Artificial lighting has become a widespread feature of modern society, transforming outdoor environments. Its primary purpose is to extend human activities into nighttime hours, providing visibility and enhancing safety. Over 80% of the world’s population, and more than 99% in regions like the U.S. and Europe, experience skies affected by artificial light. The increasing use of artificial light sources, from streetlights to building exteriors and electronic devices, has fundamentally altered natural light cycles. These changes impact human well-being and have broader ecological consequences.
Light and Human Physiology
Artificial light at night can significantly interfere with the human body’s natural 24-hour circadian rhythm, often called the body clock. This internal clock regulates biological processes like sleep-wake cycles, metabolism, hormone secretion, and alertness. The suprachiasmatic nucleus (SCN) in the hypothalamus, a structure deep within the brain, acts as the master clock, receiving light information from the eyes.
Specialized cells in the retina, called intrinsically photosensitive retinal ganglion cells (ipRGCs), detect light and send signals to the SCN. These cells are sensitive to blue wavelengths of light, typically 450 to 490 nanometers. When these photoreceptors sense light, especially blue light, they signal the SCN that it is daytime, which can delay or advance the sleep-wake cycle.
The SCN influences the pineal gland to produce melatonin, a hormone that promotes drowsiness and prepares the body for sleep. Melatonin levels naturally rise in the evening and remain high throughout the night, dropping in the morning as natural light increases. Exposure to artificial light, particularly blue-enriched light from screens and some LED lighting, can suppress melatonin production. Even low room light in the evening can significantly reduce melatonin levels and shorten its production by about 90 minutes.
This suppression of melatonin and disruption of the circadian rhythm can lead to difficulties falling asleep, staying asleep, and feeling alert during the day. Chronic misalignment of circadian rhythms has been linked to health concerns, including metabolic disorders, increased stress hormone levels, and mental health conditions such as depression. Children, women, and older adults may exhibit higher sensitivity to light-induced melatonin suppression.
Lighting’s Influence on Ecosystems
Artificial light at night, often termed light pollution, alters the natural cycles of light and darkness. This disruption affects a wide array of nocturnal animals and ecosystems. Many species rely on natural light cues for behaviors such as navigation, foraging, reproduction, and evading predators.
Light pollution can disorient migratory birds, which use moonlight and starlight to navigate. Bright city lights can draw them off course, leading to collisions with buildings and towers, resulting in millions of bird fatalities annually. This disruption can also cause birds to migrate too early or too late, causing them to miss suitable conditions for nesting or finding food.
Insects, such as moths, are attracted to artificial light sources, leading to erratic flight patterns and increased predation risk. This effect can disrupt local insect populations and affect the food chains that depend on them. For example, bats, which prey on insects, may lose food sources in brightly lit areas.
Marine life is also vulnerable to artificial light, which can interfere with breeding, nesting, feeding, and camouflage. Sea turtle hatchlings, for instance, naturally orient themselves towards the brightest horizon, typically the ocean. Artificial lights on beaches can mislead them inland, away from the sea. Zooplankton, microscopic organisms that migrate to the sea surface at night, can become confused by artificial lighting, remaining at deeper depths and disrupting their natural rhythms and food sources for fish that prey on them.
Beyond animal behavior, artificial light can affect plant physiology, including altered flowering times. Some studies suggest that urban skyglow can promote the growth of toxic algae blooms, such as those caused by cyanobacteria, impacting aquatic and surrounding wildlife.
Thoughtful Outdoor Illumination
Addressing the impacts of nighttime lighting involves making deliberate choices about outdoor illumination. One consideration is to use only the minimum amount of light necessary for a given task, rather than excessive illumination. Selecting light bulbs with the lowest appropriate brightness, measured in lumens, can achieve sufficient visibility without overlighting an area.
Directing light downwards is a practical approach to reduce light spill and glare. This can be achieved through full cutoff fixtures, designed to emit no light above the horizontal plane. Many existing fixtures can be modified with shields to direct light more effectively.
The color temperature of outdoor lighting is also important. Warmer-toned lights (3000K or lower) are preferred over cooler, bluer lights. Lights in the yellow or amber range are less disruptive to human sleep patterns and less impactful on wildlife, as many nocturnal animals are sensitive to blue wavelengths.
Incorporating controls such as timers and motion sensors can minimize unnecessary light exposure and energy consumption. Lights equipped with motion sensors activate only when needed, turning off automatically after a short period. Aligning lighting with specific needs helps reduce light pollution and promotes a healthier nighttime environment.