ABSTRACT
Humans adapt to the natural light-dark cycle according to the rotation of the Earth on its axis, and many physiological functions of humans have a circadian rhythm. However, natural light changes dynamically depending on latitude, season, and weather. Changes in natural light have a strong effect on mood and circadian rhythm in high-latitude regions in winter, when durations of sunshine are very short. In modern society, humans have succeeded in excluding night time from the life space and workplace by extensive use of artificial light. Despite the benefits of artificial light, recent studies have shown that artificial light at night has negative effects on human sleep, circadian rhythm, and health (Stevens et al. 2007).
