Vitamin D is technically not a vitamin as one’s body can produce it – it just requires sunlight. No one is suggesting that sunlight is a vitamin per se. And only a relatively short span of radiation from the sun is involved, the so-called UVB of the ultraviolet spectrum. The intensity of UVB available depends on the weather, season, location on earth, and time of day; in the United States it maximizes between 10 am and 4 pm between April and October.