Our bodies operate optimally when we maintain regularity in our sleeping and waking hours. This regularity correlates to the 24-hour dark and light cycles in nature, also known as the circadian rhythm. With modern improvements in providing inexpensive artificial light, man has increasingly disturbed the dark cycle not only for himself, but also in nature. The disrupted circadian rhythm in man has resulted in an increase of health problems: cancer, metabolic disorder, heart and neurodegenerative diseases, asthma and rheumatoid arthritis.
An instrument in a satellite launched in 2011 has measured our night world and has found it getting brighter about two percent annually, resulting in our world being about 10 percent brighter in 2016. This night glow affects all living things, including bacteria, plants and other wild life, which operate in sync with the circadian cycle. To grow plants, light energy in the visible range is needed for photosynthesis. A flowering plant’s developmental processes, such as dormancy, shoot growth and flowering are dependent on different light wavelengths and durations of uninterrupted darkness during a 24-hour cycle. Different plants species may vary in their response to daylight length. For example, shortening their daylight exposure in the fall helps Christmas cacti and poinsettias to bloom around mid-December to January.
Scientists have been able to measure the effect of light disruption on plants and on their pollinators (insects, bats, birds and animals). Researchers found flowers in an artificially-lit meadow had 62 percent fewer visitors than flowers in the dark meadow. In another study of cabbage thistles, night-lit plants produced 13 percent fewer seeds overall than counterparts in naturally dark places. If such a decrease in nocturnal pollination is not made up by daytime pollination, a flower’s population might dwindle, causing a ripple effect on all the insects, birds and animals which depend on the flower and plant for food or shelter. In temperate zones the natural circadian rhythm in plants aligns their growth to seasons as the weather changes in different seasons. These plants prepare to drop their leaves to cope with cold weather as daylight shortens with the arrival of winter. Artificial lighting that extends the day length could promote continued growth when it is unsafe for the plant to do so.
Light pollution at nighttime can produce undesirable consequences for plants. If we need light to improve safety for nighttime pedestrian and vehicular traffic, the light fixtures should be shielded so that their light output is directed toward the ground and away from plants and trees. In all cases, up-lighting and shining light over great horizontal distances should be avoided. Lights should be turned off or dimmed during off-peak hours to avoid continuous lighting of trees, which has the greatest potential for upsetting normal growth patterns. When planting trees where supplemental night lighting already exists, select varieties with low sensitivity to light or use light frequencies (fluorescent, mercury vapor and metal halides) that do the least harm.