators. A study in insects discovered an increase in light at night increased predation of aphids by a particular lady bug species; the increased light may have made it easier for the ladybugs to seek out aphids for a greater period during the day . Thus, ALAN confounds both within species interactions and interactions between species.
In conclusion, ALAN has far-reaching impacts for what appears to be almost all of the organisms on Earth. Exposure to light at night has even been shown to alter chromatin dynamics in mice, which could have genetic involvement . Interestingly, most ALAN studies examine behavioral or hormonal characteristics; thus, it seems imperative that more genetic studies are conducted in both humans, animals, and plants in order to better understand what the long-term implications of ALAN might be. Further measures must be put into place in order to mediate the repercussions of these disrupted rhythms, or we may see dangerous impacts passed on for generations.
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WORDS: Elizabeth Kantra.