Photobiology is the study of the effects of non-ionizing radiation on biological systems. The biological effect varies with the wavelength region of the radiation.
Biological reaction to light is nothing new, there are numerous examples of light induced photochemical reactions in biological systems. Vitamin D synthesis in our skin is an example of a photochemical reaction. The power density of sunlight is only 105 mW/cm2 yet when ultraviolet B (UVB) rays strikes our skin, it converts a universally present form of cholesterol, 7-dehydrocholesterol to vitamin D3. We normally experience this through our eyes which are obviously photosensitive – our vision is based upon light hitting our retinas and creating a chemical reaction that allows us to see. Throughout the course of evolution, photons have played a vital role in photo-chemically energizing certain cells.
At the cellular level, visible red and near infrared light energy stimulates cells to generate more energy and promote optimal functioning. Each cell has mitochondria, which perform the function of producing cellular energy called “ATP”. This production process involves the respiratory chain. A mitochondrial enzyme called cytochrome oxidase c then accepts photonic energy, promoting enhanced ATP production.
It is currently posited that low level visible red to near infrared light energy is absorbed by mitochondria and converted into ATP for cellular use. In addition, the process creates reactive oxygen species (ROS) that leads to gene transcription. The process also enhances ATP production by releasing nitric oxide from the mitochondrial enzyme cytochrome c oxidase  The released nitric oxide can then activate a number of beneficial cellular pathways .
[1,2] Hamblin, M. R. (2016). Shining light on the head: Photobiomodulation for brain disorders. BBA Clin, 6, 113-124