Sunlight: For Better or For Worse? A Review of Positive and Negative Effects of Sun Exposure

During the last decades new, mainly favorable, associations between sunlight and disease have been discovered, initially ascribed to vitamin D. There is, however, accumulating evidence that the formation of nitric oxide, melatonin, serotonin, endorphin, photodegradation of folic acid, immunomodulation, photoadaptation, and the effect of (sun)light on circadian clocks, are involved as well. After a systematic search in the literature, a summary is given of (recent) research on the health effects of sun exposure and the possibly involved mechanisms. In the last 200 years our exposure to sunlight has changed radically: from a more continuous to an intermittent exposure. Our exposure to light during the day and to artificial light in the evening and at night has changed as well. The present ‘epidemic’ of skin cancer is mainly caused by the increase of intermittent sun exposure, coinciding with decrease of chronic exposure. Effects of chronic and occupational exposure appear to be latitude-dependent: risk of skin cancer decreases with increasing latitude. In North-western Europe chronic exposure yields a relatively low risk of melanoma and (to a lesser degree) of basal cell carcinoma and squamous cell carcinoma. There is epidemiological and experimental evidence that chronic exposure to sunlight could contribute to the prevention of colorectal-, breast-, prostate cancer, non-Hodgkin Lymphoma, multiple sclerosis, and metabolic syndrome. The possible consequences of these findings for public health messages on sun exposure are discussed. It is concluded that both too much and too little sunlight may be harmful to our health.