During the winter months light pollution’s affect on melatonin production is especially damaging to human health.
Many studies have shown that melatonin levels fluctuate seasonally. Melatonin is a hormone that is produced with the circadian rhythm by the pineal gland. Interestingly, melatonin production is inhibited by blue light. Melatonin directly causes drowsiness, thermoregulation (your body temperature drops during sleep), changes in blood pressure, and glucose homeostasis. In addition, melatonin secretion is used to regulate seasonal biological functions such as reproduction. The body accomplishes this by using a changing day-to-night ratio that occurs as the seasons change.
In 1991, a study by Dr. Thomas A. Wehr entitled The Durations of Human Melatonin Secretion and Sleep Respond to Changes in Daylength (Photoperiod) showed through artificial photoperiods that melatonin production could be changed. Dr. Wehr questioned whether such changes in melatonin levels would lead to significant seasonal changes in human physiology and behavior. In 2010, Harvard School of Medicine and the Brigham and Women’s Hospital published a study with 116 subjects showing that even bedroom light or dim light inhibited melatonin production before bedtime. About 90 minutes of melatonin production was lost on a daily basis by electrical lighting. The authors cautioned against chronic exposure to electrical light and its effect on daily biological functions.
In October of 2011, Italian scientists published a study showing that the migration from sodium lamps to metallic halide lighting and LEDs would result in a five-fold increase in light pollutions's effect on melatonin production. The authors warned that unless lighting had stricter regulations, light pollution's known and unknown consequences to human health and the environment would increase.
During the winter months light pollution is especially acute because much of the evening is spent under artificial light.