Three types of cells help the brain tell day from night

Bright light at night interrupts the body's normal day-night cycles, called circadian rhythms, and can trigger insomnia. In fact, circadian rhythms play a major role in health. Disrupted day-night cycles have even been linked ...


Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye (about 400–700 nm, or perhaps 380–750 nm.) In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.

Three primary properties of light are:

Light, which exists in tiny "packets" called photons, exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.

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