Light has various effects such as effects on human circadian rhythm, sleep, endocrine hormone, alertness, and pupillary light reflex. These effects are called "non-visual effects" or "non-image forming effects", are induced by light signals projecting to the hypothalamus in the brain, not to the visual area. Human physiological functions adapt to the natural light-dark cycle. However, dynamic changes in natural sunlight have a negative effect on mood and circadian rhythm in high-latitude regions in winter. In modern society, it has been reported that artificial light at night has negative effects on human health via non-image forming pathways. Study on adaptation to natural light and artificial light is important topic for study in the field of physiological anthropology.
