External Light Effects Circadian Rhythms

It is becoming more evident that humans run on circadian rhythm cycles, which control our sleep-awake routines. One thing that has been shown to effect these defined cycles is the presence or absence of bright light. Researchers have now discovered, using mice as an animal model, that bright lights not only simply affect these rhythms, but can have physical, lasting effects on the body.

Eliane Lucassen and Johanna Meijer of Leiden University Medical Center in the Netherlands have produced pressing evidence that artificial, bright lights can prematurely age mice and cause various health problems, including bone-density loss, skeletal-muscle weakness, and inflammation. While previous studies have been done to hint at bright lights' effects on physical health, direct evidence was not clear.

Lucassen and Meijer implanted electrodes in mice brains to the areas associated with the circadian rhythm cycles and housed these mice in brightly lit cages for 24 weeks. Their discovers showed that neuronal patterns had shifted irregularly, which is a clear association with what happens in aging brains, and led to the mice adopting a roughly 25.5-hour day, rather than a typical 24-hour day. The researchers sought out these experiments to observe the affects humans in busy cities may experience on a daily basis, as roughly 2/3 of the world's population is exposed to light at night. 

These findings coincide with Dr. Cavanaugh's presentation on the circadian rhythm clock that is biologically innate in fruit flies. Dr. Cavanaugh explained the 4 endogenous genes--Clock, Cycle, Period, and Timeless--that are associated with the natural circadian rhythm. The experiments done in Dr. Cavanaugh's lab sought to determine if dimly lit or completely dark environments had an effect on the flies' natural sleeping cycles. He measured the amount of movement the flies had in a tube via infrared beams and found that in constant darkness, the flies did indeed move less, and therefore slept more in this environment, as compared to an environment that stimulated a typical 12-hour light period followed by a 12-hour dark period. 

Together, Meijer's and Dr. Cavanaugh's data conclude that environmental cues of light and darkness can alter the natural circadian rhythms cycles across animal models, and furthermore can have physical health consequences on said models. This could greatly impact the way in which physicians can classify risk factors when considering ailments such as cardiovascular disease, osteoporosis, cancer, and so much more. 

Reference: https://www.scientificamerican.com/article/bright-light-speeds-up-aging-in-mice/