For thousands of years in human history, the sun was the only source of blue light – one of the colors in the visible light spectrum. The natural blue light makes the sky appear blue as the blue wavelengths hit the air molecules.
Also, for us humans, a moderate amount of exposure to natural blue light has been medically proven to support the body’s natural sleep and wake cycle (circadian rhythm), promote alertness, and boost memory and cognitive function.
However, not all blue light is good.
On the light spectrum, blue light between 380nm and 445nm is harmful, which mainly comes from artificial blue light.