The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D3 and a mutagen.[1] A dietary supplement can supply vitamin D without this mutagenic effect,[2] but the supplement bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from sunlight. Vitamin D has a wide range of positive health effects, which include strengthening bones[3] and possibly inhibiting the growth of some cancers.[4][5] UV exposure also has positive effects for endorphin levels, and possibly for protection against multiple sclerosis. Visible sunlight to the eyes gives health benefits through its association with the timing of melatonin synthesis, maintenance of normal and robust circadian rhythms, and reduced risk of seasonal affective disorder.[6]
Long-term sunlight exposure is known to be associated with the development of skin cancer, skin aging, immune suppression, and eye diseases such as cataracts and macular degeneration.[7] Short-term over-exposure is the cause of sunburn, snow blindness, and solar retinopathy.
UV rays, and therefore sunlight and sunlamps, are the only listed carcinogens that are known to have health benefits,[8] and a number of public health organizations state that there needs to be a balance between the risks of having too much sunlight or too little.[9] There is a general consensus that sunburn should always be avoided.