Periodic episodes of increased sunspot activity (solar electromagnetic storms) occur with 10-11 and 5-6 year periodicities and may be associated with measurable biological events. We investigated whether this sunspot periodicity characterized the incidence of Pap smear-determined cervical epithelial histopathologies and human physiologic functions. From January 1983 through December 2003, monthly averages were obtained for solar flux and sunspot numbers; six infectious, premalignant and malignant changes in the cervical epithelium from 1,182,421 consecutive, serially independent, screening Pap smears (59°9″N, 4°29″E); and six human physiologic functions of a healthy man (oral temperature, pulse, systolic and diastolic blood pressure, respiration, and peak expiratory flow), which were measured ∼5 times daily during ∼34,500 self-measurement sessions (44°56″N, 93°8″W). After determining that sunspot numbers and solar flux, which were not annually rhythmic, occurred with a prominent 10-year and a less-prominent 5.75-year periodicity during this 21-year study span, each biological data set was analyzed with the same curve-fitting procedures. All six annually rhythmic Pap smear-detected infectious, premalignant and malignant cervical epithelial pathologies showed strong 10-year and weaker 5.75-year cycles, as did all six self-measured, annually rhythmic, physiologic functions. The phases (maxima) for the six histopathologic findings and five of six physiologic measurements were very near, or within, the first two quarters following the 10-year solar maxima. These findings add to the growing evidence that solar magnetic storm periodicities are mirrored by cyclic phase-locked rhythms of similar period length or lengths in human physiology and pathophysiology.

The incidence rate of cutaneous melanoma has been increasing faster than that of any other cancer in white-skinned populations over the past decades. The main risk factors for melanoma (i.e. exposure to sunlight, naevus count, phototype, and family history of melanoma) may not wholly explain the epidemiological trends observed for this cancer. The light-at-night theory postulates that increasing use of artificial light-at-night may contribute to the increasing breast cancer incidence through suppressed secretion of melatonin (a hormone produced in the dark and inhibited by light, which regulates circadian rhythms). Here, we postulate that this theory may also apply to melanoma and that it may explain a part of this cancer burden. Consistent with our hypothesis is evidence from experimental studies suggesting a lightening effect of melatonin on frog skin and mammal hair during seasonal changes, its antioxidant and anti-carcinogenic effects in skin melanocytes, as well as the expression of melatonin receptors in melanocytes. Also, epidemiological data suggest lower melatonin concentrations in melanoma patients compared with controls; a potential therapeutic effect of melatonin in patients with metastatic disease; a higher prevalence of melanoma in pilots and aircrews, with increased risks with higher time zones travelled; and increased melanoma risks in office workers exposed to fluorescent lighting. Moreover, melanoma incidence and seasonal patterns are consistent with a reduction of melatonin secretion with intensity of exposure to light, although it remains difficult to distinguish the effect of melatonin disruption from that of sun exposure on the basis of ecological studies. Finally, the reported associations between hormonal factors and melanoma are consistent with melatonin inhibition increasing the risk of melanoma by increasing circulating oestrogen levels. Despite the existing suggestive evidence, the light-at-night hypothesis has never been directly tested for melanoma. Very few studies examined the potential associations between melanoma risk and shift work or melatonin concentrations, and we found no studies reporting on the relationship between melanoma and number of sleeping hours, use of melatonin supplements, blindness, night-time city light levels, bedroom light levels, or clock genes polymorphisms. Therefore, since several observations support our hypothesis and very little research has been undertaken on this subject, we strongly encourage analytic epidemiological studies to test the light-at-night theory for melanoma causation.