Light is definitely a nutrient. It is essential to life and the whole endocrine system. Light has a role in triggering hormones. Vitamin D is synthesized by ultraviolet in the skin. Vitamin D receptors help proper bone development and prevent development of rickets.
Vitamin D facilitates the absorption of calcium.
Under light not closely approximating the sun, one study found calcium absorption dropped off in the elderly in the indoors in winter. But those under full-spectrum lighting had an increase in calcium absorption. Solar radiation activates other important biochemical events in our bodies involved in endocrine control, timing of our biological clocks, entrainment of 24-hour circadian rhythms, immunologic responsiveness, sexual growth and development, regulation of stress and fatigue, control of viral and cold infections, and dampening of functional disorders of the nervous system. He said the last two or three generations are the first to have spent three-fourths of their lives under artificial light. The Russians know more than Americans, perhaps, about the health effects of various kinds of light.