For over a decade, it has been acknowledged that those who work night shifts have an increased risk of developing internal cancers.1 From that paper until the present time, the primary hypothesis to explain the finding was disruption of the circadian rhythm and reduced production of melatonin. The evidence for an effect of melatonin on reducing risk of cancer is strongest for breast cancer.2 For example; breast cancer has peaks of diagnosis in spring and fall.3 The hypothesis to explain this finding is that vitamin D reduces risk of breast cancer in summer while melatonin associated with reduced sunlight in winter reduces breast cancer risk in winter. However, there does not seem to be such seasonality for other types of cancer.
A recent study from Canada found that men who worked night shifts had increased incidence rates of cancers of the bladder, colon, lung, pancreas, and prostate, and non-Hodgkin’s lymphoma.4 They found non-significant increased incidence for cancers of the esophagus, kidney, and stomach. They did not find that cancer risk was related to cumulative duration of night work. One wonders whether night shift workers in Canada tend to be night owls.
The evidence that solar ultraviolet-B (UVB) and vitamin D reduce the risk of cancers of the bladder, colon, kidney, lung, pancreas, and stomach, and non-Hodgkin’s lymphoma is relatively strong.5, 6 There is evidence that UVB reduces risk of prostate cancer, but no evidence that vitamin D reduces risk of prostate cancer.7 While the authors in their response note that farmers have an increased risk of prostate cancer, they omitted to cite their paper reporting that exposure to pesticides was an important risk factor for prostate cancer among farmers in Canada.8 My recent study of occupation and cancer incidence in Nordic countries found a modest inverse correlation for the solar UVB index associated with outdoors work and incidence of prostate cancer.6 The discrepancy between ecological studies showing reduced risk of prostate cancer associated with solar UVB and the lack of an association between serum 25-hydroxyvitamin D and prostate cancer incidence rates is puzzling.9
A study among female nurses in the United States found that night shift work for women was associated with reduced risk of melanoma and non-melanoma skin cancer.10 Risk was reduced in proportion of years of night shift work, which is a dose-dependent relation. It was also reported that those with dark brown/black hair color had significant dose-dependent reductions in all three types of skin cancer but those with light brown or red/blonde hair did not. This finding can be understood in that those with light colored hair have lighter skin and are more likely to burn in the sun, so try to avoid sun exposure.11 The authors also reported no effect that could be attributed to sun exposure in terms of residence, ultraviolet light exposure level at baseline, or location of skin cancer. While they admitted that the finding of reduced skin cancer incidence rates was in contrast to many previous studies reporting increased risk of internal cancers associated with night shift work, they proposed that there was a possible genetic component that might explain the extent of melatonin suppression for women with dark hair. In my opinion, this study strongly supports the hypothesis that night shift workers spend less time in the sun than others.
In my letter to the editor, I stated that the findings from the Canadian and U.S. studies were consistent with low solar UVB irradiance due to sleeping during the day.12
In further discussions with those who study night shift work, it was pointed out that a number of factors could explain the increased risk of internal cancers among night shift workers including low melatonin and adverse diets due to eating from vending machines.
The authors in their response pointed out that Montreal, where the study was conducted, was at 45º N, and it is impossible to produce vitamin D from solar UVB for about half of the year. In response to that point, in my study of cancer incidence rates among people in the five Nordic countries with a latitude range of 55º-70º N, there is significantly reduced risk of many types of cancer for those with the occupations with the greatest amount of time spent out of doors.6
While other risk factors for cancer among night shift workers cannot be ruled out, the easiest one to correct and the one with strong evidence is vitamin D deficiency. Based on numerous studies, night shift workers should consider taking 2000-5000 IU/d vitamin D3 and raising serum 25-hydroxyvitamin D levels to 40-60 ng/ml.13, 14