As I explained in the ‘Interesting story about Light Science’ Vol.2 (Light and Body Clock), the human body adjusts to the light and maintains the 24-hour cycle of biological rhythms, thereby controlling important activities such as sleep-wake rhythm and hormone secretion. One of the important hormones regulated by this circadian rhythm is melatonin, which is sometimes called sleeping hormone because of its ability to induce sleep.
Sunlight received through our eyes during the daytime suppresses melatonin secretion and keeps us awake. But when it gets dark at night, secretion of the sleeping hormone becomes active and makes us sleep.
Besides its roles in circadian regulation, melatonin also functions in cancer suppression. In fact, a study by the Barcelona Institute for Global Health in Spain found that people exposed to night lights had a 1.47 times higher risk of developing breast cancer and a 2.05-fold higher risk of developing prostate cancer. It is hypothesized that blue light emitted from lights at night prevents melatonin secretion, in turn disrupting circadian rhythm and inhibiting its cancer suppression function.
While artificial lighting with different spectrum from natural light offers many conveniences, clearly there are adverse effects on health due to disturbance of sleep and circadian rhythm. In fact, the research results of Seoul National University have shown that the time taken to fall asleep is reduced by 1.3 times when using illumination that resembles natural light of the evening by minimizing blue light compared to general artificial lighting. As it is difficult to completely exclude the use of night lighting in our daily lives, it is most ideal to use a comfortable light similar to a torchlight spectrum to maintain a good sleep and circadian rhythm.
<Aerial image of the city of Barcelona, Spain at night>
Credit: Image courtesy of the Earth Science and Remote Sensing Unit,