Circadian rhythm is linked to sleep, arousal and human health overall, affecting body temperature and heart rate. A 24-h natural-light cycle provides optimum lighting environment for humans. However, as people increasingly stay indoors with artificial lighting, lacking periodic characteristics, imbalance in the circadian rhythm ensues. Previous lighting-related studies to resolve such problem partially provided the colour temperatures of natural light but failed to reproduce the 24-h periodic characteristics of it. This study proposes a natural light-reproducing system that provides the daylight cycle characteristics of natural light in order to maintain the circadian rhythm. Natural light was measured through an optical measurement equipment, while the characteristics (colour temperature and short-wavelength ratio) of natural light by season and time were analysed. Subsequently, the control indicator of seasonal and hourly lighting was extracted and applied to the light-emitting diode lighting to provide lighting service, executing a daylight cycle that reflects the characteristics of natural light. After the sunset, especially, the circadian rhythm was maintained by minimizing the short-wavelength ratio of the lighting while maintaining indoor illumination.

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