Abstract
With the inherent sleep and wake cycle regulated by natural sunlight, the human body has evolved over millennia to be active during the day and to rest at night. However, maintaining an optimal 24 h cycle has become increasingly problematic in modern society as more people spend the majority of the day indoors. Many research groups have reported that inadequate artificial lighting interferes with melatonin production and disrupts the circadian rhythm. This study considered biological functions for light-emitting diodes (LEDs) of next-generation illumination, and LED packages and spectra suitable for both daytime and nighttime applications were designed. The prepared daytime human-centric (HC)-LEDs had a melanopic/photopic (M/P) ratio that was up to 26% higher than that of conventional (c)-LEDs, whereas the nighttime HC-LEDs exhibited up to a 26% lower M/P ratio compared to the c-LEDs. Nevertheless, because the HC-LED is designed to have almost the same color coordinates as the c-LED having the same correlated-color temperature (CCT), there is no change in the perceived color. To substantiate the biological effect, melatonin level data were obtained from 22 voluntary participants in c- and HC-LED lighting environments. In the HC-LED lighting environment, melatonin was suppressed by 21.9% after waking, and nocturnal melatonin secretion was increased by up to 12.2%. As human-centric lighting, our HC-LEDs are expected to become an essential element for modern life, where people spend most of their time indoors.