Long-term exposure to artificial light weakens the body’s 24-hour circadian rhythm, says a new study published in Current Biology in July 2016 by Eliane A. Lucassen et al. In addition to disrupting the biological clock, continuous light deteriorates muscle strength, bone microstructure, and innate immune response. Allowing the body to return to a natural light-dark structure allows it to restore its natural framework.
Today, most people are exposed to light both indoors and outdoors after the sun has set, due to the use of light bulbs, screens, and more. So, how does this affect overall human health?
The Circadian Rhythm
The superchiasmatic nucleus (SCN) is a region of the hypothalamus in the brain, and it controls the body’s circadian rhythm—a daily, 24-hour cycle. Neurons in the SCN dictate the rhythms that help an organism adapt to the day-to-night cycle of its environment. The SCN also synchronizes other processes in the body, including muscle function, bone metabolism, and immune responses. Organisms probably learn to adapt to circadian rhythms in order to regulate and synchronize healthy bodily systems.
According to the study, 75% of the world is now exposed to light during hours of the night. Growing numbers of people are working night shifts, and epidemiological studies report a higher prevalence of breast cancer, metabolic syndrome, osteoporosis, and bone fractures among these people. Those who are exposed to more light at night also have a higher prevalence of low sleep quality, obesity, and cardiovascular disease. Previous studies have shown that short-term abnormal exposure to light has affected both the immune and metabolic systems of animals. In this study, scientists sought to discover the effects of long-term exposure to an abnormal light-dark cycle on mice.
24 Weeks of Light
Scientists exposed mice in an experimental group to continuous light for 24 weeks, using electrodes to record rhythmic changes in the SCN. At various points, they evaluated skeletal muscle function, bone microstructure, and immune system function in the mice. Afterwards, the mice were exposed to a normal light-dark cycle (a recovery phase consisting of 12 hours of light and 12 hours of darkness per day) for another 24 weeks. A control group of mice was exposed to a normal light-dark cycle for all 48 weeks.
Circadian rhythms of mice were weakened in both groups, but the effect was much stronger in those exposed to continuous light. During the recovery phase of the experiment, rhythms of mice in the experimental group returned to normal 24-hour periods. Compared to the control group, weight and glucose levels were higher in mice exposed to continuous light, but these levels decreased once the mice recovered in a normal light-dark cycle. Firing rates of neurons in the SCN decreased once mice were exposed to darkness during the recovery phase of the experiment.
Muscle function weakened in both groups due to natural aging. The experimental group, however, had significantly lower grip strength. This difference disappeared during the recovery phase. Mice in the experimental group had less dense, thinner, and more rod-like bones—all symptoms of early osteoporosis—at the end of the experimental phase. These symptoms disappeared as well after exposure to a normal light-dark cycle. In mice of the experimental group, secretion of pro-inflammatory cytokines was higher and secretion of anti-inflammatory cytokines was lower. A pro-inflammatory state resulted in the immune systems of mice exposed to continuous light, which returned to normal function once the normal light-dark cycle was in place.
Nightly exposure to artificial light weakens neuronal rhythms in the SCN, causing not only internal clocks to change, but also other systems in the body to weaken. Long-term exposure to artificial light leads to sleep disturbance and disruptions in the autonomic nervous system and HPA axis. Moreover, bodily functions that rely on rhythms provided by the SCN—bone, muscle, and immune functions—are weakened as well. These results are seen in this experimental study in mice and correlate with epidemiological studies in humans. Scientists emphasize the necessity of a normal light-dark cycle for bodies to function properly and effectively. As recognized in this study, returning to a normal light-dark cycle can minimize the harmful effects of artificial light.
Lucassen et al., Environmental 24-hr Cycles Are Essential for Health, Current Biology (2016), http://dx.doi.org/ 10.1016/j.cub.2016.05.038