The light on the back of your hand left the sun eight minutes ago, but it is older than it looks: it was born in the sun's core and spent tens of thousands of years working its way to the surface. As it lands, it sets off a chain of responses in your body. Your skin turns 7-dehydrocholesterol, a cousin of cholesterol, into vitamin D and releases stored nitric oxide into the blood. Through the eyes, it tells the brain's internal clock that the day has started. Somewhere a strand of DNA takes a small hit, and repair enzymes arrive to cut out the damaged stretch and rebuild it.
In 1903 Niels Finsen won a Nobel Prize for curing tuberculosis of the skin with concentrated light. Around 1920 sun lamps, then plain sunlight, cured rickets in sooty cities, and Alpine clinics put tuberculosis patients on balconies in the snow. By 1928 a lamp had given mice skin cancer, and by the 1980s the message on the poster had turned around.
The effects of sunlight on the human body are hard to study. There's no standard dose: wavelength, latitude, season, hour, clothing, and a pane of glass all change what arrives, and skin color, ancestry, and genes change what a body does with it. You can't assign someone a decade of Norwegian winters. Most studies never measure sunlight itself; they measure a stand-in for it: vitamin D in the blood, questionnaires about time spent outdoors, the latitude of a town. The stand-ins fall short of the thing itself; vitamin D pills, tested in large trials, have not reproduced most of what sun-rich lives seem to carry. And no one holds a patent on the sun, so the money behind big, careful trials mostly goes to things that make a profit. Between that and the difficulty of isolating the variables and measuring what real sunlight does, our most rigorous science has drifted toward keeping actual sunlight at arm's length and testing lamps, pills, and creams in its place.
We are The Sunlight Report. We screened 157,842 scientific records and kept 784 studies for close reading. They hold a great deal of documented harm and a great deal of documented benefit, and we intend to show both. From them we aim to build the most accessible and comprehensive analysis of what science knows about how sunlight interacts with the human body, including the questions no one has answered yet.