Does winter darkness in the Arctic protect against cancer? The melatonin hypothesis revisited

The melatonin hypothesis states that excess exposure to environmental light may contribute to breast cancer risks via impaired pineal secretion of mel atonin. A corollary, not considered previously, is that a net annual increa se in oncostatic melatonin would be expected in persons who experience a li ght deficit during extended winter darkness periods; thus, hormone-dependen t cancers should occur less frequently in people who reside north, rather t han south, of the Arctic circle. Consistent with our prediction, epidemiolo gical data indicate uniformly low risks for hormone-dependent cancers in th e Arctic. The available literature on genetic, reproductive, nutritional, l ife-style, and environmental risk factors provides no obvious clues to the observed cancer patterns. Moreover, diurnal and 24-hour melatonin concentra tions in humans living in Arctic regions were reported as high in November- January, when light intensity is low. Since these observations are consiste nt with our corollary and the associated prediction, we suggest that resear ch on a melatonin-inhibited carcinogenesis in the low-risk populations of t he Arctic should be pursued.